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|
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font-family: arial, helvetica, sans-serif; font-size: x-small;
vertical-align: top; width: 33%;
color: #FFF; background-color: #666;
}
td.author { font-weight: bold; font-size: x-small; margin-left: 4em; }
td.author-text { font-size: x-small; }
/* info code from SantaKlauss at http://www.madaboutstyle.com/tooltip2.html */
a.info {
/* This is the key. */
position: relative;
z-index: 24;
text-decoration: none;
}
a.info:hover {
z-index: 25;
color: #FFF; background-color: #900;
}
a.info span { display: none; }
a.info:hover span.info {
/* The span will display just on :hover state. */
display: block;
position: absolute;
font-size: smaller;
top: 2em; left: -5em; width: 15em;
padding: 2px; border: 1px solid #333;
color: #900; background-color: #EEE;
text-align: left;
}
a { font-weight: bold; }
a:link { color: #900; background-color: transparent; }
a:visited { color: #633; background-color: transparent; }
a:active { color: #633; background-color: transparent; }
p { margin-left: 2em; margin-right: 2em; }
p.copyright { font-size: x-small; }
p.toc { font-size: small; font-weight: bold; margin-left: 3em; }
table.toc { margin: 0 0 0 3em; padding: 0; border: 0; vertical-align: text-top; }
td.toc { font-size: small; font-weight: bold; vertical-align: text-top; }
ol.text { margin-left: 2em; margin-right: 2em; }
ul.text { margin-left: 2em; margin-right: 2em; }
li { margin-left: 3em; }
/* RFC-2629 <spanx>s and <artwork>s. */
em { font-style: italic; }
strong { font-weight: bold; }
dfn { font-weight: bold; font-style: normal; }
cite { font-weight: normal; font-style: normal; }
tt { color: #036; }
tt, pre, pre dfn, pre em, pre cite, pre span {
font-family: "Courier New", Courier, monospace; font-size: small;
}
pre {
text-align: left; padding: 4px;
color: #000; background-color: #CCC;
}
pre dfn { color: #900; }
pre em { color: #66F; background-color: #FFC; font-weight: normal; }
pre .key { color: #33C; font-weight: bold; }
pre .id { color: #900; }
pre .str { color: #000; background-color: #CFF; }
pre .val { color: #066; }
pre .rep { color: #909; }
pre .oth { color: #000; background-color: #FCF; }
pre .err { background-color: #FCC; }
/* RFC-2629 <texttable>s. */
table.all, table.full, table.headers, table.none {
font-size: small; text-align: center; border-width: 2px;
vertical-align: top; border-collapse: collapse;
}
table.all, table.full { border-style: solid; border-color: black; }
table.headers, table.none { border-style: none; }
th {
font-weight: bold; border-color: black;
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table.all th, table.full th { border-style: solid; }
table.headers th { border-style: none none solid none; }
table.none th { border-style: none; }
table.all td {
border-style: solid; border-color: #333;
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}
table.full td, table.headers td, table.none td { border-style: none; }
hr { height: 1px; }
hr.insert {
width: 80%; border-style: none; border-width: 0;
color: #CCC; background-color: #CCC;
}
--></style></head><body>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<table summary="layout" border="0" cellpadding="0" cellspacing="0" width="66%"><tbody><tr><td><table summary="layout" border="0" cellpadding="2" cellspacing="1" width="100%">
<tbody><tr><td class="header">Final</td><td class="header"> specs@openid.net</td></tr>
<tr><td class="header"> </td><td class="header">December 5, 2007</td></tr>
</tbody></table></td></tr></tbody></table>
<h1><br>OpenID Authentication 2.0 - Final</h1>
<h3>Abstract</h3>
<p>
OpenID Authentication provides a way to prove that an end user
controls an Identifier. It does this without the Relying Party
needing access to end user credentials such as a password or
to other sensitive information such as an email address.
</p>
<p>
OpenID is decentralized. No central authority must approve or
register Relying Parties or OpenID Providers. An end user
can freely choose which OpenID Provider to use, and can
preserve their Identifier if they switch OpenID Providers.
</p>
<p>
While nothing in the protocol requires JavaScript or modern
browsers, the authentication scheme plays nicely with
"AJAX"-style setups. This means an end user can prove their
Identity to a Relying Party without having to leave their
current Web page.
</p>
<p>
OpenID Authentication uses only standard HTTP(S) requests and
responses, so it does not require any special capabilities of
the User-Agent or other client software. OpenID is not tied to
the use of cookies or any other specific mechanism of Relying
Party or OpenID Provider session management. Extensions to
User-Agents can simplify the end user interaction, though are
not required to utilize the protocol.
</p>
<p>
The exchange of profile information, or the exchange of other
information not covered in this specification, can be addressed
through additional service types built on top of this
protocol to create a framework. OpenID Authentication is
designed to provide a base service to enable portable,
user-centric digital identity in a free and decentralized manner.
</p><a name="toc"></a><br><hr>
<h3>Table of Contents</h3>
<p class="toc">
<a href="#anchor1">1.</a>
Requirements Notation and Conventions<br>
<a href="#terminology">2.</a>
Terminology<br>
<a href="#anchor2">3.</a>
Protocol Overview<br>
<a href="#anchor3">4.</a>
Data Formats<br>
<a href="#anchor4">4.1.</a>
Protocol Messages<br>
<a href="#btwoc">4.2.</a>
Integer Representations<br>
<a href="#anchor6">5.</a>
Communication Types<br>
<a href="#direct_comm">5.1.</a>
Direct Communication<br>
<a href="#indirect_comm">5.2.</a>
Indirect Communication<br>
<a href="#generating_signatures">6.</a>
Generating Signatures<br>
<a href="#anchor11">6.1.</a>
Procedure<br>
<a href="#sign_algos">6.2.</a>
Signature Algorithms<br>
<a href="#anchor12">7.</a>
Initiation and Discovery<br>
<a href="#initiation">7.1.</a>
Initiation<br>
<a href="#normalization">7.2.</a>
Normalization<br>
<a href="#discovery">7.3.</a>
Discovery<br>
<a href="#associations">8.</a>
Establishing Associations<br>
<a href="#anchor17">8.1.</a>
Association Session Request<br>
<a href="#anchor20">8.2.</a>
Association Session Response<br>
<a href="#assoc_types">8.3.</a>
Association Types<br>
<a href="#assoc_sess_types">8.4.</a>
Association Session Types<br>
<a href="#requesting_authentication">9.</a>
Requesting Authentication<br>
<a href="#anchor27">9.1.</a>
Request Parameters<br>
<a href="#realms">9.2.</a>
Realms<br>
<a href="#anchor28">9.3.</a>
Immediate Requests<br>
<a href="#responding_to_authentication">10.</a>
Responding to Authentication Requests<br>
<a href="#positive_assertions">10.1.</a>
Positive Assertions<br>
<a href="#negative_assertions">10.2.</a>
Negative Assertions<br>
<a href="#verification">11.</a>
Verifying Assertions<br>
<a href="#verify_return_to">11.1.</a>
Verifying the Return URL<br>
<a href="#verify_disco">11.2.</a>
Verifying Discovered Information<br>
<a href="#verify_nonce">11.3.</a>
Checking the Nonce<br>
<a href="#verifying_signatures">11.4.</a>
Verifying Signatures<br>
<a href="#identifying">11.5.</a>
Identifying the end user<br>
<a href="#extensions">12.</a>
Extensions<br>
<a href="#rp_discovery">13.</a>
Discovering OpenID Relying Parties<br>
<a href="#compat_mode">14.</a>
OpenID Authentication 1.1 Compatibility<br>
<a href="#anchor34">14.1.</a>
Changes from OpenID Authentication 1.1<br>
<a href="#anchor38">14.2.</a>
Implementing OpenID Authentication 1.1 Compatibility<br>
<a href="#security_considerations">15.</a>
Security Considerations<br>
<a href="#anchor41">15.1.</a>
Preventing Attacks<br>
<a href="#anchor43">15.2.</a>
User-Agents<br>
<a href="#anchor44">15.3.</a>
User Interface Considerations<br>
<a href="#anchor45">15.4.</a>
HTTP and HTTPS URL Identifiers<br>
<a href="#anchor46">15.5.</a>
Denial of Service Attacks<br>
<a href="#anchor47">15.6.</a>
Protocol Variants<br>
<a href="#anchor48">Appendix A.</a>
Examples<br>
<a href="#normalization_example">Appendix A.1.</a>
Normalization<br>
<a href="#anchor49">Appendix A.2.</a>
OP-Local Identifiers<br>
<a href="#XRDS_Sample">Appendix A.3.</a>
XRDS<br>
<a href="#anchor50">Appendix A.4.</a>
HTML Identifier Markup<br>
<a href="#anchor51">Appendix A.5.</a>
XRI CanonicalID<br>
<a href="#pvalue">Appendix B.</a>
Diffie-Hellman Key Exchange Default Value<br>
<a href="#anchor52">Appendix C.</a>
Acknowledgements<br>
<a href="#rfc.references1">16.</a>
Normative References<br>
<a href="#rfc.authors">§</a>
Author's Address<br>
</p>
<br clear="all">
<a name="anchor1"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.1"></a><h3>1.
Requirements Notation and Conventions</h3>
<p>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in <a class="info" href="#RFC2119">[RFC2119]<span> (</span><span class="info">Bradner, B., “Key words for use in RFCs to Indicate Requirement Levels,” .</span><span>)</span></a>.
</p>
<p>
Throughout this document, values are quoted to indicate that
they are to be taken literally. When using these values in
protocol messages, the quotes MUST NOT be used as part of the
value.
</p>
<a name="terminology"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.2"></a><h3>2.
Terminology</h3>
<p>
</p>
<blockquote class="text"><dl>
<dt>Identifier:</dt>
<dd>
An Identifier is either a "http" or "https" URI, (commonly
referred to as a "URL" within this document), or an <a class="info" href="#XRI_Syntax_2.0">XRI<span> (</span><span class="info">Reed, D. and D. McAlpin, “Extensible Resource Identifier (XRI) Syntax V2.0,” .</span><span>)</span></a> [XRI_Syntax_2.0]. This document defines
various kinds of Identifiers, designed for use in different
contexts.
</dd>
<dt>User-Agent:</dt>
<dd>
The end user's Web browser which implements HTTP/1.1 <a class="info" href="#RFC2616">[RFC2616]<span> (</span><span class="info">Fielding,
R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T.
Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .</span><span>)</span></a>.
</dd>
<dt>Relying Party:</dt>
<dd>
RP. A Web application that wants proof that the end user
controls an Identifier.
</dd>
<dt>OpenID Provider:</dt>
<dd>
OP. An OpenID Authentication server on which a Relying
Party relies for an assertion that the end user controls
an Identifier.
</dd>
<dt>OP Endpoint URL:</dt>
<dd>
The URL which accepts OpenID Authentication protocol messages,
obtained by performing discovery on the User-Supplied
Identifier. This value MUST be an absolute HTTP or HTTPS URL.
</dd>
<dt>OP Identifier:</dt>
<dd>
An Identifier for an OpenID Provider.
</dd>
<dt>User-Supplied Identifier:</dt>
<dd>
An Identifier that was presented by the end user to the
Relying Party, or selected by the user at the OpenID
Provider. During the initiation phase of the protocol,
an end user may enter either their own Identifier or an OP
Identifier. If an OP Identifier is used, the OP may then
assist the end user in selecting an Identifier to share with
the Relying Party.
</dd>
<dt>Claimed Identifier:</dt>
<dd>
An Identifier that the end user claims to own; the overall
aim of the protocol is verifying this claim. The Claimed
Identifier is either:
<ul class="text">
<li>
The Identifier obtained by <a class="info" href="#normalization">normalizing<span> (</span><span class="info">Normalization</span><span>)</span></a> the User-Supplied Identifier, if it
was an URL.
</li>
<li>
The <a class="info" href="#canonicalid">CanonicalID<span> (</span><span class="info">XRI and the CanonicalID Element</span><span>)</span></a>, if it
was an XRI.
</li>
</ul>
</dd>
<dt>OP-Local Identifier:</dt>
<dd>
An alternate Identifier for an end user that is local to a
particular OP and thus not necessarily under the end user's
control.
</dd>
</dl></blockquote><p>
</p>
<a name="anchor2"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.3"></a><h3>3.
Protocol Overview</h3>
<p>
</p>
<ol class="text">
<li>
The end user <a class="info" href="#initiation">initiates
authentication<span> (</span><span class="info">Initiation</span><span>)</span></a> by presenting a User-Supplied Identifier
to the Relying Party via their User-Agent.
</li>
<li>
After <a class="info" href="#normalization">normalizing<span> (</span><span class="info">Normalization</span><span>)</span></a> the
User-Supplied Identifier, the Relying Party <a class="info" href="#discovery">performs discovery<span> (</span><span class="info">Discovery</span><span>)</span></a> on it and
establishes the OP Endpoint URL that the end user uses for
authentication. It should be noted that the User-Supplied
Identifier may be an OP Identifier, as discussed in <a class="info" href="#discovered_info">Section 7.3.1<span> (</span><span class="info">Discovered Information</span><span>)</span></a>, which allows selection of a
Claimed Identifier at the OP or for the protocol to
proceed without a Claimed Identifier if something else
useful is being done via an <a class="info" href="#extensions">extension<span> (</span><span class="info">Extensions</span><span>)</span></a>.
</li>
<li>
(optional)
The Relying Party and the OP establish an <a class="info" href="#associations">association<span> (</span><span class="info">Establishing Associations</span><span>)</span></a> -- a shared
secret established using <a class="info" href="#RFC2631">Diffie-Hellman Key Exchange<span> (</span><span class="info">Rescorla, E., “Diffie-Hellman Key Agreement Method,” .</span><span>)</span></a> [RFC2631]. The
OP uses an association to sign subsequent messages and the
Relying Party to verify those messages; this removes the
need for subsequent direct requests to verify the
signature after each authentication request/response.
</li>
<li>
The Relying Party redirects the end user's User-Agent to
the OP with an OpenID <a class="info" href="#requesting_authentication">Authentication
request<span> (</span><span class="info">Requesting Authentication</span><span>)</span></a>.
</li>
<li>
The OP establishes whether the end user is authorized to
perform OpenID Authentication and wishes to do so. The
manner in which the end user authenticates to their OP and
any policies surrounding such authentication is out of
scope for this document.
</li>
<li>
The OP redirects the end user's User-Agent back to the
Relying Party with either an assertion that <a class="info" href="#positive_assertions">authentication is
approved<span> (</span><span class="info">Positive Assertions</span><span>)</span></a> or a message that <a class="info" href="#negative_assertions">authentication failed<span> (</span><span class="info">Negative Assertions</span><span>)</span></a>.
