CODEWITHSUNDEEP
c#signatureverifyecdsa
josagyemang
josagyemang

Reputation: 523

ECDSA Verify Signature in C# using public key and signature from Java

I have a public key and signature generated in Java which I would like to verify in C# using ECDsaCng. The public key is MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAExeg15CVOUcspdO0Pm27hPVx50thn0CGk3/3NLl08qcK+0U7cesOUUwxQetMgtUHrh0lNao5XRAAurhcBtZpo6w==

which I convert into a key that can be used by C# ECDsaCng by taking the last 64 bytes and prepending 0x45, 0x43, 0x53, 0x31,... to it.

The signature is generated in Java using SHA256. Funny thing is if I test the signature using the tool here https://kjur.github.io/jsrsasign/sample/sample-ecdsa.html, it says it is a valid signature.

I have been scouring through the net and still no joy.

Code is as below

static void VerifySignature()
{
  var publicKey = "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAExeg15CVOUcspdO0Pm27hPVx50thn0CGk3/3NLl08qcK+0U7cesOUUwxQetMgtUHrh0lNao5XRAAurhcBtZpo6w==";
  byte[] publicKeyBytes = Convert.FromBase64String(publicKey);

  var keyType = new byte[] { 0x45, 0x43, 0x53, 0x31 };
  var keyLength = new byte[] { 0x20, 0x00, 0x00, 0x00 };
  var key = keyType.Concat(keyLength).Concat(publicKeyBytes.TakeLast(64)).ToArray(); // 4543533120000000c5e835e4254e51cb2974ed0f9b6ee13d5c79d2d867d021a4dffdcd2e5d3ca9c2bed14edc7ac394530c507ad320b541eb87494d6a8e5744002eae1701b59a68eb

  // For testing in online tool
  Debug.WriteLine(ByteArrayToString(publicKeyBytes.TakeLast(65).ToArray())); //04c5e835e4254e51cb2974ed0f9b6ee13d5c79d2d867d021a4dffdcd2e5d3ca9c2bed14edc7ac394530c507ad320b541eb87494d6a8e5744002eae1701b59a68eb


  var signature = "MEQCIFNEZQRzIrvr6dtJ4j4HP8nXHSts3w3qsRt8cFXBaOGAAiAJO/EjzCZlNLQSvKBinVHfSvTEmor0dc3YX7FPMnnYCg==";
            var signatureBytes = Convert.FromBase64String(signature); // 30440220534465047322bbebe9db49e23e073fc9d71d2b6cdf0deab11b7c7055c168e1800220093bf123cc266534b412bca0629d51df4af4c49a8af475cdd85fb14f3279d80a

  var data = Encoding.UTF8.GetBytes("ABCDEFGH");

  CngKey cngKey = CngKey.Import(key, CngKeyBlobFormat.EccPublicBlob);
  ECDsaCng eCDsaCng = new ECDsaCng(cngKey);

  bool result = eCDsaCng.VerifyData(data, signatureBytes); // result is false

  string ByteArrayToString(byte[] ba)
  {
    StringBuilder hex = new StringBuilder(ba.Length * 2);
    foreach (byte b in ba)
      hex.AppendFormat("{0:x2}", b);

    return hex.ToString();
  }
}

Upvotes: 5

Views: 4474

Answers (1)

Topaco
Topaco

Reputation: 49460

The cause is a different format. Microsoft expects the format r|s while your signature is specified in the ASN.1-format (which is explained in the context of ECDSA here):

0x30|b1|0x02|b2|r|0x02|b3|s
b1 = Length of remaining data
b2 = Length of r
b3 = Length of s

Your signature 

30440220534465047322bbebe9db49e23e073fc9d71d2b6cdf0deab11b7c7055c168e1800220093bf123cc266534b412bca0629d51df4af4c49a8af475cdd85fb14f3279d80a

can be separated in the following portions

30 44 02 20 534465047322bbebe9db49e23e073fc9d71d2b6cdf0deab11b7c7055c168e180 02 20 093bf123cc266534b412bca0629d51df4af4c49a8af475cdd85fb14f3279d80a

so that the individual portions can be easily identified:

b1 = 0x44
b2 = 0x20
r  = 0x534465047322bbebe9db49e23e073fc9d71d2b6cdf0deab11b7c7055c168e180 
b3 = 0x20
s  = 0x093bf123cc266534b412bca0629d51df4af4c49a8af475cdd85fb14f3279d80a

Thus

r|s = 534465047322bbebe9db49e23e073fc9d71d2b6cdf0deab11b7c7055c168e180093bf123cc266534b412bca0629d51df4af4c49a8af475cdd85fb14f3279d80a

If you run your code using this format, verification is successful.

Upvotes: 11

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