RSA SHA256 signature generation and validation

I am generating a RSA signature in C# by using BouncyCastle and validating the signature in Go.

But for some reason the signature from the C# program will not validate in Go.

The details:

The public/private keypair is an example generated from Online RSA Key Generator

The hash byte array is one that is generated by using SHA265 and hardcoded for simplicity.

I've tried different signature algorithms for BouncyCastle, played around with key pairs but nothing seems to work.

I've also validated that the byte array of the signature matches between the two languages...

Can anyone tell me what I am missing here? Or at least how to investigate further?

My C# code to generate the signature

    static void Main(string[] args)
    {
        byte[] hashBytes = new byte[32] { 152, 154, 255, 19, 168, 20, 167, 43, 232, 133, 146, 13, 183, 80, 186, 85, 180, 249, 95, 142, 234, 71, 93, 188, 29, 147, 220, 164, 248, 83, 196, 80 };
        String privateKeyPemStr = @"-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----";

        StringReader strReader = new StringReader(privateKeyPemStr);
        PemReader pemReader = new PemReader(strReader);
        AsymmetricCipherKeyPair keyPair = (AsymmetricCipherKeyPair)pemReader.ReadObject();
        RsaKeyParameters privateKey = (RsaKeyParameters)keyPair.Private;

        
        ISigner sig = SignerUtilities.GetSigner("SHA256withRSA");
        sig.Init(true, privateKey);
        sig.BlockUpdate(hashBytes, 0, hashBytes.Length);
        byte[] signedBytes = sig.GenerateSignature();

        var signedStr = Convert.ToBase64String(signedBytes);
        
        Console.WriteLine(signedStr);
        Console.ReadLine();            
    }

This is my Go code to validate the signature. I copy the result of the C# code into the signature variable

func main() {
pubKeyStr := `-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCIw68jARnmvTh+xvMcv5iugtoH
Xt60NWEebfbghLTFuTlQvK0exY5hxnN/uD2UVc/S3QGnBQn0AfynhxlEZkedpYBb
5RWoVChGZMHu7hbZukMjByjxec0LjtuEQhY4m18XaVNmAQWD/EiROMGTghMwykkQ
+SBtx4Gl/O/BB6F4SQIDAQAB
-----END PUBLIC KEY-----`
pemBlockPub, _ := pem.Decode([]byte(pubKeyStr))
pub, _ := x509.ParsePKIXPublicKey(pemBlockPub.Bytes)
publicKey, _ := pub.(*rsa.PublicKey)

signatureStr := "YJxDTSMnFb4uh/orsUjHTHEsW1dkxuStsGP0PmjmObJhog/7OQfWgBcBZ58w0qWoknLGMVBBgZTgJtKq1ZSSTsx9uXhNKEhNEI3a+7ZhmPiHp6JRLbftsEoGKe7FKU8vXkp6Bo90qMOoJz54YI2xue8EA9b5PTgjkGbDbKdimF8="
signatureBytes, err := base64.StdEncoding.DecodeString(signatureStr)
hashBytes := [32]byte{152, 154, 255, 19, 168, 20, 167, 43, 232, 133, 146, 13, 183, 80, 186, 85, 180, 249, 95, 142, 234, 71, 93, 188, 29, 147, 220, 164, 248, 83, 196, 80}

err := rsa.VerifyPKCS1v15(publicKey, crypto.SHA256, hashBytes[:], signatureBytes)

if err != nil {
	fmt.Printf("err: %v\n", err)
} else {
	fmt.Printf("ok")
}

}

If I use the following code in C# to validate the signature, it is fine.