</li>
<li>
The Relying Party <a class="info" href="#verification">verifies<span> (</span><span class="info">Verifying Assertions</span><span>)</span></a> the information received from the OP including
checking the Return URL, verifying the discovered
information, checking the nonce, and verifying the
signature by using either the shared key established
during the association or by sending a direct request
to the OP.
</li>
</ol><p>
</p>
<a name="anchor3"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.4"></a><h3>4.
Data Formats</h3>
<a name="anchor4"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.4.1"></a><h3>4.1.
Protocol Messages</h3>
<p>
The OpenID Authentication protocol messages are
mappings of plain-text keys to plain-text values. The keys and
values permit the full Unicode character set (UCS). When the
keys and values need to be converted to/from bytes, they
MUST be encoded using <a class="info" href="#RFC3629">UTF-8<span> (</span><span class="info">Yergeau, F., “UTF-8, a transformation format of Unicode and ISO 10646,” .</span><span>)</span></a> [RFC3629].
</p>
<p>
Messages MUST NOT contain multiple parameters with the same name.
</p>
<p>
Throughout this document, all OpenID message parameters are
REQUIRED, unless specifically marked as OPTIONAL.
</p>
<a name="kvform"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.4.1.1"></a><h3>4.1.1.
Key-Value Form Encoding</h3>
<p>
A message in Key-Value form is a sequence of lines. Each
line begins with a key, followed by a colon, and the value
associated with the key. The line is terminated by a
single newline (UCS codepoint 10, "\n"). A key or value
MUST NOT contain a newline and a key also MUST NOT contain
a colon.
</p>
<p>
Additional characters, including whitespace, MUST NOT be
added before or after the colon or newline. The message
MUST be encoded in UTF-8 to produce a byte string.
</p>
<p>
Key-Value Form encoding is used for signature calculation
and for <a class="info" href="#direct_response">direct
responses<span> (</span><span class="info">Direct Response</span><span>)</span></a> to Relying Parties.
</p>
<a name="http_encoding"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.4.1.2"></a><h3>4.1.2.
HTTP Encoding</h3>
<p>
When a message is sent to an HTTP server, it MUST be encoded
using a form encoding specified in Section 17.13.4 of
<a class="info" href="#HTML401">[HTML401]<span> (</span><span class="info">W3C, “HTML 4.01 Specification,” .</span><span>)</span></a>. Likewise, if the "Content-Type"
header is included in the request headers, its value MUST
also be such an encoding.
</p>
<p>
All of the keys in the request message MUST be prefixed
with "openid.". This prefix prevents interference with
other parameters that are passed along with the OpenID
Authentication message. When a message is sent as a POST,
OpenID parameters MUST only be sent in, and extracted
from, the POST body.
</p>
<p>
All messages that are sent as HTTP requests (GET or POST)
MUST contain the following fields:
</p>
<ul class="text">
<li>
openid.ns
<blockquote class="text">
<p>
Value: "http://specs.openid.net/auth/2.0"
</p>
<p>
This particular value MUST be present for the
request to be a valid OpenID Authentication 2.0
request. Future versions of the specification may
define different values in order to allow message
recipients to properly interpret the request.
</p>
<p>
If this value is absent or set to one of
"http://openid.net/signon/1.1" or
"http://openid.net/signon/1.0", then this message
SHOULD be interpreted using <a class="info" href="#compat_mode">OpenID Authentication 1.1
Compatibility mode<span> (</span><span class="info">OpenID Authentication 1.1 Compatibility</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.mode
<blockquote class="text">
<p>
Value: Specified individually for each message
type.
</p>
<p>
The "openid.mode" parameter allows the recipient
of the message to know what kind of message it is
processing. If "openid.mode" is absent, the party
processing the message SHOULD assume that the
request is not an OpenID message.
</p>
</blockquote>
</li>
</ul><p>
</p>
<p>
This model applies to messages from the User-Agent to both
the Relying Party and the OP, as well as messages from the
Relying Party to the OP.
</p>
<a name="anchor5"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.4.1.3"></a><h3>4.1.3.
Example</h3>
<p>
Non-normative
</p>
<p>
The following examples encode the following information:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre>Key | Value
--------+---------------------------
mode | error
error | This is an example message
</pre></div>
<p>
Key-Value Form encoded:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre>mode:error
error:This is an example message
</pre></div>
<p>
x-www-urlencoded, as in a HTTP POST body or in a URL's
query string (<a class="info" href="#RFC3986">[RFC3986]<span> (</span><span class="info">Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> section 3):
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre>openid.mode=error&openid.error=This%20is%20an%20example%20message</pre></div>
<a name="btwoc"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.4.2"></a><h3>4.2.
Integer Representations</h3>
<p>
Arbitrary precision integers MUST be encoded as big-endian
signed two's complement binary strings. Henceforth, "btwoc"
is a function that takes an arbitrary precision integer and
returns its shortest big-endian two's complement
representation. All integers that are used with
Diffie-Hellman Key Exchange are positive. This means that
the left-most bit of the two's complement representation
MUST be zero. If it is not, implementations MUST add a zero
byte at the front of the string.
</p>
<p>Non-normative example:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre>Base 10 number | btwoc string representation
---------------+----------------------------
0 | "\x00"
127 | "\x7F"
128 | "\x00\x80"
255 | "\x00\xFF"
32768 | "\x00\x80\x00"
</pre></div>
<a name="anchor6"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5"></a><h3>5.
Communication Types</h3>
<a name="direct_comm"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5.1"></a><h3>5.1.
Direct Communication</h3>
<p>
Direct communication is initiated by a Relying Party to an
OP endpoint URL. It is used for <a class="info" href="#associations">establishing associations<span> (</span><span class="info">Establishing Associations</span><span>)</span></a> and
<a class="info" href="#check_auth">verifying authentication
assertions<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>.
</p>
<a name="direct_request"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5.1.1"></a><h3>5.1.1.
Direct Request</h3>
<p>
The message MUST be encoded as a POST body, as specified
by <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
<p>
All direct requests are HTTP POSTs, and so
contain the required fields listed in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
<a name="direct_response"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5.1.2"></a><h3>5.1.2.
Direct Response</h3>
<p>
The body of a response to a <a class="info" href="#direct_request">Direct Request<span> (</span><span class="info">Direct Request</span><span>)</span></a> consists of
an HTTP Response body in <a class="info" href="#kvform">Key-Value
Form<span> (</span><span class="info">Key-Value Form Encoding</span><span>)</span></a>. The content-type of the response SHOULD be
"text/plain".
</p>
<p>
All Key-Value form message MUST contain the following field:
</p>
<ul class="text">
<li>
ns
<blockquote class="text">
<p>
Value: "http://specs.openid.net/auth/2.0"
</p>
<p>
This particular value MUST be present for the
response to be a valid OpenID 2.0 response. Future
versions of the specification may define different
values in order to allow message recipients to
properly interpret the request.
</p>
<p>
If this value is absent or set to one of
"http://openid.net/signon/1.1" or
"http://openid.net/signon/1.0", then this message
SHOULD be interpreted using <a class="info" href="#compat_mode">OpenID Authentication 1.1
Compatibility mode<span> (</span><span class="info">OpenID Authentication 1.1 Compatibility</span><span>)</span></a>.
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="anchor7"></a><br><hr>
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<a name="rfc.section.5.1.2.1"></a><h3>5.1.2.1.
Successful Responses</h3>
<p>
A server receiving a valid request MUST send a
response with an HTTP status code of 200.
</p>
<a name="anchor8"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5.1.2.2"></a><h3>5.1.2.2.
Error Responses</h3>
<p>
If a request is malformed or contains invalid arguments,
the server MUST send a response with a status code of
400. The response body MUST be a <a class="info" href="#kvform">Key-Value Form<span> (</span><span class="info">Key-Value Form Encoding</span><span>)</span></a> message with the
following fields:
</p>
<p>
</p>
<ul class="text">
<li>
ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#direct_response">Section 5.1.2<span> (</span><span class="info">Direct Response</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
error
<blockquote class="text">
<p>
Value: A human-readable message indicating the cause
of the error.
</p>
</blockquote>
</li>
<li>
contact
<blockquote class="text">
<p>
Value: (optional) Contact address for the
administrator of the sever. The contact address
may take any form, as it is intended to be
displayed to a person.
</p>
</blockquote>
</li>
<li>
reference
<blockquote class="text">
<p>
Value: (optional) A reference token, such
as a support ticket number or a URL to a news
blog, etc.
</p>
</blockquote>
</li>
</ul><p>
The OP MAY add additional fields to this response.
</p>
<a name="indirect_comm"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5.2"></a><h3>5.2.
Indirect Communication</h3>
<p>
In indirect communication, messages are passed through the
User-Agent. This can be initiated by either the Relying
Party or the OP. Indirect communication is used for <a class="info" href="#requesting_authentication">authentication
requests<span> (</span><span class="info">Requesting Authentication</span><span>)</span></a> and <a class="info" href="#responding_to_authentication">authentication
responses<span> (</span><span class="info">Responding to Authentication Requests</span><span>)</span></a>.
</p>
<p>
There are two methods for indirect communication: HTTP
redirects and HTML form submission.
Both form submission and redirection require that the sender
know a recipient URL and that the recipient URL expect
indirect messages, as specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>. The initiator of the communication chooses which method
of indirect communication is appropriate depending on
capabilities, message size, or other external factors.
</p>
<p>
All indirect messages arrive as HTTP requests, and so
contain the required fields listed in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
<a name="anchor9"></a><br><hr>
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<a name="rfc.section.5.2.1"></a><h3>5.2.1.
HTTP Redirect</h3>
<p>
Data can be transferred by issuing a 302, 303, or 307 HTTP
Redirect to the end user's User-Agent. The redirect URL is
the URL of the receiver with the OpenID Authentication
message appended to the query string, as specified in
<a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
<a name="anchor10"></a><br><hr>
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<a name="rfc.section.5.2.2"></a><h3>5.2.2.
HTML FORM Redirection</h3>
<p>
A mapping of keys to values can be transferred by
returning an HTML page to the User-Agent that contains an
HTML form element. Form submission MAY be automated,
for example by using JavaScript.
</p>
<p>
The <form> element's "action" attribute value MUST
be the URL of the receiver. Each Key-Value pair MUST be
included in the form as an <input> element. The key
MUST be encoded as the "name" attribute and the value as
the "value" attribute, such that the User-Agent will
generate a message as specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a> when the form is submitted. The
form MUST include a submit button.
</p>
<a name="indirect_error"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.5.2.3"></a><h3>5.2.3.
Indirect Error Responses</h3>
<p>
In the case of a malformed request, or one that contains
invalid arguments, the OpenID Provider MUST redirect the
User-Agent to the "openid.return_to" URL value if the
value is present and it is a valid URL.
</p>
<p>
</p>
<ul class="text">
<li>
openid.ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.mode
<blockquote class="text">
<p>
Value: "error"
</p>
</blockquote>
</li>
<li>
openid.error
<blockquote class="text">
<p>
Value: A human-readable message indicating the cause
of the error.
</p>
</blockquote>
</li>
<li>
openid.contact
<blockquote class="text">
<p>
Value: (optional) Contact address for the
administrator of the sever. The contact address
may take any form, as it is intended to be
displayed to a person.
</p>
</blockquote>
</li>
<li>
openid.reference
<blockquote class="text">
<p>
Value: (optional) A reference token, such as a
support ticket number or a URL to a news blog,
etc.
</p>
</blockquote>
</li>
</ul><p>
The server MAY add additional keys to this response.
</p>
<p>
If the malformed or invalid message is received by the Relying
Party, or "openid.return_to" is not present or its value is not
a valid URL, the server SHOULD return a response to the
end user indicating the error and that it is unable to continue.
</p>
<a name="generating_signatures"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.6"></a><h3>6.
Generating Signatures</h3>
<p>
The most common usage of an association is as a Message
Authentication Code (MAC) key used to sign OpenID
Authentication messages.
</p>
<p>
When generating MAC keys, the recommendations in <a class="info" href="#RFC1750">[RFC1750]<span> (</span><span class="info">Eastlake, D., Crocker, S., and J. Schiller, “Randomness Recommendations for Security,” .</span><span>)</span></a> SHOULD be followed.
</p>
<a name="anchor11"></a><br><hr>
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<a name="rfc.section.6.1"></a><h3>6.1.
Procedure</h3>
<p>
To generate a message signature:
</p>
<ol class="text">
<li>
Determine the list of keys to be signed, according to
the message to be signed (See <a class="info" href="#positive_assertions">Section 10.1<span> (</span><span class="info">Positive Assertions</span><span>)</span></a>). The list of keys to be
signed MUST be part of the message. The list is stored
with the key "openid.signed". The value is a
comma-separated list of keys, with the "openid." prefix
stripped. This algorithm is only capable of signing keys
that start with "openid."
</li>
<li>
Iterate through the list of keys to be signed in the
order they appear in "openid.signed" list. For each
key, find the value in the message whose key is equal to
the signed list key prefixed with "openid."
</li>
<li>
Convert the list of key/value pairs to be signed to an
octet string by encoding with <a class="info" href="#kvform">Key-Value Form Encoding<span> (</span><span class="info">Key-Value Form Encoding</span><span>)</span></a>.
</li>
<li>
Determine the signature algorithm from the <a class="info" href="#associations">association type<span> (</span><span class="info">Establishing Associations</span><span>)</span></a>. Apply
the <a class="info" href="#sign_algos">signature algorithm<span> (</span><span class="info">Signature Algorithms</span><span>)</span></a>
to the octet string.
</li>
</ol><p>
</p>
<a name="sign_algos"></a><br><hr>
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<a name="rfc.section.6.2"></a><h3>6.2.
Signature Algorithms</h3>
<p>
OpenID Authentication supports two signature algorithms:
</p>
<ul class="text">
<li>HMAC-SHA1 - 160 bit key length (<a class="info" href="#RFC2104">[RFC2104]<span> (</span><span class="info">Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” .</span><span>)</span></a> and <a class="info" href="#RFC3174">[RFC3174]<span> (</span><span class="info">Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” .</span><span>)</span></a>)
</li>
<li>HMAC-SHA256 - 256 bit key length (<a class="info" href="#RFC2104">[RFC2104]<span> (</span><span class="info">Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” .</span><span>)</span></a> and <a class="info" href="#FIPS180-2">[FIPS180‑2]<span> (</span><span class="info">U.S.
