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diff --git a/src/DotNetOpenAuth.Core/Messaging/DataBag.cs b/src/DotNetOpenAuth.Core/Messaging/DataBag.cs
new file mode 100644
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+++ b/src/DotNetOpenAuth.Core/Messaging/DataBag.cs
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+//-----------------------------------------------------------------------
+// <copyright file="DataBag.cs" company="Andrew Arnott">
+//     Copyright (c) Andrew Arnott. All rights reserved.
+// </copyright>
+//-----------------------------------------------------------------------
+
+namespace DotNetOpenAuth.Messaging {
+	using System;
+	using System.Collections.Generic;
+	using System.Diagnostics.CodeAnalysis;
+	using System.Diagnostics.Contracts;
+
+	/// <summary>
+	/// A collection of message parts that will be serialized into a single string,
+	/// to be set into a larger message.
+	/// </summary>
+	internal abstract class DataBag : IMessage {
+		/// <summary>
+		/// The default version for DataBags.
+		/// </summary>
+		private static readonly Version DefaultVersion = new Version(1, 0);
+
+		/// <summary>
+		/// The backing field for the <see cref="IMessage.Version"/> property.
+		/// </summary>
+		private Version version;
+
+		/// <summary>
+		/// A dictionary to contain extra message data.
+		/// </summary>
+		private Dictionary<string, string> extraData = new Dictionary<string, string>();
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="DataBag"/> class.
+		/// </summary>
+		protected DataBag()
+			: this(DefaultVersion) {
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="DataBag"/> class.
+		/// </summary>
+		/// <param name="version">The DataBag version.</param>
+		protected DataBag(Version version) {
+			Contract.Requires(version != null);
+			this.version = version;
+		}
+
+		#region IMessage Properties
+
+		/// <summary>
+		/// Gets the version of the protocol or extension this message is prepared to implement.
+		/// </summary>
+		/// <remarks>
+		/// Implementations of this interface should ensure that this property never returns null.
+		/// </remarks>
+		Version IMessage.Version {
+			get { return this.version; }
+		}
+
+		/// <summary>
+		/// Gets the extra, non-standard Protocol parameters included in the message.
+		/// </summary>
+		/// <value></value>
+		/// <remarks>
+		/// Implementations of this interface should ensure that this property never returns null.
+		/// </remarks>
+		public IDictionary<string, string> ExtraData {
+			get { return this.extraData; }
+		}
+
+		#endregion
+
+		/// <summary>
+		/// Gets or sets the nonce.
+		/// </summary>
+		/// <value>The nonce.</value>
+		[MessagePart]
+		internal byte[] Nonce { get; set; }
+
+		/// <summary>
+		/// Gets or sets the UTC creation date of this token.
+		/// </summary>
+		/// <value>The UTC creation date.</value>
+		[MessagePart("ts", IsRequired = true, Encoder = typeof(TimestampEncoder))]
+		internal DateTime UtcCreationDate { get; set; }
+
+		/// <summary>
+		/// Gets or sets the signature.
+		/// </summary>
+		/// <value>The signature.</value>
+		internal byte[] Signature { get; set; }
+
+		/// <summary>
+		/// Gets or sets the message that delivered this DataBag instance to this host.
+		/// </summary>
+		protected internal IProtocolMessage ContainingMessage { get; set; }
+
+		/// <summary>
+		/// Gets the type of this instance.
+		/// </summary>
+		/// <value>The type of the bag.</value>
+		/// <remarks>
+		/// This ensures that one token cannot be misused as another kind of token.
+		/// </remarks>
+		[SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode", Justification = "Accessed by reflection")]
+		[MessagePart("t", IsRequired = true, AllowEmpty = false)]
+		private Type BagType {
+			get { return this.GetType(); }
+		}
+
+		#region IMessage Methods
+
+		/// <summary>
+		/// Checks the message state for conformity to the protocol specification
+		/// and throws an exception if the message is invalid.
+		/// </summary>
+		/// <remarks>
+		/// 	<para>Some messages have required fields, or combinations of fields that must relate to each other
+		/// in specialized ways.  After deserializing a message, this method checks the state of the
+		/// message to see if it conforms to the protocol.</para>
+		/// 	<para>Note that this property should <i>not</i> check signatures or perform any state checks
+		/// outside this scope of this particular message.</para>
+		/// </remarks>
+		/// <exception cref="ProtocolException">Thrown if the message is invalid.</exception>
+		void IMessage.EnsureValidMessage() {
+			this.EnsureValidMessage();
+		}
+
+		#endregion
+
+		/// <summary>
+		/// Checks the message state for conformity to the protocol specification
+		/// and throws an exception if the message is invalid.
+		/// </summary>
+		/// <remarks>
+		/// 	<para>Some messages have required fields, or combinations of fields that must relate to each other
+		/// in specialized ways.  After deserializing a message, this method checks the state of the
+		/// message to see if it conforms to the protocol.</para>
+		/// 	<para>Note that this property should <i>not</i> check signatures or perform any state checks
+		/// outside this scope of this particular message.</para>
+		/// </remarks>
+		/// <exception cref="ProtocolException">Thrown if the message is invalid.</exception>
+		protected virtual void EnsureValidMessage() {
+		}
+	}
+}