ValidateHmac.cs

#r "System.Security"
using System.Net;
using System.Text;
using System.Security.Cryptography;

private const string Sha1Prefix = "sha1=";

public static async Task<HttpResponseMessage> Run(HttpRequestMessage req, TraceWriter log)
{
   log.Info("C# HTTP trigger function processed a request.");
   
       string signatureWithPrefix = req.Headers.GetValues("X-Hub-Signature").FirstOrDefault();
   log.Info("Signature: " + signatureWithPrefix);

   string sharedSecret = req.Headers.GetValues("X-LogicApp-Secret").FirstOrDefault();
   log.Info("Secret: " + sharedSecret);

   var payload = await req.Content.ReadAsStringAsync();
   log.Info("Payload: " + payload);

   bool isValid = IsGithubPushAllowed(payload, signatureWithPrefix, sharedSecret);

   return isValid == false
      ? req.CreateResponse(HttpStatusCode.BadRequest, "Missing or invalid Github security headers!")
      : req.CreateResponse(HttpStatusCode.OK);
}

public static bool IsGithubPushAllowed(string payload, string signatureWithPrefix, string sharedSecret)
{
   if (string.IsNullOrWhiteSpace(payload))
   {
      throw new ArgumentNullException(nameof(payload));
   }

   if (string.IsNullOrWhiteSpace(signatureWithPrefix))
   {
      throw new ArgumentNullException(nameof(signatureWithPrefix));
   }

   if (signatureWithPrefix.StartsWith(Sha1Prefix, StringComparison.OrdinalIgnoreCase))
   {
      var signature = signatureWithPrefix.Substring(Sha1Prefix.Length);
      var secret = Encoding.ASCII.GetBytes(sharedSecret);
      var payloadBytes = Encoding.ASCII.GetBytes(payload);
      using (var hmSha1 = new HMACSHA1(secret))
      {
         var hash = hmSha1.ComputeHash(payloadBytes);
         var hashString = ToHexString(hash);
         if (hashString.Equals(signature))
         {
            return true;
         }
      }
   }

   return false;
}

public static string ToHexString(byte[] bytes)
{
   var builder = new StringBuilder(bytes.Length * 2);
   foreach (byte b in bytes)
   {
      builder.AppendFormat("{0:x2}", b);
   }
   return builder.ToString();
}