Department of Commerce and National Institute of Standards and
Technology, “Secure Hash Signature Standard,” .</span><span>)</span></a>
</li>
</ul><p>
If supported, the use of HMAC-SHA256 is RECOMMENDED.
</p>
<a name="anchor12"></a><br><hr>
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<a name="rfc.section.7"></a><h3>7.
Initiation and Discovery</h3>
<a name="initiation"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.1"></a><h3>7.1.
Initiation</h3>
<p>
To initiate OpenID Authentication, the Relying Party SHOULD
present the end user with a form that has a field for
entering a User-Supplied Identifier.
</p>
<p>
The form field's "name" attribute SHOULD have the value
"openid_identifier", so that User-Agents can automatically
determine that this is an OpenID form. Browser extensions or
other software that support OpenID Authentication may not
detect a Relying Party's support if this attribute is not
set appropriately.
</p>
<a name="normalization"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.2"></a><h3>7.2.
Normalization</h3>
<p>
The end user's input MUST be normalized into an
Identifier, as follows:
</p>
<p>
</p>
<ol class="text">
<li>
If the user's input starts with the "xri://" prefix,
it MUST be stripped off, so that XRIs are used in the
canonical form.
</li>
<li>
If the first character of the resulting string is an
XRI Global Context Symbol ("=", "@", "+", "$", "!") or "(", as
defined in Section 2.2.1 of <a class="info" href="#XRI_Syntax_2.0">[XRI_Syntax_2.0]<span> (</span><span class="info">Reed, D. and D. McAlpin, “Extensible Resource Identifier (XRI) Syntax V2.0,” .</span><span>)</span></a>,
then the input SHOULD be treated as an XRI.
</li>
<li>
Otherwise, the input SHOULD be treated as an http URL; if it
does not include a "http" or "https" scheme, the Identifier
MUST be prefixed with the string "http://". If the URL
contains a fragment part, it MUST be stripped off
together with the fragment delimiter character "#". See
<a class="info" href="#http_s_identifiers">Section 11.5.2<span> (</span><span class="info">HTTP and HTTPS URL Identifiers</span><span>)</span></a> for more information.
</li>
<li>
URL Identifiers MUST then be further normalized by both
following redirects when retrieving their content and
finally applying the rules in Section 6 of <a class="info" href="#RFC3986">[RFC3986]<span> (</span><span class="info">Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> to the final destination URL. This
final URL MUST be noted by the Relying Party as the
Claimed Identifier and be used when <a class="info" href="#requesting_authentication">requesting
authentication<span> (</span><span class="info">Requesting Authentication</span><span>)</span></a>.
</li>
</ol><p>
See <a class="info" href="#normalization_example">normalization
example<span> (</span><span class="info">Normalization</span><span>)</span></a>.
</p>
<a name="discovery"></a><br><hr>
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<a name="rfc.section.7.3"></a><h3>7.3.
Discovery</h3>
<p>
Discovery is the process where the Relying Party uses the
Identifier to look up ("discover") the necessary information
for initiating requests. OpenID Authentication has three
paths through which to do discovery:
</p>
<p>
</p>
<ol class="text">
<li>
If the identifier is an XRI, <a class="info" href="#XRI_Resolution_2.0">[XRI_Resolution_2.0]<span> (</span><span class="info">Wachob,
G., Reed, D., Chasen, L., Tan, W., and S. Churchill, “Extensible
Resource Identifier (XRI) Resolution V2.0 - Committee Draft 02,” .</span><span>)</span></a>
will yield an XRDS document that
contains the necessary information. It should also be
noted that Relying Parties can take advantage of
XRI Proxy Resolvers, such as the one provided by
XDI.org at http://www.xri.net. This will remove the need for the RPs to
perform XRI Resolution locally.
</li>
<li>
If it is a URL, the <a class="info" href="#Yadis">Yadis
protocol<span> (</span><span class="info">Miller, J., “Yadis Specification 1.0,” .</span><span>)</span></a> [Yadis] SHALL be first attempted. If it
succeeds, the result is again an XRDS document.
</li>
<li>
If the Yadis protocol fails and no valid XRDS document
is retrieved, or no <a class="info" href="#service_elements">Service Elements<span> (</span><span class="info">OpenID Service Elements</span><span>)</span></a> are
found in the XRDS document, the URL is retrieved and
<a class="info" href="#html_disco">HTML-Based discovery<span> (</span><span class="info">HTML-Based Discovery</span><span>)</span></a>
SHALL be attempted.
</li>
</ol><p>
</p>
<a name="discovered_info"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.3.1"></a><h3>7.3.1.
Discovered Information</h3>
<p>
Upon successful completion of discovery, the Relying Party
will have one or more sets of the following information
(see the <a class="info" href="#terminology">Terminology
section<span> (</span><span class="info">Terminology</span><span>)</span></a> for definitions). If more than one set of
the following information has been discovered, the
precedence rules defined in <a class="info" href="#XRI_Resolution_2.0">[XRI_Resolution_2.0]<span> (</span><span class="info">Wachob,
G., Reed, D., Chasen, L., Tan, W., and S. Churchill, “Extensible
Resource Identifier (XRI) Resolution V2.0 - Committee Draft 02,” .</span><span>)</span></a>
are to be applied.
</p>
<ul class="text">
<li>OP Endpoint URL
</li>
<li>Protocol Version
</li>
</ul><p>
If the end user did not enter an OP Identifier, the
following information will also be present:
</p>
<ul class="text">
<li>Claimed Identifier
</li>
<li>OP-Local Identifier
</li>
</ul><p>
If the end user entered an OP Identifier, there is no
Claimed Identifier. For the purposes of making OpenID
Authentication requests, the value
"http://specs.openid.net/auth/2.0/identifier_select"
MUST be used as both the Claimed Identifier and the
OP-Local Identifier when an OP Identifier is entered.
</p>
<a name="anchor13"></a><br><hr>
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<a name="rfc.section.7.3.2"></a><h3>7.3.2.
XRDS-Based Discovery</h3>
<p>
If XRI or Yadis discovery was used, the result will be an
XRDS Document. This is an XML document with entries for
services that are related to the Identifier. It is
defined in <a class="info" href="#XRI_Resolution_2.0">Section 3
of<span> (</span><span class="info">Wachob,
G., Reed, D., Chasen, L., Tan, W., and S. Churchill, “Extensible
Resource Identifier (XRI) Resolution V2.0 - Committee Draft 02,” .</span><span>)</span></a> [XRI_Resolution_2.0]. See <a class="info" href="#XRDS_Sample">Appendix A.3<span> (</span><span class="info">XRDS</span><span>)</span></a> for an
example XRDS document.
</p>
<a name="service_elements"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.3.2.1"></a><h3>7.3.2.1.
OpenID Service Elements</h3>
<a name="anchor14"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.3.2.1.1"></a><h3>7.3.2.1.1.
OP Identifier Element</h3>
<p>
An OP Identifier Element is an <xrd:Service>
element with the following information:
</p>
<blockquote class="text"><dl>
<dt></dt>
<dd>
An <xrd:Type> tag whose text content is
"http://specs.openid.net/auth/2.0/server".
</dd>
<dt></dt>
<dd>
An <xrd:URI> tag whose text content is the
OP Endpoint URL
</dd>
</dl></blockquote><p>
</p>
<a name="anchor15"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.3.2.1.2"></a><h3>7.3.2.1.2.
Claimed Identifier Element</h3>
<p>
A Claimed Identifier Element is an
<xrd:Service> element with the following
information:
</p>
<blockquote class="text"><dl>
<dt></dt>
<dd>
An <xrd:Type> tag whose text content is
"http://specs.openid.net/auth/2.0/signon".
</dd>
<dt></dt>
<dd>
An <xrd:URI> tag whose text content is the
OP Endpoint URL.
</dd>
<dt></dt>
<dd>
An <xrd:LocalID> tag (optional) whose text
content is the OP-Local Identifier.
</dd>
</dl></blockquote><p>
</p>
<a name="extracting_auth"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.3.2.2"></a><h3>7.3.2.2.
Extracting Authentication Data</h3>
<p>
Once the Relying Party has obtained an XRDS document, it
MUST first search the document (following the rules
described in <a class="info" href="#XRI_Resolution_2.0">[XRI_Resolution_2.0]<span> (</span><span class="info">Wachob,
G., Reed, D., Chasen, L., Tan, W., and S. Churchill, “Extensible
Resource Identifier (XRI) Resolution V2.0 - Committee Draft 02,” .</span><span>)</span></a>) for
an OP Identifier Element. If none is found, the RP will search
for a Claimed Identifier Element.
</p>
<a name="canonicalid"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.7.3.2.3"></a><h3>7.3.2.3.
XRI and the CanonicalID Element</h3>
<p>
When the Identifier is an XRI, the <xrd:XRD>
element that contains the OpenID Authentication
<xrd:Service> element MUST also contain a
<CanonicalID> element. The content of this element
MUST be used as the Claimed Identifier (see <a class="info" href="#identifying">Section 11.5<span> (</span><span class="info">Identifying the end user</span><span>)</span></a>). This is a vital security
consideration because a primary purpose of the
<CanonicalID> element is to assert a persistent
identifier that will never be reassigned, thus
preventing the possibility of an XRI being ("taken
over") by a new registrant.
</p>
<p>
The Relying Party MUST confirm that the provider of the
XRD that contains the <CanonicalID> element is
authoritative for that Canonical ID and that this XRDS
document is authoritative for the OpenID Service
Element. Relying Parties should either do this manually
or ensure that their resolver does this.
</p>
<p>
When using XRI resolution, the Canonical ID MUST be
used as the Claimed Identifier. For an XRI to be a
valid Identifier, both the <ProviderID> and
<CanonicalID> MUST be present in the discovered
XRDS document.
</p>
<p>
When using URL Identifiers, the CanonicalID
element MUST be ignored if present.
</p>
<a name="anchor16"></a><br><hr>
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<a name="rfc.section.7.3.2.4"></a><h3>7.3.2.4.
Additional Information</h3>
<p>
The "openid" namespace is no longer used as of OpenID
Authentication 2.0. The "xrd" namespace is
"xri://$xrd*($v*2.0)".
</p>
<p>
For compatibility with deployed code, it is RECOMMENDED
that Relying Parties also accept
"http://openid.net/signon/1.0" or
"http://openid.net/signon/1.1" for the value of
<xrd:Type>, as described in the <a class="info" href="#compat_mode">OpenID Authentication 1.1
Compatibility mode<span> (</span><span class="info">OpenID Authentication 1.1 Compatibility</span><span>)</span></a> section. It is RECOMMENDED
that Relying Parties supporting OpenID Authentication
2.0 choose to use, if available, endpoints with the type
"http://specs.openid.net/auth/2.0/server" and
"http://specs.openid.net/auth/2.0/signon", in
this order, as specified in <a class="info" href="#extracting_auth">Section 7.3.2.2<span> (</span><span class="info">Extracting Authentication Data</span><span>)</span></a>
</p>
<p>
If an OP supports extensions (<a class="info" href="#extensions">Section 12<span> (</span><span class="info">Extensions</span><span>)</span></a>), the extensions SHOULD be listed as additional
<xrd:Type> child elements of the
<xrd:Service> element.
</p>
<a name="html_disco"></a><br><hr>
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<a name="rfc.section.7.3.3"></a><h3>7.3.3.
HTML-Based Discovery</h3>
<p>
HTML-Based discovery MUST be supported by Relying
Parties. HTML-Based discovery is only usable for discovery
of Claimed Identifiers. OP Identifiers must be XRIs or
URLs that support XRDS discovery.
</p>
<p>
To use HTML-Based discovery, an HTML document MUST be
available at the URL of the Claimed Identifier. Within the
HEAD element of the document:
</p>
<blockquote class="text">
<p>
A LINK element MUST be included with attributes
"rel" set to "openid2.provider" and "href" set to an OP
Endpoint URL
</p>
<p>
A LINK element MAY be included with attributes
"rel" set to "openid2.local_id" and "href" set to the
end user's OP-Local Identifier
</p>
</blockquote><p>
</p>
<p>
The protocol version when HTML discovery is performed is
"http://specs.openid.net/auth/2.0/signon".
</p>
<p>
The host of the HTML document MAY be different from the
end user's OP's host.
</p>
<p>
The "openid2.provider" and "openid2.local_id" URLs MUST NOT
include entities other than "&amp;", "&lt;",
"&gt;", and "&quot;". Other characters that would
not be valid in the HTML document or that cannot be
represented in the document's character encoding MUST be
escaped using the percent-encoding (%xx) mechanism
described in <a class="info" href="#RFC3986">[RFC3986]<span> (</span><span class="info">Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a>.
</p>
<p>
As discussed in the <a class="info" href="#compat_mode">OpenID Authentication 1.1
Compatibility mode<span> (</span><span class="info">OpenID Authentication 1.1 Compatibility</span><span>)</span></a> section, these discovery tags
are not the same as in previous versions of the protocol.
While the same data is conveyed, the names have changed which
allows a Relying Party to determine the protocol version
being used. A Relying Party MAY encounter a Claimed Identifier
which uses HTML-Based Discovery to advertise both version 1.1
and 2.0 Providers.
</p>
<a name="associations"></a><br><hr>
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<a name="rfc.section.8"></a><h3>8.
Establishing Associations</h3>
<p>
An association between the Relying Party and the OpenID Provider
establishes a shared secret between them, which is used to verify
subsequent protocol messages and reduce round trips.
</p>
<p>
It is RECOMMENDED that a Relying Party form associations if it
is possible for it to do so. If a Relying Party is incapable
of creating or storing associations, <a class="info" href="#check_auth">Section 11.4.2<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>
provides an alternate verification mechanism referred to as
Stateless Mode.
</p>
<a name="anchor17"></a><br><hr>
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<a name="rfc.section.8.1"></a><h3>8.1.
Association Session Request</h3>
<p>
An association session is initiated by a <a class="info" href="#direct_comm">direct request<span> (</span><span class="info">Direct Communication</span><span>)</span></a> from a Relying
Party to an OP Endpoint URL with the "openid.mode" key
having the value of "associate".
</p>
<a name="anchor18"></a><br><hr>
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<a name="rfc.section.8.1.1"></a><h3>8.1.1.
Common Request Parameters</h3>
<p>
These parameters are common to all association requests:
</p>
<p>
</p>
<ul class="text">
<li>openid.ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>openid.mode
<blockquote class="text">
<p> Value: "associate"
</p>
</blockquote>
</li>
<li>openid.assoc_type
<blockquote class="text">
<p> The preferred association type. The association
type defines the algorithm to be used to sign
subsequent messages.
