Golang's RSA signature doesn't match Java's

Question

I am trying to create a RSA signature using the PKCS #1v15 standard. The data is first hashed using MD5, and the signature' algorithm is SHA256. But for some reason, both outputs are different and the server is throwing an error for the Golang code. Any advice would be appreciated.

Java code:

For context:

RSA_ALGORITHM_SIGN = "SHA256WithRSA";

CHARSET = "UTF-8";

private static String getSign(String data, RSAPrivateKey privateKey) {
    try {
        String encodeStr = DigestUtils.md5Hex(data);
        System.out.println(encodeStr);
        Signature signature = Signature.getInstance(RSA_ALGORITHM_SIGN);
        signature.initSign(privateKey);
        signature.update(encodeStr.getBytes(CHARSET));
        byte[] sign = signature.sign();
        System.out.println(sign.length);
        return Base64.encodeBase64URLSafeString(sign);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

Golang code:

func GetSignature(data []byte, pk *rsa.PrivateKey) (string, error) {
    m := md5.New()
    _, err := m.Write(data)
    if err != nil {
        return "", err
    }
    encoded := hex.EncodeToString(m.Sum(nil))
    fmt.Println(encoded)
    signature, err := rsa.SignPKCS1v15(rand.Reader, pk, crypto.SHA256, []byte(encoded))
    if err != nil {
        return "", err
    }
    return base64.URLEncoding.EncodeToString(signature), nil
}

I was hoping for the server I am using to not throw an error. However, no matter how I change the Golang code, I am getting signature errors.

Answer 1

So, I missed something in the Java method.

In the Java code, the work process is MD5 -> HEX -> SHA256 -> Signature. In my earlier Golang code, I implemented MD5 -> HEX -> Signature.

Finally managed to fix it. Thank you to all those, who shared their collective input. Really helped me steer in the right direction.