Initial version

DataTransfer.Client/Services/HandshakeCodec.cs

@@ -0,0 +1,53 @@
+namespace DataTransfer.Client.Services;
+
+/// <summary>
+///     Plaintext messages of the commit-then-reveal key exchange (see <see cref="SecureChannel" />). They are relayed
+///     like frames, but before the channel is encrypted. Layout: version (byte) | type (byte) | payload.
+/// </summary>
+internal static class HandshakeCodec
+{
+    public const int CommitmentBytes = 32;
+
+    // A P-256 SPKI key has 91 bytes; anything much larger is not a key.
+    private const int MaxPublicKeyBytes = 512;
+    private const byte Version = 1;
+
+    private enum MessageType : byte
+    {
+        Commitment = 1,
+        PublicKey = 2
+    }
+
+    public static byte[] EncodeCommitment(byte[] commitment) => Encode(MessageType.Commitment, commitment);
+
+    public static byte[] EncodePublicKey(byte[] publicKey) => Encode(MessageType.PublicKey, publicKey);
+
+    /// <exception cref="InvalidDataException">The message is not a commitment.</exception>
+    public static byte[] DecodeCommitment(byte[] message) =>
+        Decode(message, MessageType.Commitment, CommitmentBytes, CommitmentBytes);
+
+    /// <exception cref="InvalidDataException">The message is not a public key.</exception>
+    public static byte[] DecodePublicKey(byte[] message) => Decode(message, MessageType.PublicKey, 1, MaxPublicKeyBytes);
+
+    private static byte[] Encode(MessageType type, byte[] payload)
+    {
+        var message = new byte[payload.Length + 2];
+        message[0] = Version;
+        message[1] = (byte)type;
+        payload.CopyTo(message, 2);
+        return message;
+    }
+
+    private static byte[] Decode(byte[] message, MessageType type, int minPayloadBytes, int maxPayloadBytes)
+    {
+        ArgumentNullException.ThrowIfNull(message);
+
+        var payloadBytes = message.Length - 2;
+        if (payloadBytes < minPayloadBytes || payloadBytes > maxPayloadBytes || message[0] != Version || message[1] != (byte)type)
+        {
+            throw new InvalidDataException($"Expected a valid key-exchange message of type {type}.");
+        }
+
+        return message[2..];
+    }
+}