</p>
<p> Value: A valid association type from <a class="info" href="#assoc_types">Section 8.3<span> (</span><span class="info">Association Types</span><span>)</span></a>
</p>
</blockquote>
</li>
<li>openid.session_type
<blockquote class="text">
<p>
The preferred association session type. This
defines the method used to encrypt the association's
MAC key in transit.
</p>
<p>
Value: A valid association session type from
<a class="info" href="#assoc_sess_types">Section 8.4<span> (</span><span class="info">Association Session Types</span><span>)</span></a>.
</p>
<p>
Note: Unless using transport layer encryption,
"no-encryption" MUST NOT be used.
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="anchor19"></a><br><hr>
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<a name="rfc.section.8.1.2"></a><h3>8.1.2.
Diffie-Hellman Request Parameters</h3>
<p>
The following parameters are common to requests whose
requested association session type is "DH-SHA1" or
"DH-SHA256":
</p>
<p>
</p>
<ul class="text">
<li>
openid.dh_modulus
<blockquote class="text">
<p>Value: base64(btwoc(p))
</p>
<p>Default: See <a class="info" href="#pvalue">Appendix B<span> (</span><span class="info">Diffie-Hellman Key Exchange Default Value</span><span>)</span></a>
</p>
</blockquote>
</li>
<li>
openid.dh_gen
<blockquote class="text">
<p>Value: base64(btwoc(g))
</p>
<p>Default: g = 2
</p>
</blockquote>
</li>
<li>
openid.dh_consumer_public
<blockquote class="text">
<p>Value: base64(btwoc(g ^ xa mod p))
</p>
</blockquote>
</li>
</ul><p>
</p>
<p>
See <a class="info" href="#dh_sessions">Section 8.4.2<span> (</span><span class="info">Diffie-Hellman Association Sessions</span><span>)</span></a> for more information on
these parameters.
</p>
<p>
NOTE: The 'btwoc' function is defined in <a class="info" href="#btwoc">Section 4.2<span> (</span><span class="info">Integer Representations</span><span>)</span></a>.
</p>
<a name="anchor20"></a><br><hr>
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<a name="rfc.section.8.2"></a><h3>8.2.
Association Session Response</h3>
<p>
An association session response is a direct response from the
OP to the Relying Party in <a class="info" href="#kvform">Key-Value
Form<span> (</span><span class="info">Key-Value Form Encoding</span><span>)</span></a>.
</p>
<a name="anchor21"></a><br><hr>
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<a name="rfc.section.8.2.1"></a><h3>8.2.1.
Common Response Parameters</h3>
<p>
</p>
<ul class="text">
<li>
ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#direct_response">Section 5.1.2<span> (</span><span class="info">Direct Response</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
assoc_handle
<blockquote class="text">
<p>
The association handle is used as a key to refer
to this association in subsequent messages.
</p>
<p>
Value: A string 255 characters or less in length.
It MUST consist only of ASCII characters in the
range 33-126 inclusive (printable non-whitespace
characters).
</p>
</blockquote>
</li>
<li>
session_type
<blockquote class="text">
<p>
The value of the "openid.session_type" parameter
from the request. If the OP is unwilling or
unable to support this association type, it MUST
return an <a class="info" href="#refuse_assoc">unsuccessful
response<span> (</span><span class="info">Unsuccessful Response Parameters</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
assoc_type
<blockquote class="text">
<p>
The value of the "openid.assoc_type" parameter
from the request. If the OP is unwilling or
unable to support this association type, it MUST
return an <a class="info" href="#refuse_assoc">unsuccessful
response<span> (</span><span class="info">Unsuccessful Response Parameters</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
expires_in
<blockquote class="text">
<p>
The lifetime, in seconds, of this association.
The Relying Party MUST NOT use the association
after this time has passed.
</p>
<p>
Value: An integer, represented in base 10 ASCII.
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="anchor22"></a><br><hr>
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<a name="rfc.section.8.2.2"></a><h3>8.2.2.
Unencrypted Response Parameters</h3>
<p>
</p>
<ul class="text">
<li>
mac_key
<blockquote class="text">
<p>
The MAC key (shared secret) for this
association, <a class="info" href="#RFC3548">Base 64<span> (</span><span class="info">Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” .</span><span>)</span></a> [RFC3548]
encoded.
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="anchor23"></a><br><hr>
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<a name="rfc.section.8.2.3"></a><h3>8.2.3.
Diffie-Hellman Response Parameters</h3>
<p>
</p>
<ul class="text">
<li>
dh_server_public
<blockquote class="text">
<p>
Value: base64(btwoc(g ^ xb mod p))
</p>
<p>
Description: The OP's Diffie-Hellman public key.
</p>
</blockquote>
</li>
<li>
enc_mac_key
<blockquote class="text">
<p>
Value: base64(H(btwoc(g ^ (xa * xb) mod p)) XOR MAC key)
</p>
<p>
Description: The MAC key (shared secret),
encrypted with the secret Diffie-Hellman value. H
is either "SHA1" or "SHA256" depending on the
session type.
</p>
</blockquote>
</li>
</ul><p>
</p>
<p>
NOTE: The 'btwoc' function is defined in <a class="info" href="#btwoc">Section 4.2<span> (</span><span class="info">Integer Representations</span><span>)</span></a>
</p>
<a name="refuse_assoc"></a><br><hr>
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<a name="rfc.section.8.2.4"></a><h3>8.2.4.
Unsuccessful Response Parameters</h3>
<p>
If the OP does not support a session type or association
type, it MUST respond with a direct error message
indicating that the association request failed. If there
is another association session type or association type
that is supported, the OP SHOULD include that information
in the response.
</p>
<p>
</p>
<ul class="text">
<li>
ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#direct_response">Section 5.1.2<span> (</span><span class="info">Direct Response</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
error
<blockquote class="text">
<p>
Value: A human-readable message indicating why the
association request failed.
</p>
</blockquote>
</li>
<li>
error_code
<blockquote class="text">
<p>
Value: "unsupported-type"
</p>
</blockquote>
</li>
<li>
session_type
<blockquote class="text">
<p>
Value: (optional) A valid association session type
from <a class="info" href="#assoc_sess_types">Section 8.4<span> (</span><span class="info">Association Session Types</span><span>)</span></a> that the
OP supports.
</p>
</blockquote>
</li>
<li>
assoc_type
<blockquote class="text">
<p>
Value: (optional) An association type supported by
the OP from <a class="info" href="#assoc_types">Section 8.3<span> (</span><span class="info">Association Types</span><span>)</span></a>.
</p>
</blockquote>
</li>
</ul><p>
</p>
<p>
Upon receipt of an "unsupported-type" response, the
Relying Party MAY make another request with the specified
association session type and association type. If no
association is established, the Relying Party MAY continue
the authentication process in <a class="info" href="#check_auth">Direct Verification<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>.
</p>
<a name="assoc_types"></a><br><hr>
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<a name="rfc.section.8.3"></a><h3>8.3.
Association Types</h3>
<a name="anchor24"></a><br><hr>
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<a name="rfc.section.8.3.1"></a><h3>8.3.1.
HMAC-SHA1</h3>
<p>
An association of type "HMAC-SHA1" uses the <a class="info" href="#sign_algos">HMAC-SHA1<span> (</span><span class="info">Signature Algorithms</span><span>)</span></a> signature algorithm.
</p>
<a name="anchor25"></a><br><hr>
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<a name="rfc.section.8.3.2"></a><h3>8.3.2.
HMAC-SHA256</h3>
<p>
An association of type "HMAC-SHA256" uses the <a class="info" href="#sign_algos">HMAC-SHA256<span> (</span><span class="info">Signature Algorithms</span><span>)</span></a> signature
algorithm.
</p>
<a name="assoc_sess_types"></a><br><hr>
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<a name="rfc.section.8.4"></a><h3>8.4.
Association Session Types</h3>
<p>
OpenID Authentication defines three valid association
session types: "no-encryption", "DH-SHA1", and "DH-SHA256".
</p>
<a name="anchor26"></a><br><hr>
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<a name="rfc.section.8.4.1"></a><h3>8.4.1.
No-Encryption Association Sessions</h3>
<p>
In a "no-encryption" association session, the OP sends
the association MAC key in plain-text to the Relying Party.
This makes it possible for an eavesdropper to intercept
the key, and forge messages to this Relying Party when not
using transport layer encryption. Therefore, "no-encryption"
association sessions MUST NOT be used unless the messages
are using transport layer encryption. See <a class="info" href="#preventing_eavesdropping">Section 15.1.1<span> (</span><span class="info">Eavesdropping Attacks</span><span>)</span></a>
for more information.
</p>
<p>
The MAC key sent by the OP MUST be the length specified
for the requested association type, as specified in <a class="info" href="#sign_algos">Section 6.2<span> (</span><span class="info">Signature Algorithms</span><span>)</span></a>.
</p>
<a name="dh_sessions"></a><br><hr>
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<a name="rfc.section.8.4.2"></a><h3>8.4.2.
Diffie-Hellman Association Sessions</h3>
<p>
The "DH-SHA1" and "DH-SHA256" association types use
Diffie-Hellman Key Exchange to securely transmit the
shared secret.
</p>
<p>
The MAC key MUST be the same length as the output of H,
the hash function - 160 bits (20 bytes) for DH-SHA1 or 256
bits (32 bytes) for DH-SHA256, as well as the output of
the signature algorithm of this association.
</p>
<p>
The Relying Party specifies a modulus, p, and a generator,
g. The Relying Party chooses a random private key xa and
OpenID Provider chooses a random private key xb, both in
the range [1 .. p-1]. The shared secret used to encrypt
the MAC key is thus g ^ (xa * xb) mod p = (g ^ xa) ^ xb
mod p = (g ^ xb) ^ xa mod p. For more information, see
<a class="info" href="#RFC2631">[RFC2631]<span> (</span><span class="info">Rescorla, E., “Diffie-Hellman Key Agreement Method,” .</span><span>)</span></a>. For information on the
selection of random values, see <a class="info" href="#RFC1750">[RFC1750]<span> (</span><span class="info">Eastlake, D., Crocker, S., and J. Schiller, “Randomness Recommendations for Security,” .</span><span>)</span></a>.
</p>
<a name="requesting_authentication"></a><br><hr>
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<a name="rfc.section.9"></a><h3>9.
Requesting Authentication</h3>
<p>
Once the Relying Party has successfully performed discovery
and (optionally) created an association with the discovered
OP Endpoint URL, it can send an authentication request to the
OP to obtain an assertion. An authentication request is an
<a class="info" href="#indirect_comm">indirect request<span> (</span><span class="info">Indirect Communication</span><span>)</span></a>.
</p>
<a name="anchor27"></a><br><hr>
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<a name="rfc.section.9.1"></a><h3>9.1.
Request Parameters</h3>
<p>
</p>
<ul class="text">
<li>
openid.ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.mode
<blockquote class="text">
<p>
Value: "checkid_immediate" or "checkid_setup"
</p>
<p>
Note: If the Relying Party wishes the end user to be
able to interact with the OP, "checkid_setup"
should be used. An example of a situation where
interaction between the end user and the OP is not
desired is when the authentication request is
happening asynchronously in JavaScript.
</p>
</blockquote>
</li>
<li>
openid.claimed_id
<blockquote class="text">
<p>
Value: (optional) The Claimed Identifier.
</p>
<p>
"openid.claimed_id" and "openid.identity" SHALL
be either both present or both absent. If neither
value is present, the assertion is not about an
identifier, and will contain other information in
its payload, using
<a class="info" href="#extensions">extensions<span> (</span><span class="info">Extensions</span><span>)</span></a>.
</p>
<p>
It is RECOMMENDED that OPs accept XRI identifiers
with or without the "xri://" prefix, as specified
in the <a class="info" href="#normalization">Normalization<span> (</span><span class="info">Normalization</span><span>)</span></a> section.
</p>
</blockquote>
</li>
<li>
openid.identity
<blockquote class="text">
<p>
Value: (optional) The OP-Local Identifier.
</p>
<p>
If a different OP-Local Identifier is not specified,
the claimed identifier MUST be used as the value for
openid.identity.
</p>
<p>
Note: If this is set to the special value
"http://specs.openid.net/auth/2.0/identifier_select"
then the OP SHOULD choose an Identifier that belongs
to the end user. This parameter MAY be omitted if
the request is not about an identifier (for instance
if an extension is in use that makes the request
meaningful without it; see openid.claimed_id above).
</p>
</blockquote>
</li>
<li>
openid.assoc_handle
<blockquote class="text">
<p>
Value: (optional) A handle for an association
between the Relying Party and the OP that SHOULD be
used to sign the response.
</p>
<p>
Note: If no association handle is sent, the
transaction will take place in <a class="info" href="#check_auth">Stateless Mode<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.return_to
<blockquote class="text">
<p>
Value: (optional) URL to which the OP SHOULD return
the User-Agent with the response indicating the
status of the request.
</p>
<p>
Note: If this value is not sent in the request it
signifies that the Relying Party does not wish
for the end user to be returned.
</p>
<p>
Note: The return_to URL MAY be used as a mechanism
for the Relying Party to attach context about the
authentication request to the authentication
response. This document does not define a mechanism
by which the RP can ensure that query parameters are
not modified by outside parties; such a mechanism
can be defined by the RP itself.
</p>
</blockquote>
</li>
<li>
openid.realm
<blockquote class="text">
<p>
Value: (optional) URL pattern the OP SHOULD ask the
end user to trust. See <a class="info" href="#realms">Section 9.2<span> (</span><span class="info">Realms</span><span>)</span></a>.
This value MUST be sent if openid.return_to is
omitted.
</p>
<p>
Default: return_to URL
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="realms"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.9.2"></a><h3>9.2.
Realms</h3>
<p>
A "realm" is a pattern that represents the part of URL-space
for which an OpenID Authentication request is valid. A realm
is designed to give the end user an indication of the scope
of the authentication request. OPs SHOULD present the realm
when requesting the end user's approval for an authentication
request. The realm SHOULD be used by OPs to uniquely identify
Relying Parties. For example, OPs MAY use the realm to allow
the end user to automate approval of authentication requests.
</p>
<p>
A realm pattern is a URL, with the following changes:
</p>
<ul class="text">
<li>
A realm MUST NOT contain a URI fragment
</li>
<li>
A realm MAY contain a wild-card at the beginning of the
URL authority section. A wild-card consists of the
characters "*." prepended to the DNS name in the
authority section of the URL.
</li>
</ul><p>
</p>
<p>
A URL matches a realm if:
</p>
<ul class="text">
<li>
The URL scheme and port of the URL are identical to those
in the realm. See <a class="info" href="#RFC3986">RFC
3986<span> (</span><span class="info">Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> [RFC3986], section 3.1 for rules about URI matching.
