Generating ECDSA signature with Node.js/crypto - node.js

Question

I have code that generates a concatenated (r-s) signature for the ECDSA signature using jsrsasign and a key in JWK format:
const sig = new Signature({ alg: 'SHA256withECDSA' });
sig.init(KEYUTIL.getKey(key));
sig.updateHex(dataBuffer.toString('hex'));
const asn1hexSig = sig.sign();
const concatSig = ECDSA.asn1SigToConcatSig(asn1hexSig);
return new Buffer(concatSig, 'hex');
Seems to work. I also have code that uses SubtleCrypto to achieve the same thing:
importEcdsaKey(key, 'sign') // importKey JWK -> raw
.then((privateKey) => subtle.sign(
{ name: 'ECDSA', hash: {name: 'SHA-256'} },
privateKey,
dataBuffer
))
These both return 128-byte buffers; and they cross-verify (i.e. I can verify jsrsasign signatures with SubtleCrypto and vice versa). However, when I use the Sign class in the Node.js crypto module, I seem to get something quite different.
key = require('jwk-to-pem')(key, {'private': true});
const sign = require('crypto').createSign('sha256');
sign.update(dataBuffer);
return sign.sign(key);
Here I get a buffer of variable length, roughly 70 bytes; it does not cross-verify with jsrsa (which bails complaining about an invalid length for an r-s signature).
How can I get an r-s signature, as generated by jsrsasign and SubtleCrypto, using Node crypto?

Answer 1

The answer turns out to be that the Node crypto module generates ASN.1/DER signatures, while other APIs like jsrsasign and SubtleCrypto produce a “concatenated” signature. In both cases, the signature is a concatenation of (r, s). The difference is that ASN.1 does so with the minimum number of bytes, plus some payload length data; while the P1363 format uses two 32-bit hex encoded integers, zero-padding them if necessary.
The below solution assumes that the “canonical” format is the concatenated style used by SubtleCrypto.
const asn1 = require('asn1.js');
const BN = require('bn.js');
const crypto = require('crypto');
const EcdsaDerSig = asn1.define('ECPrivateKey', function() {
return this.seq().obj(
this.key('r').int(),
this.key('s').int()
);
});
function asn1SigSigToConcatSig(asn1SigBuffer) {
const rsSig = EcdsaDerSig.decode(asn1SigBuffer, 'der');
return Buffer.concat([
rsSig.r.toArrayLike(Buffer, 'be', 32),
rsSig.s.toArrayLike(Buffer, 'be', 32)
]);
}
function concatSigToAsn1SigSig(concatSigBuffer) {
const r = new BN(concatSigBuffer.slice(0, 32).toString('hex'), 16, 'be');
const s = new BN(concatSigBuffer.slice(32).toString('hex'), 16, 'be');
return EcdsaDerSig.encode({r, s}, 'der');
}
function ecdsaSign(hashBuffer, key) {
const sign = crypto.createSign('sha256');
sign.update(asBuffer(hashBuffer));
const asn1SigBuffer = sign.sign(key, 'buffer');
return asn1SigSigToConcatSig(asn1SigBuffer);
}
function ecdsaVerify(data, signature, key) {
const verify = crypto.createVerify('SHA256');
verify.update(data);
const asn1sig = concatSigToAsn1Sig(signature);
return verify.verify(key, new Buffer(asn1sig, 'hex'));
}
Figured it out thanks to
https://crypto.stackexchange.com/questions/1795/how-can-i-convert-a-der-ecdsa-signature-to-asn-1
ECDSA signatures between Node.js and WebCrypto appear to be incompatible?