        String publicKeyPemStr = @"-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCIw68jARnmvTh+xvMcv5iugtoH
Xt60NWEebfbghLTFuTlQvK0exY5hxnN/uD2UVc/S3QGnBQn0AfynhxlEZkedpYBb
5RWoVChGZMHu7hbZukMjByjxec0LjtuEQhY4m18XaVNmAQWD/EiROMGTghMwykkQ
+SBtx4Gl/O/BB6F4SQIDAQAB
-----END PUBLIC KEY-----";
        
        strReader = new StringReader(publicKeyPemStr);
        pemReader = new PemReader(strReader);
        RsaKeyParameters publicKey = (RsaKeyParameters)((AsymmetricKeyParameter)pemReader.ReadObject());

        sig = SignerUtilities.GetSigner("SHA256withRSA");
        sig.Init(false, publicKey);
        sig.BlockUpdate(hashBytes, 0, hashBytes.Length);

        if (sig.VerifySignature(signedBytes))
        {
            Console.WriteLine("Ok");
        }
        else
        {
            Console.WriteLine("NOK");
        }

        Console.ReadLine();

Did some additional tests, the following Go code produces a totally different signature string from the C# code although the private key is the same. Where is the difference??

func main() {
privKeyStr := `-----BEGIN RSA PRIVATE KEY-----
MIICXAIBAAKBgQCIw68jARnmvTh+xvMcv5iugtoHXt60NWEebfbghLTFuTlQvK0e
xY5hxnN/uD2UVc/S3QGnBQn0AfynhxlEZkedpYBb5RWoVChGZMHu7hbZukMjByjx
ec0LjtuEQhY4m18XaVNmAQWD/EiROMGTghMwykkQ+SBtx4Gl/O/BB6F4SQIDAQAB
AoGAJLarODFee6OGG/paXvhMC2TTFLFyBVxjAuEwKdtWD9IGQdc0fhM4gqTccofJ
+B0FGiz7+ZMPtfImme5ZaRQv2wx7KOPbOdAyYxC7nLFCHYqDWZJ8/cCoS+hPJFd5
9OeGLGz3QKfEEPtYEAw4+E/UjilYAtRNREkISkYoB9Va8PUCQQDDHMPRSCfXbfZV
ufmlRZj2bH8sjVaSBbJIw+y9HKJ3ORRnKGjtIZ/+z70EwMwtbbQKnl71SruO5HB9
AUTtRka/AkEAs3GWQhplPbuH/fAlaEPy5GQilUNRt76NMsgtIFWPMXnt82cxTUUR
RIKwX7M96WBppPZ2Dy7uLrX8O+3fr6BK9wJATZ0lsBy57JKLiTJ/wmTbIjuqozhe
FZw6fYOiqt+3KSIFobuLcbkMgjp1AG0JS5D2K7swHvdpgMASl0dn+dMY1QJBAIPw
9QbN2bs2dJvnQ9oSfDoq1rLhuOheF/xK68Nmpc8/VBMwwTOLoVK6tWzoopFC7ur4
vX4Uh9WYwkpecab1OakCQD0SCs4zIggA/2NkUx8J5H07/drbm9uH+98EQUvzjTdz
qSW2jJPCA4GYYmtnnDRduukjmkJlbaaTdEH5YLCilF0=
-----END RSA PRIVATE KEY-----`

pemBlockPriv, _ := pem.Decode([]byte(privKeyStr))
privateKey, _ := x509.ParsePKCS1PrivateKey(pemBlockPriv.Bytes)

hashBytes := [32]byte{152, 154, 255, 19, 168, 20, 167, 43, 232, 133, 146, 13, 183, 80, 186, 85, 180, 249, 95, 142, 234, 71, 93, 188, 29, 147, 220, 164, 248, 83, 196, 80}

signatureByte, _ := rsa.SignPKCS1v15(rand.Reader, privateKey, crypto.SHA256, hashBytes[:])
signatureStr := base64.StdEncoding.EncodeToString(signatureByte)

fmt.Printf("%v\n", signatureStr)
}

BouncyCastle will automatically calculate the sha256 of your message when you calculate the signature with sig.GenerateSignature(). What you should pass into sig.BlockUpdate() is the message that you want sha256 and rsa signed. So essentially you've used sha256 twice on your message.

You can see this here: https://play.golang.org/p/mplEnmNbs9. On line 27 I added another call to sha256 on your hashBytes and then passed that to rsa.VerifyPKCS1v15() and works fine.