</li>
<li>
The URL's path is equal to or a sub-directory of the
realm's path.
</li>
<li>
Either:
<ol class="text">
<li>
The realm's domain contains the wild-card characters
"*.", and the trailing part of the URL's domain is
identical to the part of the realm following the
"*." wildcard, or
</li>
<li>
The URL's domain is identical to the realm's domain
</li>
</ol>
</li>
</ul><p>
When present, the "openid.return_to" URL MUST match the
"openid.realm", or the OP MUST return an <a class="info" href="#indirect_error">indirect error response<span> (</span><span class="info">Indirect Error Responses</span><span>)</span></a>.
</p>
<p>
It is RECOMMENDED that OPs protect their users from making
assertions with overly-general realms, like http://*.com/ or
http://*.co.uk/. Overly general realms can be dangerous when
the realm is used for identifying a particular Relying
Party. Whether a realm is overly-general is at the
discretion of the OP.
</p>
<a name="return_to_verification"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.9.2.1"></a><h3>9.2.1.
Using the Realm for Return URL Verification</h3>
<p>
OpenID providers SHOULD verify that the return_to URL
specified in the request is an OpenID relying party
endpoint. To verify a return_to URL, obtain the relying
party endpoints for the realm by performing <a class="info" href="#rp_discovery">discovery on the relying
party<span> (</span><span class="info">Discovering OpenID Relying Parties</span><span>)</span></a>. As always when performing discovery, the
discovered URL is the URL of the last HTTP response,
following redirects. If any redirects are followed when
performing discovery on the realm, verification has
failed. If discovery has successfuly completed, check to
make sure that the return_to URL matches one of the
relying party endpoints.
</p>
<p>
A realm may contain a wildcard, and so may not be a valid
URL. In that case, perform discovery on the URL obtained
by substituting "www" for the wildcard in the realm.
</p>
<p>
To match a return_to URL against a relying party endpoint,
use the same rules as for matching the return_to URL
against the realm, treating the relying party's endpoint
URL as the realm. Relying party endpoint URLs MUST NOT
contain a domain wildcard, and SHOULD be as specific as
possible.
</p>
<p>
If verification is attempted and fails, the provider
SHOULD NOT send a positive assertion to that return_to
URL.
</p>
<p>
Providers MAY cache verified return_to URLs.
</p>
<a name="anchor28"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.9.3"></a><h3>9.3.
Immediate Requests</h3>
<p>
When requesting authentication, the Relying Party MAY
request that the OP not interact with the end user. In
this case the OP MUST respond immediately with either an
assertion that authentication is successful, or a response
indicating that the request cannot be completed without
further user interaction. This is accomplished by an
authentication request with "openid.mode" set to
"checkid_immediate".
</p>
<a name="responding_to_authentication"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.10"></a><h3>10.
Responding to Authentication Requests</h3>
<p>
When an authentication request comes from the User-Agent via
<a class="info" href="#indirect_comm">indirect communication<span> (</span><span class="info">Indirect Communication</span><span>)</span></a>,
the OP SHOULD determine that an authorized end user wishes to
complete the authentication. If an authorized end user wishes
to complete the authentication, the OP SHOULD send a <a class="info" href="#positive_assertions">positive assertion<span> (</span><span class="info">Positive Assertions</span><span>)</span></a> to the
Relying Party.
</p>
<p>
Methods of identifying authorized end users and obtaining
approval to return an OpenID Authentication assertion are
beyond the scope of this specification. See <a class="info" href="#rp_mitm_proxy">Section 15.1.2.1<span> (</span><span class="info">Rogue Relying Party Proxying</span><span>)</span></a> for OpenID Provider security
considerations.
</p>
<p>
If the relying party requested OP-driven identifier selection
by setting "openid.identity" to
"http://specs.openid.net/auth/2.0/identifier_select"
and there are Identifiers for which the end user is authorized
to issue authentication responses, the OP SHOULD allow the end
user to choose which Identifier to use.
</p>
<p>
If the Relying Party supplied an association handle with the
authentication request, the OP SHOULD attempt to look up an
association based on that handle. If the association is
missing or expired, the OP SHOULD send the
"openid.invalidate_handle" parameter as part of the response
with the value of the request's "openid.assoc_handle"
parameter, and SHOULD proceed as if no association handle was
specified.
</p>
<p>
If no association handle is specified, the OP SHOULD use a
private association for signing the response. The OP MUST
store this association and MUST respond to later requests to
check the signature of the response via <a class="info" href="#check_auth">Direct Verification<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>.
</p>
<p>
Relying Parties SHOULD accept and verify assertions about
Identifiers for which they have not requested authentication.
OPs SHOULD use private associations for signing unsolicited
positive assertions.
</p>
<p>
If the "openid.return_to" value is omitted in the request, the
Relying Party does not wish to receive an authentication
assertion from the OP. This can be useful when using
extensions to transfer data from the Relying Party to the OP.
</p>
<a name="positive_assertions"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.10.1"></a><h3>10.1.
Positive Assertions</h3>
<p>
Positive assertions are <a class="info" href="#indirect_comm">indirect responses<span> (</span><span class="info">Indirect Communication</span><span>)</span></a> with the following fields:
</p>
<p>
</p>
<ul class="text">
<li>
openid.ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.mode
<blockquote class="text">
<p>Value: "id_res"
</p>
</blockquote>
</li>
<li>
openid.op_endpoint
<blockquote class="text">
<p>
The OP Endpoint URL.
</p>
</blockquote>
</li>
<li>
openid.claimed_id
<blockquote class="text">
<p>
Value: (optional) The Claimed Identifier.
"openid.claimed_id" and "openid.identity" SHALL
be either both present or both absent.
</p>
<p>
Note: The end user MAY choose to use an OP-Local
Identifier as a Claimed Identifier.
</p>
<p>
Note: If neither Identifier is present in the assertion,
it is not about an identifier, and will contain
other information in its payload, using <a class="info" href="#extensions">extensions<span> (</span><span class="info">Extensions</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.identity
<blockquote class="text">
<p>
Value: (optional) The OP-Local Identifier
</p>
<p>
Note: OpenID Providers MAY assist the end user in
selecting the Claimed and OP-Local Identifiers about
which the assertion is made. The openid.identity
field MAY be omitted if an extension is in use that
makes the response meaningful without it
(see openid.claimed_id above).
</p>
</blockquote>
</li>
<li>
openid.return_to
<blockquote class="text">
<p>
Value: Verbatim copy of the return_to URL parameter
sent in the request.
</p>
</blockquote>
</li>
<li>
openid.response_nonce
<blockquote class="text">
<p>
Value: A string 255 characters or less in length,
that MUST be unique to this particular successful
authentication response. The nonce MUST start with the
current time on the server, and MAY contain additional
ASCII characters in the range 33-126 inclusive
(printable non-whitespace characters), as necessary to
make each response unique. The date and time MUST be
formatted as specified in section 5.6 of
<a class="info" href="#RFC3339">[RFC3339]<span> (</span><span class="info">Klyne, G. and C. Newman, “Date and Time on the Internet: Timestamps,” .</span><span>)</span></a>, with the following restrictions:
</p>
<ul class="text">
<li>
All times must be in the UTC
timezone, indicated with a "Z".
</li>
<li>
No fractional seconds are allowed
</li>
</ul>
For example: 2005-05-15T17:11:51ZUNIQUE
</blockquote>
</li>
<li>
openid.invalidate_handle
<blockquote class="text">
<p>
Value: (optional) If the Relying Party sent an
invalid association handle with the request, it
SHOULD be included here.
</p>
</blockquote>
</li>
<li>
openid.assoc_handle
<blockquote class="text">
<p>
Value: The handle for the association that was used
to sign this assertion.
</p>
</blockquote>
</li>
<li>
openid.signed
<blockquote class="text">
<p>
Value: Comma-separated list of signed fields.
</p>
<p>
Note: This entry consists of the fields without the
"openid." prefix that the signature covers. This
list MUST contain at least "op_endpoint",
"return_to" "response_nonce" and "assoc_handle", and
if present in the response, "claimed_id" and
"identity". Additional keys MAY be signed as part of
the message. See <a class="info" href="#generating_signatures">Generating
Signatures<span> (</span><span class="info">Generating Signatures</span><span>)</span></a>.
</p>
<p>
For example,
"op_endpoint,identity,claimed_id,return_to,assoc_handle,response_nonce".
</p>
</blockquote>
</li>
<li>
openid.sig
<blockquote class="text">
<p>
Value: Base 64 encoded signature calculated as
specified in <a class="info" href="#generating_signatures">Section 6<span> (</span><span class="info">Generating Signatures</span><span>)</span></a>.
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="negative_assertions"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.10.2"></a><h3>10.2.
Negative Assertions</h3>
<p>
If the OP is unable to identify the end user or the end
user does not or cannot approve the authentication request,
the OP SHOULD send a negative assertion to the Relying
Party as an <a class="info" href="#indirect_comm">indirect
response<span> (</span><span class="info">Indirect Communication</span><span>)</span></a>.
</p>
<p>
When receiving a negative assertion in response to a
"checkid_immediate" mode request, Relying Parties SHOULD
construct a new authentication request using "checkid_setup"
mode. Details about how this differs from OpenID
Authentication 1.1 can be found in <a class="info" href="#compat_mode">Section 14<span> (</span><span class="info">OpenID Authentication 1.1 Compatibility</span><span>)</span></a>.
</p>
<a name="anchor29"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.10.2.1"></a><h3>10.2.1.
In Response to Immediate Requests</h3>
<p>
If the request was an immediate request, there is no chance
for the end user to interact with pages on the OP to provide
identifying credentials or approval of a request.
A negative assertion of an immediate request takes the
following form:
</p>
<ul class="text">
<li>
openid.ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.mode
<blockquote class="text">
<p>Value: "setup_needed"
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="anchor30"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.10.2.2"></a><h3>10.2.2.
In Response to Non-Immediate Requests</h3>
<p>
Since the OP may display pages to the end user and
request credentials from the end user, a negative response
to a request that is not immediate is definitive. It
takes the following form:
</p>
<ul class="text">
<li>
openid.ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#http_encoding">Section 4.1.2<span> (</span><span class="info">HTTP Encoding</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
openid.mode
<blockquote class="text">
<p>
Value: "cancel"
</p>
</blockquote>
</li>
</ul><p>
</p>
<p>
Often, if the user does not wish to or cannot complete the
authentication request, the OpenID authentication process
will be aborted and the Relying Party will not get a
cancel mode response (the end user may quit or press the
back button in their User-Agent instead of continuing).
If a RP receives the "cancel" response, authentication was
unsuccessful and the RP MUST treat the end user as
non-authenticated.
</p>
<a name="verification"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11"></a><h3>11.
Verifying Assertions</h3>
<p>
When the Relying Party receives a positive assertion, it MUST
verify the following before accepting the assertion:
</p>
<ul class="text">
<li>
The value of "openid.return_to" matches the URL of the
current request (<a class="info" href="#verify_return_to">Section 11.1<span> (</span><span class="info">Verifying the Return URL</span><span>)</span></a>)
</li>
<li>
Discovered information matches the information in the
assertion (<a class="info" href="#verify_disco">Section 11.2<span> (</span><span class="info">Verifying Discovered Information</span><span>)</span></a>)
</li>
<li>
An assertion has not yet been accepted from this OP with
the same value for "openid.response_nonce" (<a class="info" href="#verify_nonce">Section 11.3<span> (</span><span class="info">Checking the Nonce</span><span>)</span></a>)
</li>
<li>
The signature on the assertion is valid and all fields
that are required to be signed are signed (<a class="info" href="#verifying_signatures">Section 11.4<span> (</span><span class="info">Verifying Signatures</span><span>)</span></a>)
</li>
</ul><p>
If all four of these conditions are met, assertion is now
verified. If the assertion contained a Claimed Identifier, the
user is now authenticated with that identifier.
</p>
<a name="verify_return_to"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.1"></a><h3>11.1.
Verifying the Return URL</h3>
<p>
To verify that the "openid.return_to" URL matches the URL
that is processing this assertion:
</p>
<ul class="text">
<li>
The URL scheme, authority, and path MUST be the same
between the two URLs.
</li>
<li>
Any query parameters that are present in the
"openid.return_to" URL MUST also be present with the
same values in the URL of the HTTP request the RP
received.
</li>
</ul><p>
</p>
<a name="verify_disco"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.2"></a><h3>11.2.
Verifying Discovered Information</h3>
<p>
If the Claimed Identifier in the assertion is a URL and
contains a fragment, the fragment part and the fragment
delimiter character "#" MUST NOT be used for the purposes
of verifying the discovered information.
</p>
<p>
If the Claimed Identifier is included in the assertion, it
MUST have been <a class="info" href="#discovery">discovered<span> (</span><span class="info">Discovery</span><span>)</span></a> by
the Relying Party and the information in the assertion MUST
be present in the discovered information. The Claimed
Identifier MUST NOT be an OP Identifier.
</p>
<p>
If the Claimed Identifier was not previously discovered
by the Relying Party (the "openid.identity" in the request
was "http://specs.openid.net/auth/2.0/identifier_select" or
a different Identifier, or if the OP is sending an unsolicited
positive assertion), the Relying Party MUST perform discovery
on the Claimed Identifier in the response to make sure that
the OP is authorized to make assertions about the Claimed
Identifier.
</p>
<p>
If no Claimed Identifier is present in the response, the
assertion is not about an identifier and the RP MUST NOT use
the User-supplied Identifier associated with the current
OpenID authentication transaction to identify the user.
Extension information in the assertion MAY still be used.
</p><br><hr class="insert">
<table class="full" align="center" border="0" cellpadding="2" cellspacing="2">
<col align="left"><col align="left">
<tbody><tr><th align="left">Discovered Value</th><th align="left">Response Field</th></tr>
<tr>
<td align="left">Claimed Identifier</td>
<td align="left">openid.claimed_id</td>
</tr>
<tr>
<td align="left">OP-Local Identifier</td>
<td align="left">openid.identity</td>
</tr>
<tr>
<td align="left">OP Endpoint URL</td>
<td align="left">openid.op_endpoint</td>
</tr>
<tr>
<td align="left">Protocol Version</td>
<td align="left">openid.ns</td>
</tr>
</tbody></table>
<p style="text-align: center;">
This table shows the mapping of <a class="info" href="#discovered_info">discovered information<span> (</span><span class="info">Discovered Information</span><span>)</span></a>
into fields in the <a class="info" href="#positive_assertions">OpenID Authentication 2.0
"id_res" response<span> (</span><span class="info">Positive Assertions</span><span>)</span></a>
</p><table align="center" border="0" cellpadding="0" cellspacing="2"><tbody><tr><td align="center"><font face="monaco, MS Sans Serif" size="1"><b> Discovered Information to Authentication Response Mapping </b></font><br></td></tr></tbody></table><hr class="insert">
<p>
If using a discovery mechanism that yields an XRDS document,
the protocol version, OP Endpoint URL and the OP-Local
Identifier (if different than the Claimed Identifier) MUST
be present in one <xrd:Service> element. There MAY be
unused fields in that <xrd:Service> element.
