From 3163811279a9d074a8d33bbf6ae4b6f776c0c46d Mon Sep 17 00:00:00 2001
From: Matthew Endsley <mendsley@gmail.com>
Date: Wed, 13 Jan 2021 00:18:45 -0800
Subject: [PATCH] Add the psuedoranomm function P_SHA256

---
 Hazel/Dtls/PrfSha256.cs | 56 +++++++++++++++++++++++++++++++++++++++++
 Hazel/Hazel.csproj      |  1 +
 2 files changed, 57 insertions(+)
 create mode 100644 Hazel/Dtls/PrfSha256.cs

diff --git a/Hazel/Dtls/PrfSha256.cs b/Hazel/Dtls/PrfSha256.cs
new file mode 100644
index 0000000..683e55f
--- /dev/null
+++ b/Hazel/Dtls/PrfSha256.cs
@@ -0,0 +1,56 @@
+using System.Text;
+using System.Security.Cryptography;
+
+namespace Hazel.Dtls
+{
+    /// <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)
+        {
+            ByteSpan writer = output;
+
+            ///TODO(mendsley): Look into pre-calculating all of these
+            ByteSpan labelAsBytes = Encoding.ASCII.GetBytes(label);
+
+            byte[] roundSeed = new byte[labelAsBytes.Length + initialSeed.Length];
+            labelAsBytes.CopyTo(roundSeed);
+            initialSeed.CopyTo(roundSeed, labelAsBytes.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);
+                }
+            }
+        }
+    }
+}
diff --git a/Hazel/Hazel.csproj b/Hazel/Hazel.csproj
index 1f130f7..e75bb1e 100644
--- a/Hazel/Hazel.csproj
+++ b/Hazel/Hazel.csproj
@@ -80,6 +80,7 @@
     <Compile Include="DisconnectedEventArgs.cs" />
     <Compile Include="Dtls\Handshake.cs" />
     <Compile Include="Dtls\IHandshakeCipherSuite.cs" />
+    <Compile Include="Dtls\PrfSha256.cs" />
     <Compile Include="Dtls\X25519EcdheRsaSha256.cs" />
     <Compile Include="FewerThreads\HazelThreadPool.cs" />
     <Compile Include="FewerThreads\ThreadLimitedUdpConnectionListener.cs" />
-- 
2.39.5