1 files changed, 84 insertions, 0 deletions
diff --git a/Tools/Hazel-Networking/Hazel/Dtls/PrfSha256.cs b/Tools/Hazel-Networking/Hazel/Dtls/PrfSha256.cs
new file mode 100644
index 0000000..1fa7f17
--- /dev/null
+++ b/Tools/Hazel-Networking/Hazel/Dtls/PrfSha256.cs
@@ -0,0 +1,84 @@
+using System.Text;
+using System.Security.Cryptography;
+
+namespace Hazel.Dtls
+{
+    /// <summary>
+    /// Common Psuedorandom Function labels for TLS
+    /// </summary>
+    public struct PrfLabel
+    {
+        public static readonly ByteSpan MASTER_SECRET = LabelToBytes("master secert");
+        public static readonly ByteSpan KEY_EXPANSION = LabelToBytes("key expansion");
+        public static readonly ByteSpan CLIENT_FINISHED = LabelToBytes("client finished");
+        public static readonly ByteSpan SERVER_FINISHED = LabelToBytes("server finished");
+
+        /// <summary>
+        /// Convert a text label to a byte sequence
+        /// </summary>
+        public static ByteSpan LabelToBytes(string label)
+        {
+            return Encoding.ASCII.GetBytes(label);
+        }
+    }
+
+    /// <summary>
+    /// The P_SHA256 Psuedorandom Function
+    /// </summary>
+    public struct PrfSha256
+    {
+        /// <summary>
+        /// Expand a secret key
+        /// </summary>
+        /// <param name="output">Output span. Length determines how much data to generate</param>
+        /// <param name="key">Original key to expand</param>
+        /// <param name="label">Label (treated as a salt)</param>
+        /// <param name="initialSeed">Seed for expansion (treated as a salt)</param>
+        public static void ExpandSecret(ByteSpan output, ByteSpan key, string label, ByteSpan initialSeed)
+        {
+            ExpandSecret(output, key, PrfLabel.LabelToBytes(label), initialSeed);
+        }
+
+        /// <summary>
+        /// Expand a secret key
+        /// </summary>
+        /// <param name="output">Output span. Length determines how much data to generate</param>
+        /// <param name="key">Original key to expand</param>
+        /// <param name="label">Label (treated as a salt)</param>
+        /// <param name="initialSeed">Seed for expansion (treated as a salt)</param>
+        public static void ExpandSecret(ByteSpan output, ByteSpan key, ByteSpan label, ByteSpan initialSeed)
+        {
+            ByteSpan writer = output;
+
+            byte[] roundSeed = new byte[label.Length + initialSeed.Length];
+            label.CopyTo(roundSeed);
+            initialSeed.CopyTo(roundSeed, label.Length);
+
+            byte[] hashA = roundSeed;
+
+            using (HMACSHA256 hmac = new HMACSHA256(key.ToArray()))
+            {
+                byte[] input = new byte[hmac.OutputBlockSize + roundSeed.Length];
+                new ByteSpan(roundSeed).CopyTo(input, hmac.OutputBlockSize);
+
+                while (writer.Length > 0)
+                {
+                    // Update hashA
+                    hashA = hmac.ComputeHash(hashA);
+
+                    // generate hash input
+                    new ByteSpan(hashA).CopyTo(input);
+
+                    ByteSpan roundOutput = hmac.ComputeHash(input);
+                    if (roundOutput.Length > writer.Length)
+                    {
+                        roundOutput = roundOutput.Slice(0, writer.Length);
+                    }
+
+                    roundOutput.CopyTo(writer);
+                    writer = writer.Slice(roundOutput.Length);
+                }
+            }
+        }
+    }
+}