</p>
<p>Non-normative example:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre><Service xmlns="xri://$xrd*($v*2.0)">
<Type>http://specs.openid.net/auth/2.0/signon</Type>
<URI>http://provider.example.com/openid</URI>
<URI>https://provider.example.com/openid</URI>
</Service>
</pre></div>
<p>
In this example XRDS snippet, the <xrd:Service>
element has two <xrd:URI> elements, which map to OP
Endpoint URLs as per <a class="info" href="#discovered_info">Section 7.3.1<span> (</span><span class="info">Discovered Information</span><span>)</span></a>. If
an assertion has either value for "openid.op_endpoint",
then that field matches this <xrd:Service>
element. The other <xrd:URI> element is unused.
</p>
<a name="verify_nonce"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.3"></a><h3>11.3.
Checking the Nonce</h3>
<p>
To prevent replay attacks, the agent checking the signature
keeps track of the nonce values included in positive
assertions and never accepts the same value more than once
for the same OP Endpoint URL.
</p>
<p>
</p>
<ul class="text">
<li>
When using "check_authentication", the OP MUST NOT issue
more than one successful response to a request with the
same value for "openid.response_nonce".
</li>
<li>
When the Relying Party checks the signature on an
assertion, the Relying Party SHOULD ensure that an
assertion has not yet been accepted with the same value
for "openid.response_nonce" from the same OP Endpoint
URL.
</li>
</ul><p>
</p>
<p>
The time-stamp MAY be used to reject responses that are too
far away from the current time, limiting the amount of time
that nonces must be stored to prevent attacks. The
acceptable range is out of the scope of this
specification. A larger range requires storing more nonces
for a longer time. A shorter range increases the chance that
clock-skew and transaction time will cause a spurious
rejection.
</p>
<a name="verifying_signatures"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.4"></a><h3>11.4.
Verifying Signatures</h3>
<p>
If the Relying Party has stored an association with the
association handle specified in the assertion, it MUST check
the signature on the assertion itself. If it does not have
an association stored, it MUST request that the OP verify
the signature via <a class="info" href="#check_auth">Direct
Verification<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>.
</p>
<a name="anchor31"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.4.1"></a><h3>11.4.1.
Verifying with an Association</h3>
<p>
The Relying Party follows the same procedure that the
OP followed in <a class="info" href="#generating_signatures">generating the signature<span> (</span><span class="info">Generating Signatures</span><span>)</span></a>, and then compares the
signature in the response to the signature it
generated. If the signatures do not match, the assertion
is invalid.
</p>
<p>
If an authentication request included an association
handle for an association between the OP and the Relying
party, and the OP no longer wishes to use that handle
(because it has expired or the secret has been
compromised, for instance), the OP will send a response
that must be verified directly with the OP, as specified
in <a class="info" href="#check_auth">Section 11.4.2<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>. In that instance, the OP
will include the field "openid.invalidate_handle" set to
the association handle that the Relying Party included
with the original request.
</p>
<a name="check_auth"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.4.2"></a><h3>11.4.2.
Verifying Directly with the OpenID Provider</h3>
<p>
To have the signature verification performed by the OP,
the Relying Party sends a <a class="info" href="#direct_request">direct request<span> (</span><span class="info">Direct Request</span><span>)</span></a> to the OP. To verify the signature,
the OP uses a private association that was generated when
it issued the <a class="info" href="#positive_assertions">positive assertion<span> (</span><span class="info">Positive Assertions</span><span>)</span></a>.
</p>
<a name="anchor32"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.4.2.1"></a><h3>11.4.2.1.
Request Parameters</h3>
<p>
</p>
<ul class="text">
<li>
openid.mode
<blockquote class="text">
<p>Value: "check_authentication"
</p>
</blockquote>
</li>
<li>
Exact copies of all fields from the authentication
response, except for "openid.mode".
</li>
</ul><p>
</p>
<p>
For verifying signatures an OP MUST only use private
associations and MUST NOT use associations that have
shared keys. If the verification request contains a
handle for a shared association, it means the Relying
Party no longer knows the shared secret, or an entity
other than the RP (e.g. an attacker) has established
this association with the OP.
</p>
<p>
To prevent replay attacks, the OP MUST NOT issue more
than one verification response for each authentication
response it had previously issued. An authentication
response and its matching verification request may
be identified by their "openid.response_nonce" values.
</p>
<a name="anchor33"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.4.2.2"></a><h3>11.4.2.2.
Response Parameters</h3>
<p>
</p>
<ul class="text">
<li>
ns
<blockquote class="text">
<p>
As specified in <a class="info" href="#direct_response">Section 5.1.2<span> (</span><span class="info">Direct Response</span><span>)</span></a>.
</p>
</blockquote>
</li>
<li>
is_valid
<blockquote class="text">
<p>
Value: "true" or "false"; asserts whether the
signature of the verification request is valid.
</p>
</blockquote>
</li>
<li>
invalidate_handle
<blockquote class="text">
<p>
Value: (optional) The "invalidate_handle" value
sent in the verification request, if the OP confirms
it is invalid.
</p>
<p>
Description: If present in a verification response
with "is_valid" set to "true", the Relying Party
SHOULD remove the corresponding association from its
store and SHOULD NOT send further authentication
requests with this handle.
</p>
<p>
Note: This two-step process for invalidating
associations is necessary to prevent an attacker
from invalidating an association at will by adding
"invalidate_handle" parameters to an authentication
response.
</p>
</blockquote>
</li>
</ul><p>
</p>
<a name="identifying"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.5"></a><h3>11.5.
Identifying the end user</h3>
<p>
The Claimed Identifier in a successful authentication
response SHOULD be used by the Relying Party as a key for
local storage of information about the user. The Claimed
Identifier MAY be used as a user-visible Identifier. When
displaying URL Identifiers, the fragment MAY be omitted.
</p>
<a name="identifier_recycling"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.5.1"></a><h3>11.5.1.
Identifier Recycling</h3>
<p>
OpenID Providers with large user bases can use fragments
to recycle URL Identifiers if it is so desired. When
reassigning a URL Identifier to a new end user OPs should
generate a new, unique fragment part.
</p>
<p>
The full URL with the fragment part constitutes the Claimed
Identifier in positive assertions, therefore Relying Parties
will distinguish between the current and previous owners of
the fragment-less URL.
</p>
<p>
This mechanism allows the (presumably short, memorable)
recycled URL Identifiers without the fragment to be used by
end users at login time and by Relying Parties for display
purposes.
</p>
<a name="http_s_identifiers"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.11.5.2"></a><h3>11.5.2.
HTTP and HTTPS URL Identifiers</h3>
<p>
Relying Parties MUST differentiate between URL Identifiers
that have different schemes. When end user input is
processed into a URL, it is processed into a HTTP URL. If
the same end user controls the same URL, differing only by
scheme, and it is desired that the Identifier be the HTTPS
URL, it is RECOMMENDED that a redirect be issued from the
HTTP URL to the HTTPS URL. Because the HTTP and HTTPS URLs
are not equivalent and the Identifier that is used is the
URL after following redirects, there is no foreseen
reduction in security when using this scheme. If an
attacker could gain control of the HTTP URL, it would have
no effect on the HTTPS URL, since the HTTP URL is not ever
used as an Identifier except to initiate the discovery
process.
</p>
<a name="extensions"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.12"></a><h3>12.
Extensions</h3>
<p>
An Extension to OpenID Authentication is a protocol that
"piggybacks" on the authentication request and response. Extensions
are useful for providing extra information about an
authentication request or response as well as providing extra
information about the subject of the authentication response.
</p>
<p>
OpenID extensions are identified by a Type URI. The Type URI
MAY be used as the value of an <xrd:Type> element of an
OpenID <xrd:Service> element in an XRDS document
associated with a Claimed Identifier. The Type URI is also
used to associate key-value pairs in messages with the extension.
</p>
<p>
To associate keys and values in a message with an extension,
the key MUST be associated with the Type URI. To associate
keys with a Type URI, establish an alias by adding a key
prefixed with "openid.ns." and ending with the alias text
whose value is the Type URI. Once an alias has been
established, all pairs in the message whose keys start with
"openid." followed by the alias text, followed by a period or
the end of the key are associated with that extension.
This mechanism is similar to the XML namespaces.
</p>
<p>
A namespace alias MUST NOT contain a period and MUST NOT be
the same as another namespace alias in the same message. A
namespace alias also MUST NOT be in the following list of
disallowed aliases:
</p>
<ul class="text">
<li>assoc_handle
</li>
<li>assoc_type
</li>
<li>claimed_id
</li>
<li>contact
</li>
<li>delegate
</li>
<li>dh_consumer_public
</li>
<li>dh_gen
</li>
<li>dh_modulus
</li>
<li>error
</li>
<li>identity
</li>
<li>invalidate_handle
</li>
<li>mode
</li>
<li>ns
</li>
<li>op_endpoint
</li>
<li>openid
</li>
<li>realm
</li>
<li>reference
</li>
<li>response_nonce
</li>
<li>return_to
</li>
<li>server
</li>
<li>session_type
</li>
<li>sig
</li>
<li>signed
</li>
<li>trust_root
</li>
</ul><p>
A namespace MUST NOT be assigned more than one alias in the
same message. If a message is a response to another message,
the response MAY use a different alias to refer to the same
namespace.
</p>
<p>Non-normative example:
</p>
<p>An extension's type URI is
"<http://example.com/ext/1.0>".
</p>
<blockquote class="text">
<p>openid.ns.x=http://example.com/ext/1.0
</p>
<p>openid.x=example
</p>
<p>openid.x.foo=bar
</p>
<p>openid.xx=notx
</p>
</blockquote><p>
In this example, the keys "openid.x" and "openid.x.foo" are
associated with the extension; the "openid.xx" key is not.
</p>
<p>
Extensions MUST NOT define multiple parameters with the same name.
Extensions that need to send multiple values for the same parameter
name must define their own conventions for doing so.
</p>
<a name="rp_discovery"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.13"></a><h3>13.
Discovering OpenID Relying Parties</h3>
<p>
Relying Party discovery allows for software agents to discover
sites that support OpenID. It also allows OpenID providers to
automatically verify that a return_to URL in an OpenID request
is an OpenID relying party endpoint for the specified realm.
</p>
<p>
Relying Parties SHOULD use the Yadis protocol to publish their
valid return_to URLs. The relying party MAY publish this
information at any URL, and SHOULD publish it under the realm
so that providers can verify return_to URLs.
</p>
<p>
A Relying Party discovery XRDS document MUST contain one or more
<xrd:Service> elements:
</p>
<ul class="text">
<li>
Containing at least one <xrd:URI> element.
</li>
<li>
Where all <xrd:URI> tags contain a URL that accepts
OpenID 2.0 Authentication responses.
</li>
<li>
Containing a <xrd:Type> tag whose content is
"http://specs.openid.net/auth/2.0/return_to".
</li>
</ul><p>
</p>
<p>Non-normative example:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre><Service xmlns="xri://$xrd*($v*2.0)">
<Type>http://specs.openid.net/auth/2.0/return_to</Type>
<URI>http://consumer.example.com/return</URI>
</Service>
</pre></div>
<a name="compat_mode"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14"></a><h3>14.
OpenID Authentication 1.1 Compatibility</h3>
<p>
This section describes how to interact with OpenID
Authentication 1.1 Relying Parties and OPs. OpenID
Authentication 2.0 implementations SHOULD support OpenID
Authentication 1.1 compatibility, unless security
considerations make it undesirable.
</p>
<a name="anchor34"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14.1"></a><h3>14.1.
Changes from OpenID Authentication 1.1</h3>
<p>
(non-normative)
</p>
<p>
This specification is based on the original specification for
OpenID Authentication as written by Brad Fitzpatrick. That
specification did not have a version number, but was called
OpenID 1.0, and then OpenID 1.1 when it was revised. The
protocol outlined in this specification is intended to be
backwards-compatible with the revised OpenID protocol. The
changes to the specification are outlined in this section.
</p>
<a name="anchor35"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14.1.1"></a><h3>14.1.1.
Updated Initiation and Discovery</h3>
<p>
</p>
<ul class="text">
<li>
Supports OP Identifiers. This new variation of the
protocol flow is initiated by an end user entering an OP
Identifier instead of their own Identifier. This allows
the OP to assist the end user in selecting an
Identifier.
</li>
<li>
Supports the use of XRIs as Identifiers. XRIs may be
used as Identifiers for both end users and OPs, and
provide automatic mapping from one or more reassignable
i-names to a synonymous persistent Canonical ID that
will never be reassigned.
</li>
<li>
When URLs are used as Identifiers, they are normalized
according to the guidelines in <a class="info" href="#RFC3986">[RFC3986]<span> (</span><span class="info">Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a>,
for better compatibility with the existing Web infrastructure.
</li>
<li>
Uses the Yadis protocol for discovery. This allows for
using multiple OPs for a single Identifier, for
load-balancing and fallback in the case of OP
failure. Additionally, it allows for discovery of
supported extensions and other associated services.
</li>
</ul><p>
</p>
<a name="anchor36"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14.1.2"></a><h3>14.1.2.
Security improvements</h3>
<p>
A nonce is now part of the protocol for built-in protection
against replay attacks, which was previously implemented
out-of-band by each library or application.
</p>
<p>
A new association type, HMAC-SHA256, and a new association
session type, DH-SHA256, allow for stronger signatures on
authentication assertions.
</p>
<p>
An actual <a class="info" href="#security_considerations">Security
Considerations section<span> (</span><span class="info">Security Considerations</span><span>)</span></a> which looks at protecting
the protocol from end-to-end.
</p>
<a name="anchor37"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14.1.3"></a><h3>14.1.3.
Extensions</h3>
<p>
Extensions are now an officially supported mechanism to
support data exchange and other Relying Party-OP
communication along with the authentication
exchange. Extensions allow for the exchange of arbitrary
attributes, as well as for protocol extensions,
such as the inclusion of additional information about the
Relying Party in the authentication request.
