Extracting (r,s) and Verifying ECDSA signature remotely

Question

I am trying to sign content using a java client and then verifying the same on a server (nodejs). My client signature function uses ECDSA and returns a byte[]. I have access to the x and y coordinate values comprising the publicKey on the server.

public static byte[] sign(String plainText, PrivateKey privateKey) throws Exception {
    java.security.Signature dsa = java.security.Signature.getInstance("SHA1withECDSA");
    dsa.initSign(privateKey);
    dsa.update(plainText.getBytes(UTF_8));
    return dsa.sign();
}

Is it possible to find the r and s values that comprise the signature? How do I convert the byte[] obtained from above into a (r,s) pair or a DER-encoded signature as hex? On the node server side I am using elliptic for the signature verification.

Edit:

Thank you Dave for the comments, I am using the methods indicated in this SO answer:

public static BigInteger extractR(byte[] signature) throws Exception {
    int startR = (signature[1] & 0x80) != 0 ? 3 : 2;
    int lengthR = signature[startR + 1];
    return new BigInteger(Arrays.copyOfRange(signature, startR + 2, startR + 2 + lengthR));
}

public static BigInteger extractS(byte[] signature) throws Exception {
    int startR = (signature[1] & 0x80) != 0 ? 3 : 2;
    int lengthR = signature[startR + 1];
    int startS = startR + 2 + lengthR;
    int lengthS = signature[startS + 1];
    return new BigInteger(Arrays.copyOfRange(signature, startS + 2, startS + 2 + lengthS));
}

Knowing the x,y, r and s values, I am trying to verify the message this is a test string on the node server.

Message : this is a test string
Curve Parameters: secp256k1
Public Key:
   X : 52552626316292256179275635993655485173638967401615704770864787021340356427096
   Y : 115577290317206876914379725139810202736866562857077399175416156471449711434272
Signature details:
   R : [0, -63, -80, -50, -87, -56, 93, 19, 82, 46, 51, 14, -75, 103, 115, 126, 21, 94, 43, 102, -21, -86, -29, -5, 25, 14, -6, -116, 120, -54, -66, 2, -78]
   S : [0, -40, -119, 77, -14, 113, -105, -117, 93, 70, -107, -3, 63, 12, 77, -48, 59, -47, -7, -126, -60, -109, 95, -6, -66, -120, -8, -103, 122, 40, 24, -31, 89]

For verification using the elliptic module I have the following

var EC_Instance = new EC();
var signature = {
    r : new Buffer([0, -63, ..., 2, -78]),
    s : new Buffer([0, -40, ..., -31, 89])
};
var x = "52552626316292256179275635993655485173638967401615704770864787021340356427096";
var y = "115577290317206876914379725139810202736866562857077399175416156471449711434272";
EC_Instance.importPublicKey(x, y); // calls ec.keyFromPublic(pub, 'hex')
var verification_true = EC_Instance.verify("this is a test string", signature);

And EC_Instance is an object of the class containing the following:

constructor() {
    // Require the elliptic library for curve cryptography
    var EC = require('elliptic').ec;
    var ec = new EC('secp256k1');
    this.ec = ec;
}

importPublicKey(x, y) {
    var pub = { x: x.toString('hex'), y: y.toString('hex') };
    var key = this.ec.keyFromPublic(pub, 'hex');
    this.key = key;
    return key;
}

verify(message, signature) {
    return this.key.verify(message, signature);
}

Answer 1

It's probably the hash function. SHA-1 should not be used anymore for signature operations.

I presume that for the node.js code that the SHA-256 hash method is used, although it is next to impossible to verify this with the current documentation (hashing is hardly even mentioned).

Note that - with any signatures - the hash is a configuration parameter; it should (and for EC it cannot, at least without actual verification) be determined from the signature itself.