Add HOTP Support (#6)

* Add HOTP

* Fix Issue

* Fix incorrect RFC number

* Change incorrect function name

* Add VerifyHotp function

* Sign assembly

* Update README

* Fix small typos

1 files changed, 101 insertions, 0 deletions

diff --git a/src/Otp.NET/Hotp.cs b/src/Otp.NET/Hotp.cs
new file mode 100644
index 0000000..ee804b7
--- /dev/null
+++ b/src/Otp.NET/Hotp.cs
@@ -0,0 +1,101 @@
+/*
+Credits to Devin Martin and the original OtpSharp library:
+https://bitbucket.org/devinmartin/otp-sharp/overview 
+
+Copyright (C) 2012 Devin Martin
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included
+in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+*/
+
+using System;
+using System.Globalization;
+
+namespace OtpNet
+{
+    /// <summary>
+    /// Calculate HMAC-Based One-Time-Passwords (HOTP) from a secret key
+    /// </summary>
+    /// <remarks>
+    /// The specifications for this are found in RFC 4226
+    /// http://tools.ietf.org/html/rfc4226
+    /// </remarks>
+    public class Hotp: Otp
+    {
+        private readonly int hotpSize;
+
+        /// <summary>
+        /// Create a HOTP instance
+        /// </summary>
+        /// <param name="secretKey">The secret key to use in HOTP calculations</param>
+        /// <param name="mode">The hash mode to use</param>
+        /// <param name="hotpSize">The number of digits that the returning HOTP should have.  The default is 6.</param>
+        public Hotp(byte[] secretKey, OtpHashMode mode = OtpHashMode.Sha1, int hotpSize = 6)
+            : base(secretKey, mode)
+        {
+            VerifyParameters(hotpSize);
+
+            this.hotpSize = hotpSize;
+        }
+        private static void VerifyParameters(int hotpSize)
+        {
+            if (!(hotpSize >= 6))
+                throw new ArgumentOutOfRangeException("hotpSize");
+            if (!(hotpSize <= 8))
+                throw new ArgumentOutOfRangeException("hotpSize");
+        }
+        /// <summary>
+        /// Takes a counter and then computes a HOTP value
+        /// </summary>
+        /// <param name="timestamp">The timestamp to use for the HOTP calculation</param>
+        /// <returns>a HOTP value</returns>
+        public string ComputeHOTP(long counter)
+        {
+            return this.Compute(counter, this.hashMode);
+        }
+        /// <summary>
+        /// Verify a value that has been provided with the calculated value
+        /// </summary>
+        /// <param name="hotp">the trial HOTP value</param>
+        /// <param name="counter">The counter value to verify/param>
+        /// <returns>True if there is a match.</returns>
+        public bool VerifyHotp(string hotp, long counter)
+        {
+            if (hotp == ComputeHOTP(counter))
+            {
+                return true;
+            } else
+            {
+                return false;
+            }
+        }
+        /// <summary>
+        /// Takes a time step and computes a HOTP code
+        /// </summary>
+        /// <param name="counter">counter</param>
+        /// <param name="mode">The hash mode to use</param>
+        /// <returns>HOTP calculated code</returns>
+        protected override string Compute(long counter, OtpHashMode mode)
+        {
+            var data = KeyUtilities.GetBigEndianBytes(counter);
+            var otp = this.CalculateOtp(data, mode);
+            return Digits(otp, this.hotpSize);
+        }
+
+    }
+}
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