</p>
<p>
Because extensions can transfer arbitrary data, the
Identifier is now optional in authentication messages.
</p>
<a name="anchor38"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14.2"></a><h3>14.2.
Implementing OpenID Authentication 1.1 Compatibility</h3>
<p>
All messages in OpenID Authentication 1.1 omit the
"openid.ns" parameter, which is an easy way for an RP to
determine if the message is from an OpenID Authentication
1.1 endpoint. OpenID Authentication 1.1 supports only
HMAC-SHA1 associations.
</p>
<p>
Error responses in OpenID Authentication 1.1 did not define
"contact" or "reference". OpenID Authentication 1.1 did
allow for the addition of extra fields in error
responses. It is RECOMMENDED for contact and reference to be
sent even when using OpenID Authentication 1.1, since they
may be useful for debugging and do not affect compatibility.
</p>
<a name="anchor39"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.14.2.1"></a><h3>14.2.1.
Relying Parties</h3>
<p>
</p>
<ul class="text">
<li>
When HTML discovery is performed, the OP endpoint URL
is marked by the link relationship "openid.server"
rather than "openid2.provider". The end user's
OP-Local Identifier is marked by the link relationship
"openid.delegate" rather than "openid2.local_id". The
protocol version is in this case
"http://openid.net/signon/1.1". HTML allows multiple
link relationships to be specified for a single link,
so if an OP provides both OpenID Authentication 1.1
and OpenID Authentication 2.0, "openid2.provider" and
"openid.server" may appear in the same "rel"
attribute.
</li>
<li>
When XRDS-based discovery is performed, the end user's
OP-Local Identifier appears in the
<openid:Delegate> tag of the OpenID
<xrd:Service> element rather than in the
<xrd:LocalID> tag. In order to support
currently-deployed discovery code, both tags MAY
appear in the <xrd:Service> element.
</li>
<li>
Relying Parties SHOULD extract and use OpenID
Authentication 1.x service elements from XRDS
documents, if Yadis succeeds on an URL
Identifier. Such service elements are identified by
<xrd:Type> tags whose text contents are
"http://openid.net/server/1.0" or
"http://openid.net/server/1.1". Although this is not
specified in the previous version of the protocol, it
is a generally accepted practice of advertising OpenID
Authentication 1.x services through Yadis.
</li>
<li>
"openid.claimed_id" is not defined by OpenID
Authentication 1.1. Relying Parties MAY send the value
when making requests, but MUST NOT depend on the value
being present in authentication responses. When the
OP-Local Identifier ("openid.identity") is
different from the Claimed Identifier, the Relying
Party MUST keep track of what Claimed Identifier was
used to discover the OP-Local Identifier, for
example by keeping it in session state. Although the
Claimed Identifier will not be present in the
response, it MUST be used as the identifier for the
user.
</li>
<li>
"openid.identity" MUST be sent in a <a class="info" href="#responding_to_authentication">authentication
request<span> (</span><span class="info">Responding to Authentication Requests</span><span>)</span></a>.
</li>
<li>
Relying Parties MUST send a blank session_type parameter
in "no-encryption" association requests.
</li>
<li>
In OpenID Authentication 1.1, the "no-encryption"
association session type is represented by a blank or
missing "openid.session_type" parameter. Relying
Parties MUST NOT send requests with
"openid.session_type" set to "no-encryption".
</li>
<li>
In <a class="info" href="#requesting_authentication">authentication
requests<span> (</span><span class="info">Requesting Authentication</span><span>)</span></a>, the "openid.identity" parameter
SHOULD NOT be the special value
"http://specs.openid.net/auth/2.0/identifier_select",
because OpenID Authentication 1.1 does not support the
use of OP Identifiers.
</li>
<li>
The "openid.realm" parameter in authentication requests
was known as "openid.trust_root". The syntax and meaning
are identical.
</li>
<li>
When responding with a negative assertion to a
"checkid_immediate" mode authentication request, the
"user_setup_url" parameter MUST be returned. This is a
URL that the end user may visit to complete the
request. The OP MAY redirect the end user to
this URL, or provide the end user with a link that
points to this URL.
</li>
<li>
The Relying Party MUST accept an <a class="info" href="#positive_assertions">authentication
response<span> (</span><span class="info">Positive Assertions</span><span>)</span></a> that is missing the
"openid.response_nonce" parameter. It SHOULD
implement a method for preventing replay attacks.
</li>
<li>
Relying Parties MUST accept
<a class="info" href="#positive_assertions">authentication responses<span> (</span><span class="info">Positive Assertions</span><span>)</span></a> that are missing the "openid.op_endpoint" parameter.
</li>
</ul><p>
</p>
<a name="anchor40"></a><br><hr>
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<a name="rfc.section.14.2.2"></a><h3>14.2.2.
OpenID Providers</h3>
<p>
</p>
<ul class="text">
<li>
"openid.identity" MUST be sent in a <a class="info" href="#positive_assertions">positive authentication
assertion<span> (</span><span class="info">Positive Assertions</span><span>)</span></a>.
</li>
<li>
In OpenID Authentication 1.1, the "no-encryption"
association session type is represented by a blank or
missing "openid.session_type" parameter. OPs MUST NOT
send responses with "openid.session_type" set to
"no-encryption".
</li>
<li>
OPs MAY choose to return a successful "no-encryption"
response to any association request. As above, the
"openid.session_type" parameter MUST be blank or
omitted from the response.
</li>
<li>
OPs MUST accept association requests with no assoc_type
parameter, and assume them to be of type HMAC-SHA1.
</li>
<li>
Unsuccessful association attempts MAY be responded with
direct error messages or with "no-encryption" positive
association responses.
</li>
<li>
The "openid.realm" parameter in authentication requests
was known as "openid.trust_root". The syntax and meaning
are identical.
</li>
<li>
When responding with a negative assertion to a
"checkid_immediate" mode authentication request, the
"user_setup_url" parameter MUST be returned. This is a URL
that the end user may visit to complete the request. The
Relying Party may redirect the end user to this URL, or
provide the end user with a link that points to this
URL.
</li>
<li>
OPs MUST NOT send the "openid.op_endpoint" parameter in
<a class="info" href="#positive_assertions">authentication responses<span> (</span><span class="info">Positive Assertions</span><span>)</span></a>, since it is not part of the OpenID Authentication
1.1 protocol.
</li>
</ul><p>
</p>
<a name="security_considerations"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15"></a><h3>15.
Security Considerations</h3>
<a name="anchor41"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.1"></a><h3>15.1.
Preventing Attacks</h3>
<a name="preventing_eavesdropping"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.1.1"></a><h3>15.1.1.
Eavesdropping Attacks</h3>
<p>
There is one place in this protocol that is vulnerable
to eavesdropping attacks.
</p>
<ul class="text">
<li>
If the nonce were not checked, an eavesdropper could also
intercept a successful authentication assertion and re-use it.
</li>
</ul><p>
</p>
<p>
This attack can be prevented by using transport layer encryption
for these connections to prevent eavesdropping. In addition,
if not using TLS this attack can still be prevented by
checking the nonce while performing message verification.
When doing so, the positive authentication assertion cannot
be re-used.
</p>
<a name="anchor42"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.1.2"></a><h3>15.1.2.
Man-in-the-Middle Attacks</h3>
<p>
Associations prevent tampering of signed fields by a man
in the middle except during discovery, association
sessions and <a class="info" href="#check_auth">Direct
Verification<span> (</span><span class="info">Verifying Directly with the OpenID Provider</span><span>)</span></a>. Altering signed fields without the
shared secret requires breaking the MAC. Currently no
tractable attack is known on the MACs used in this
protocol. The quality of the protection provided by the
MAC depends on the randomness of the shared MAC key, so it
is important that an unguessable value be used.
</p>
<p>
If DNS resolution or the transport layer is compromised
signatures on messages are not adequate, since the
attacker can impersonate the OP and issue its own
associations, or its own decisions in Stateless Mode. If
an attacker can tamper with the discovery process they can
specify any OP, and so does not have to impersonate the
OP. Additionally, if an attacker can compromise the
integrity of the information returned during the discovery
process, by altering the XRDS document, the need for a man
in the middle is removed. One method to prevent this sort
of attack is by digitally signing the XRDS file as per
<a class="info" href="#RFC3275">XMLDSIG<span> (</span><span class="info">Eastlake 3rd, D., Reagle Jr., J., and D. Solo, “(Extensible Markup Language) XML-Signature Syntax and Processing,” .</span><span>)</span></a> [RFC3275]. The keying material
is not specified, since the RP ultimately needs to make
its own decision whether to trust keys used for such
signature.
</p>
<p>
Using SSL with certificates signed by a trusted authority
prevents these kinds of attacks by verifying the results
of the DNS look-up against the certificate. Once the
validity of the certificate has been established,
tampering is not possible. Impersonating an SSL server
requires forging or stealing a certificate, which is
significantly harder than the network based attacks.
</p>
<p>
In order to get protection from SSL, SSL must be used for
all parts of the interaction, including interaction with
the end user through the User-Agent. While the protocol
does not require SSL be used, its use is strongly
RECOMMENDED. Current best practices dictate that an OP
SHOULD use SSL, with a certificate signed by a trusted
authority, to secure its Endpoint URL as well as the
interactions with the end user's User-Agent. In addition,
SSL, with a certificate signed by a trusted authority,
SHOULD be used so that a Relying Party can fetch the end
user's URL in a secure manner. Following its own security
policies, a Relying Party MAY choose to not complete, or
even begin, a transaction if SSL is not being correctly
used at these various endpoints.
</p>
<a name="rp_mitm_proxy"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.1.2.1"></a><h3>15.1.2.1.
Rogue Relying Party Proxying</h3>
<p>
A special type of man-in-the-middle attack is one where
the Relying Party is a rogue party acting as a MITM. The
RP would perform discovery on the End User's Claimed
Identifier and instead of redirecting the User Agent to
the OP, would instead proxy the OP through itself. This
would thus allow the RP to capture credentials the End
User provides to the OP. While there are multiple ways to
prevent this sort of attack, the specifics are outside the
scope of this document. Each method of prevention
requires that the OP establish a secure channel with the
End User.
</p>
<a name="anchor43"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.2"></a><h3>15.2.
User-Agents</h3>
<p>
Since this protocol is intended to be used interactively,
User-Agents will primarily be common Web browsers. Web
browsers or their hosts may be infected with spyware or
other malware, which limits the strength of the
authentication assertion, since untrusted software makes it
impossible to know whether the authentication decision has
been made with the end user's approval. With that said, many
web applications and protocols today rely on the security of
the Web browser and their hosts.
</p>
<p>
Cross-site-scripting attacks against OPs may be used to the
same effect. For the best security, OPs should not depend
on scripting. This enables User-Agents that do not support
scripting, or have scripting disabled, to still employ the
protocol.
</p>
<a name="anchor44"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.3"></a><h3>15.3.
User Interface Considerations</h3>
<p>
The Relying Party SHOULD redirect the end user to the OP
Endpoint URL in a top-level browser window with all controls
visible. This allows better protection for the end user
against OP look-alike sites (phishing).
</p>
<p>
OpenID Providers SHOULD educate their end users about the
potential for OpenID phishing attacks and SHOULD equip their
end users with the tools to defeat such attacks, for example
browser plug-ins that verify the authenticity of the OP's
Authentication Service Endpoint URL.
</p>
<a name="anchor45"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.4"></a><h3>15.4.
HTTP and HTTPS URL Identifiers</h3>
<p>
While these types of Identifiers have been <a class="info" href="#http_s_identifiers">previously discussed<span> (</span><span class="info">HTTP and HTTPS URL Identifiers</span><span>)</span></a>,
they are worth mentioning again. As previously stated, the
RECOMMENDED method of an End User expressing control over a
URL differing only be scheme is to setup a redirect from the
HTTP URL to the HTTPS URL. Relying Parties will never store
the HTTP URL as during the discovery and initiation phase
will follow the redirect and use the HTTPS URL as the
Claimed Identifier.
</p>
<p>
End users with concerns over this recommendation should
directly enter their HTTPS URL at each Relying Party. This
thus removes the step where the Relying Party follows the
redirect to the HTTPS URL. The single security
consideration currently seen is if an attacker were to
compromise the integrity of the HTTP URL by removing the
redirect and pointing the Identifier at a rogue OP. This
however will alter the user experience, is detectable by
anti-phishing technologies, and the security of the
Identifier itself is a fundamental principle within OpenID.
</p>
<a name="anchor46"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.5"></a><h3>15.5.
Denial of Service Attacks</h3>
<p>
Within the protocol there are places where a rogue RP could
launch a denial of service attack against an OP since there
is nothing in OpenID protocol messages that allows the OP to
quickly check that it is a genuine request. This can be
done by the RP repeatedly requesting associations,
authentication, or verification of a signature.
</p>
<p>
The potentially most severe attack is during the association
phase as each message requires the OP to execute a discrete
exponentiation. Since the RP has the ability to specify
modulus and generator per message, an attacker can even
force the OP to perform this exponentiation in real time
prior to responding for each message.
</p>
<p>
While this could be particularly harmful, OpenID Providers
can easily use generic IP based rate-limiting and banning
techniques to help combat these sorts of attacks. OPs can
also look at banning requests based on the "openid.realm"
and "openid.return_to" values.
</p>
<a name="anchor47"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.15.6"></a><h3>15.6.
Protocol Variants</h3>
<p>
The following are known variations in the protocol which may
or may not directly affect the security of the use of the
protocol. It is imagined that these values could be used in
the creation of security profiles for this protocol. The
following list of variants are from the perspective of an
OpenID Provider.
</p><table class="full" align="center" border="0" cellpadding="2" cellspacing="2">
<col align="left"><col align="left"><col align="left">
<tbody><tr><th align="left">Number</th><th align="left">Variant</th><th align="left">Values</th></tr>
<tr>
<td align="left">1.</td>
<td align="left">
Are wildcards allowed in realms?
</td>
<td align="left">
One of Yes/No
</td>
</tr>
<tr>
<td align="left">2.</td>
<td align="left">
Require prior association? Does the OP require the RP
first create an association before requesting
authentication?
</td>
<td align="left">
One of Yes/No
</td>
</tr>
<tr>
<td align="left">3.</td>
<td align="left">
Types of claimed identifiers accepted.
</td>
<td align="left">
Set of HTTP/HTTPS/XRI
</td>
</tr>
<tr>
<td align="left">4.</td>
<td align="left">
Are self-issued certificates allowed for authentication?
This applies to all SSL traffic. If 'no' here, then OP
*probably* requires all HTTPS identifiers to chain up to
known trust roots, but that's intentionally not implied.
