C# HMACSHA256 Problem - Matching Facebook Signed Request Implementation

Question

Hi Everyone,

I thought I would try and get the new Signed Request logic added to my facebook canvas application, to make this "easy" on myself I went to the facebook PHP sdk over at GitHub and took a look at the unit tests.

http://github.com/facebook/php-sdk/blob/master/tests/tests.php

My actual problem is that I cannot get the hash included in the request to match the hash I calculate using the application secret, and the data sent within the request.

How this is meant to work is described here:
developers.facebook.com/docs/authentication/canvas

Thanks
James

private string VALID_SIGNED_REQUEST = "ZcZocIFknCpcTLhwsRwwH5nL6oq7OmKWJx41xRTi59E.eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiIsImV4cGlyZXMiOiIxMjczMzU5NjAwIiwib2F1dGhfdG9rZW4iOiIyNTQ3NTIwNzMxNTJ8Mi5JX2VURmtjVEtTelg1bm8zakk0cjFRX18uMzYwMC4xMjczMzU5NjAwLTE2Nzc4NDYzODV8dUk3R3dybUJVZWQ4c2VaWjA1SmJkekdGVXBrLiIsInNlc3Npb25fa2V5IjoiMi5JX2VURmtjVEtTelg1bm8zakk0cjFRX18uMzYwMC4xMjczMzU5NjAwLTE2Nzc4NDYzODUiLCJ1c2VyX2lkIjoiMTY3Nzg0NjM4NSJ9";

private string NON_TOSSED_SIGNED_REQUEST = "laEjO-az9kzgFOUldy1G7EyaP6tMQEsbFIDrB1RUamE.eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiJ9";

public void SignedRequestExample()
{
 var Encoding = new UTF8Encoding();

 string ApplicationSecret = "904270b68a2cc3d54485323652da4d14"; 

 string SignedRequest = VALID_SIGNED_REQUEST;
 string ExpectedSignature = SignedRequest.Substring(0, SignedRequest.IndexOf('.'));
 string Payload = SignedRequest.Substring(SignedRequest.IndexOf('.') + 1);

 // Back & Forth with Signature
 byte[] ActualSignature = FromUrlBase64String(ExpectedSignature);
 string TestSignature = ToUrlBase64String(ActualSignature);

 // Back & Forth With Data
 byte[] ActualPayload = FromUrlBase64String(Payload);
 string Json = Encoding.GetString(ActualPayload);
 string TestPayload = ToUrlBase64String(ActualPayload);

 // Attempt to get same hash
 var Hmac = SignWithHMAC(ActualPayload, Encoding.GetBytes(ApplicationSecret));
 var HmacBase64 = ToUrlBase64String(Hmac);            
 var HmacHex = BytesToHex(Hmac);

 if (HmacBase64 != ExpectedSignature)
 {
  // YAY
 }
 else
 {
  // BOO
 }
}

private static string BytesToHex(byte[] input)
{
 StringBuilder sb = new StringBuilder();

 foreach (byte b in input)
 {
  sb.Append(string.Format("{0:x2}", b));
 }
 return sb.ToString();
}
private string ToUrlBase64String(byte[] Input)
{
 return Convert.ToBase64String(Input).Replace("=", String.Empty).Replace('+', '-').Replace('/', '_');
}

// http://tools.ietf.org/html/rfc4648#section-5            
private byte[] FromUrlBase64String(string Base64UrlSafe)
{
 Base64UrlSafe = Base64UrlSafe.PadRight(Base64UrlSafe.Length + (4 - Base64UrlSafe.Length % 4) % 4, '=');
 Base64UrlSafe = Base64UrlSafe.Replace('-', '+').Replace('_', '/');
 return Convert.FromBase64String(Base64UrlSafe);
}

private byte[] SignWithHMAC(byte[] dataToSign, byte[] keyBody)
{
 using (var hmac = new HMACSHA256(keyBody))
 {
  hmac.ComputeHash(dataToSign);
  /*
  CryptoStream cs = new CryptoStream(System.IO.Stream.Null, hmac, CryptoStreamMode.Write);
  cs.Write(dataToSign, 0, dataToSign.Length);
  cs.Flush();
  cs.Close();
  byte[] hashResult = hmac.Hash;
  */
  return hmac.Hash;
 }
}

public string Base64ToHex(string input)
{
 StringBuilder sb = new StringBuilder();
 byte[] inputBytes = Convert.FromBase64String(input);
 foreach (byte b in inputBytes)
 {
  sb.Append(string.Format("{0:x2}", b));
 }
 return sb.ToString();
}

Answer thanks to Rasmus below, to assist anyone else here is the updated (cleaned up code):

/// Example signed_request variable from PHPSDK Unit Testing
private string VALID_SIGNED_REQUEST = "ZcZocIFknCpcTLhwsRwwH5nL6oq7OmKWJx41xRTi59E.eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiIsImV4cGlyZXMiOiIxMjczMzU5NjAwIiwib2F1dGhfdG9rZW4iOiIyNTQ3NTIwNzMxNTJ8Mi5JX2VURmtjVEtTelg1bm8zakk0cjFRX18uMzYwMC4xMjczMzU5NjAwLTE2Nzc4NDYzODV8dUk3R3dybUJVZWQ4c2VaWjA1SmJkekdGVXBrLiIsInNlc3Npb25fa2V5IjoiMi5JX2VURmtjVEtTelg1bm8zakk0cjFRX18uMzYwMC4xMjczMzU5NjAwLTE2Nzc4NDYzODUiLCJ1c2VyX2lkIjoiMTY3Nzg0NjM4NSJ9";

public bool ValidateSignedRequest()
{            
    string applicationSecret = "904270b68a2cc3d54485323652da4d14";
    string[] signedRequest = VALID_SIGNED_REQUEST.Split('.');            
    string expectedSignature = signedRequest[0];
    string payload = signedRequest[1];

    // Attempt to get same hash
    var Hmac = SignWithHmac(UTF8Encoding.UTF8.GetBytes(payload), UTF8Encoding.UTF8.GetBytes(applicationSecret));
    var HmacBase64 = ToUrlBase64String(Hmac);

    return (HmacBase64 == expectedSignature);           
}


private string ToUrlBase64String(byte[] Input)
{
    return Convert.ToBase64String(Input).Replace("=", String.Empty)
                                        .Replace('+', '-')
                                        .Replace('/', '_');
}

private byte[] SignWithHmac(byte[] dataToSign, byte[] keyBody)
{
    using (var hmacAlgorithm = new HMACSHA256(keyBody))
    {
        hmacAlgorithm.ComputeHash(dataToSign);
        return hmacAlgorithm.Hash;
    }
}

Answer 1

You are not supposed to base64-decode the payload before calculating the HMAC.

Use this line:

var Hmac = SignWithHMAC(Encoding.GetBytes(Payload), Encoding.GetBytes(ApplicationSecret));

and it should work.

A few more pointers:

• Instead of fiddling with Substring() and IndexOf() try using String.Split()
• You have switched the YAY and BOO comments around
• C# code is more readable if you follow the common rule of starting the names of local variables with lowercase (like this: var applicationSecret = "...";)

Answer 2

Hi!

I would like to know how to get a valid signed_request???

I only can get the non_tossen_signed_request form...

Thanks!