</td>
<td align="left">
One of Yes/No
</td>
</tr>
<tr>
<td align="left">5.</td>
<td align="left">
Must the XRDS file be signed? Signature on the XRDS as per
XMLDSIG. Keying material not specified, since the RP
ultimately needs to make own decision whether to trust
keys used for such signature.
</td>
<td align="left">
One of Yes/No
</td>
</tr>
<tr>
<td align="left">6.</td>
<td align="left">
Must the XRDS file be retrieved over secure channel? This does not
imply SSL?
</td>
<td align="left">
One of Yes/No
</td>
</tr>
<tr>
<td align="left">7.</td>
<td align="left">
What types of session types can be used when creating
associations?
</td>
<td align="left">
Set of no-encryption/DH-SHA1/DH-SHA256
</td>
</tr>
<tr>
<td align="left">8.</td>
<td align="left">
Must the RP have an XRDS document?
</td>
<td align="left">
One of Yes/No
</td>
</tr>
<tr>
<td align="left">9.</td>
<td align="left">
What association types the OP agrees to use for
signatures?
</td>
<td align="left">
Set of HMAC-SHA1/HMAC-SHA256
</td>
</tr>
<tr>
<td align="left">10.</td>
<td align="left">
Must the association request take place over secure channel?
</td>
<td align="left">
One of Yes/No
</td>
</tr>
</tbody></table>
<p style="text-align: center;">
Identified security variants.
</p>
<a name="anchor48"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.A"></a><h3>Appendix A.
Examples</h3>
<p>Non-normative
</p>
<a name="normalization_example"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.A.1"></a><h3>Appendix A.1.
Normalization</h3>
<p>
See section 6 of <a class="info" href="#RFC3986">[RFC3986]<span> (</span><span class="info">Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .</span><span>)</span></a> for
textual URL normalization details and more examples.
</p><br><hr class="insert">
<table class="full" align="center" border="0" cellpadding="2" cellspacing="2">
<col align="left"><col align="left"><col align="left"><col align="left">
<tbody><tr><th align="left">User's Input</th><th align="left">Identifier</th><th align="left">Type</th><th align="left">Discussion</th></tr>
<tr>
<td align="left">example.com</td>
<td align="left">http://example.com/</td>
<td align="left">URL</td>
<td align="left">A URI with a missing scheme is normalized to a http URI</td>
</tr>
<tr>
<td align="left">http://example.com</td>
<td align="left">http://example.com/</td>
<td align="left">URL</td>
<td align="left">An empty path component is normalized to a slash</td>
</tr>
<tr>
<td align="left">https://example.com/</td>
<td align="left">https://example.com/</td>
<td align="left">URL</td>
<td align="left">https URIs remain https URIs</td>
</tr>
<tr>
<td align="left">http://example.com/user</td>
<td align="left">http://example.com/user</td>
<td align="left">URL</td>
<td align="left">No trailing slash is added to non-empty path components</td>
</tr>
<tr>
<td align="left">http://example.com/user/</td>
<td align="left">http://example.com/user/</td>
<td align="left">URL</td>
<td align="left">Trailing slashes are preserved on non-empty path components</td>
</tr>
<tr>
<td align="left">http://example.com/</td>
<td align="left">http://example.com/</td>
<td align="left">URL</td>
<td align="left">Trailing slashes are preserved when the path is empty</td>
</tr>
<tr>
<td align="left">=example</td>
<td align="left">=example</td>
<td align="left">XRI</td>
<td align="left">Normalized XRIs start with a global context symbol</td>
</tr>
<tr>
<td align="left">xri://=example</td>
<td align="left">=example</td>
<td align="left">XRI</td>
<td align="left">Normalized XRIs start with a global context symbol</td>
</tr>
</tbody></table>
<table align="center" border="0" cellpadding="0" cellspacing="2"><tbody><tr><td align="center"><font face="monaco, MS Sans Serif" size="1"><b> User's Input to Identifier Normalization </b></font><br></td></tr></tbody></table><hr class="insert">
<a name="anchor49"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.A.2"></a><h3>Appendix A.2.
OP-Local Identifiers</h3>
<p>
An end user wants to use "http://www.example.com/" as their
Claimed Identifier. The end user has an account with
Example Provider, which functions as an OpenID Provider. The
end user's OP-Local Identifier is
"https://exampleuser.exampleprovider.com/".
</p>
<p>
In this scenario, with the proper configuration of Yadis or
HTML-Based Discovery (see <a class="info" href="#discovery">Section 7.3<span> (</span><span class="info">Discovery</span><span>)</span></a> and
<a class="info" href="#XRDS_Sample">Appendix A.3<span> (</span><span class="info">XRDS</span><span>)</span></a> below), a Relying Party will
discover the following information about the end user:
</p>
<blockquote class="text"><dl>
<dt>Claimed Identifier</dt>
<dd>
http://www.example.com/
</dd>
<dt>OP-Local Identifier</dt>
<dd>
https://exampleuser.exampleprovider.com/
</dd>
</dl></blockquote><p>
</p>
<a name="XRDS_Sample"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.A.3"></a><h3>Appendix A.3.
XRDS</h3>
<p>
For an end user to use "http://www.example.com/" as
their Identifier, but have Relying Parties actually
verify "https://exampleuser.exampleprovider.com/" with the OP
Endpoint URL
"https://www.exampleprovider.com/endpoint/", the
following XML snippet should be present in the final XRD
element in the XRDS file when discovery is preformed on
"http://www.example.com/":
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre><Service xmlns="xri://$xrd*($v*2.0)">
<Type>http://specs.openid.net/auth/2.0/signon</Type>
<URI>https://www.exampleprovider.com/endpoint/</URI>
<LocalID>https://exampleuser.exampleprovider.com/</LocalID>
</Service>
</pre></div>
<a name="anchor50"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.A.4"></a><h3>Appendix A.4.
HTML Identifier Markup</h3>
<p>
To use "http://www.example.com/" as their Identifier, but have
Relying Parties actually verify
"http://exampleuser.livejournal.com/" with the OpenID
Provider located at
"http://www.livejournal.com/openid/server.bml", the
following markup should be present in the <head>
of the HTML document located by the identifier URL:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre><link rel="openid2.provider openid.server"
href="http://www.livejournal.com/openid/server.bml"/>
<link rel="openid2.local_id openid.delegate"
href="http://exampleuser.livejournal.com/"/>
</pre></div>
<a name="anchor51"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.A.5"></a><h3>Appendix A.5.
XRI CanonicalID</h3>
<p>
For example, if the XRI i-names =example and =exmpl both
yield an XRDS document with the CanonicalID
xri://(example)!1234 then those Identifiers should be
treated as equivalent. For applications with user accounts,
the persistent Canonical ID xri://(example)!1234 should be
used the primary key for the account. Although the
i-names =example and =exmpl may also be stored for reference
as display names, they are reassignable identifiers and
should not be used as persistent keys.
</p>
<a name="pvalue"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.B"></a><h3>Appendix B.
Diffie-Hellman Key Exchange Default Value</h3>
<p>
This is a confirmed-prime number, used as the default
modulus for Diffie-Hellman Key Exchange. In hexadecimal:
</p><div style="display: table; width: 0pt; margin-left: 3em; margin-right: auto;"><pre>DCF93A0B883972EC0E19989AC5A2CE310E1D37717E8D9571BB7623731866E61E
F75A2E27898B057F9891C2E27A639C3F29B60814581CD3B2CA3986D268370557
7D45C2E7E52DC81C7A171876E5CEA74B1448BFDFAF18828EFD2519F14E45E382
6634AF1949E5B535CC829A483B8A76223E5D490A257F05BDFF16F2FB22C583AB
</pre></div>
<a name="anchor52"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<a name="rfc.section.C"></a><h3>Appendix C.
Acknowledgements</h3>
<p>
The OpenID Community would like to thank the following people
for the work they've done in the drafting and editing of this
specification. If you want to know the nitty gritty of who
actually wrote what, feel free to look at our SVN repository
or even use "svn blame". :)
http://openid.net/svn/specifications/authentication/2.0/
</p>
<p>
</p>
<blockquote class="text">
<p>Barry Ferg (barry@sxip.com)
</p>
<p>Brad Fitzpatrick (brad@danga.com) <author>
</p>
<p>Carl Howells (chowells@janrain.com)
</p>
<p>David Recordon (david@sixapart.com) <author/editor>
</p>
<p>Dick Hardt (dick@sxip.com) <author>
</p>
<p>Drummond Reed (drummond.reed@cordance.net)
</p>
<p>Hans Granqvist (hgranqvist@verisign.com)
</p>
<p>Johannes Ernst (jernst@netmesh.us)
</p>
<p>Johnny Bufu (johnny@sxip.com) <editor>
</p>
<p>Josh Hoyt (josh@janrain.com) <author/editor>
</p>
<p>Kevin Turner (kevin@janrain.com)
</p>
<p>Marius Scurtescu (marius@sxip.com)
</p>
<p>Martin Atkins (mart@degeneration.co.uk)
</p>
<p>Mike Glover (mpg4@janrain.com)
</p>
</blockquote><p>
</p>
<a name="rfc.references1"></a><br><hr>
<table summary="layout" class="TOCbug" align="right" cellpadding="0" cellspacing="2"><tbody><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></tbody></table>
<h3>16. Normative References</h3>
<table border="0" width="99%">
<tbody><tr><td class="author-text" valign="top"><a name="FIPS180-2">[FIPS180-2]</a></td>
<td class="author-text">U.S. Department of Commerce and National Institute of Standards
and Technology, “<a href="http://csrc.nist.gov/publications/fips/fips180-2/fips180-2withchangenotice.pdf">Secure Hash Signature Standard</a>,” FIPS 180-2.<p>
Defines Secure Hash Algorithm 256 (SHA256)
</p>
</td></tr>
<tr><td class="author-text" valign="top"><a name="HTML401">[HTML401]</a></td>
<td class="author-text">W3C, “<a href="http://www.w3.org/TR/html401">HTML 4.01 Specification</a>.”</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC1750">[RFC1750]</a></td>
<td class="author-text">Eastlake, D., Crocker, S., and J. Schiller, “<a href="ftp://ftp.isi.edu/in-notes/rfc1750.txt">Randomness Recommendations for Security</a>,” RFC 1750.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2104">[RFC2104]</a></td>
<td class="author-text">Krawczyk, H., Bellare, M., and R. Canetti, “<a href="ftp://ftp.isi.edu/in-notes/rfc2104.txt">HMAC: Keyed-Hashing for Message Authentication</a>,” RFC 2104.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2119">[RFC2119]</a></td>
<td class="author-text">Bradner, B., “<a href="ftp://ftp.isi.edu/in-notes/rfc2119.txt">Key words for use in RFCs to Indicate Requirement Levels</a>,” RFC 2119.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2616">[RFC2616]</a></td>
<td class="author-text">Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “<a href="ftp://ftp.isi.edu/in-notes/rfc2616.txt">Hypertext Transfer Protocol -- HTTP/1.1</a>,” RFC 2616.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC2631">[RFC2631]</a></td>
<td class="author-text">Rescorla, E., “<a href="ftp://ftp.isi.edu/in-notes/rfc2631.txt">Diffie-Hellman Key Agreement Method</a>,” RFC 2631.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3174">[RFC3174]</a></td>
<td class="author-text">Eastlake, D. and P. Jones, “<a href="ftp://ftp.isi.edu/in-notes/rfc3174.txt">US Secure Hash Algorithm 1 (SHA1)</a>,” RFC 3174.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3275">[RFC3275]</a></td>
<td class="author-text">Eastlake 3rd, D., Reagle Jr., J., and D. Solo, “<a href="ftp://ftp.isi.edu/in-notes/rfc3275.txt">(Extensible Markup Language) XML-Signature Syntax and
Processing</a>,” RFC 3275.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3339">[RFC3339]</a></td>
<td class="author-text">Klyne, G. and C. Newman, “<a href="ftp://ftp.isi.edu/in-notes/rfc3339.txt">Date and Time on the Internet: Timestamps</a>,” RFC 3339.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3548">[RFC3548]</a></td>
<td class="author-text">Josefsson, S., “<a href="ftp://ftp.isi.edu/in-notes/rfc3548.txt">The Base16, Base32, and Base64 Data Encodings</a>,” RFC 3548.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3629">[RFC3629]</a></td>
<td class="author-text">Yergeau, F., “<a href="ftp://ftp.isi.edu/in-notes/rfc3629.txt">UTF-8, a transformation format of Unicode and ISO 10646</a>,” RFC 3629.</td></tr>
<tr><td class="author-text" valign="top"><a name="RFC3986">[RFC3986]</a></td>
<td class="author-text">Berners-Lee, T., “<a href="ftp://ftp.isi.edu/in-notes/rfc3986.txt">Uniform Resource Identifiers (URI): Generic Syntax</a>,” RFC 3986.</td></tr>
<tr><td class="author-text" valign="top"><a name="XRI_Resolution_2.0">[XRI_Resolution_2.0]</a></td>
<td class="author-text">Wachob, G., Reed, D., Chasen, L., Tan, W., and S. Churchill, “<a href="http://www.oasis-open.org/committees/download.php/17293">Extensible Resource Identifier (XRI) Resolution V2.0
- Committee Draft 02</a>” (<a href="http://docs.oasis-open.org/xri/2.0/specs/cd02/xri-resolution-V2.0-cd-02.html">HTML</a>, <a href="http://docs.oasis-open.org/xri/2.0/specs/cd02/xri-resolution-V2.0-cd-02.pdf">PDF</a>).</td></tr>
<tr><td class="author-text" valign="top"><a name="XRI_Syntax_2.0">[XRI_Syntax_2.0]</a></td>
<td class="author-text">Reed, D. and D. McAlpin, “<a href="http://www.oasis-open.org/committees/download.php/15376">Extensible Resource Identifier (XRI) Syntax V2.0</a>” (<a href="http://www.oasis-open.org/committees/download.php/15376">HTML</a>, <a href="http://www.oasis-open.org/committees/download.php/15377">PDF</a>).</td></tr>
<tr><td class="author-text" valign="top"><a name="Yadis">[Yadis]</a></td>
<td class="author-text">Miller, J., “Yadis Specification 1.0” (<a href="http://yadis.org/papers/yadis-v1.0.pdf">PDF</a>, <a href="http://yadis.org/papers/yadis-v1.0.odt">ODT</a>).</td></tr>
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<h3>Author's Address</h3>
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<tbody><tr><td class="author-text"> </td>
<td class="author-text">specs@openid.net</td></tr>
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