Convert AWS KMS ECDSA_SHA_256 Signature from DER encoded ANS.1 format to JWT base64url encoded R || S format in NodeJS/Javascript

I am trying to create JWT Signature in NodeJS with ES256 algorithm using AWS KMS Customer Managed Keys.

The signature created using AWS KMS with cryptographic Signing Algorithms ECDSA_SHA_256 is not JWT accepted R || S format. As per AWS doc, Signature will be in DER encoded ANS X9.62–2005 format (https://docs.aws.amazon.com/kms/latest/APIReference/API_Sign.html#API_Sign_ResponseSyntax).

I tried to convert the AWS KMS Sign to JWT R||S format using below code in NodeJS with ans1js(https://www.npmjs.com/package/asn1js), But the R and S length is not consistent to be 32 + 32 rather it varies 33 most of the time.

function toArrayBuffer(buffer) {
    const ab = new ArrayBuffer(buffer.length);
    const view = new Uint8Array(ab);
    for (let i = 0; i < buffer.length; ++i) {
        view[i] = buffer[i];
    }
    return ab;
}

//call this with your signature buffer
function parseBERSignature(sig) {
    const { result } = asn1js.fromBER(toArrayBuffer(sig));

    const part1 = result.valueBlock.value[0];
    const part2 = result.valueBlock.value[1];

    let r = Buffer.from(part1.valueBlock.valueHex);
    let s = Buffer.from(part2.valueBlock.valueHex);

    console.log("R value", r);
    console.log("S value", s);

    console.log("R value", r.toString('base64'));
    console.log("S value", s.toString('base64'));

    console.log("R length", r.length);
    console.log("S length", s.length);

    return base64url.fromBase64(Buffer.concat([r, s]).toString('base64'));

} 

Complete Code for Signature creation:

const base64url = require('base64url')
const AWS = require('aws-sdk');
const kms = new AWS.KMS();
const asn1js = require('asn1js')

const keyid = "9001e08c-b7bc-4f53-9eca-ec034904cdd5";

const header = {
    "typ": "JWT",
    "alg": "ES256",
    "kid": keyid
}

const payload = {
    "sub": "name",
    "status": "valid",
    "aud": "name"
}

exports.handler = async function (event, context, callback) {
    console.log("Hello, new World");

    payload.iat = Math.floor(Date.now() / 1000);

    console.log("header", header);
    console.log("payload", payload);

    const jwtHeader = base64url(JSON.stringify(header));
    const jwtPayload = base64url(JSON.stringify(payload));

    console.log("jwtHeader", jwtHeader);
    console.log("jwtPayload", jwtPayload);

    const message = Buffer.from(jwtHeader + "." + jwtPayload);

    const messageDigest = createHash(message);

    let kmsResponse = await kms.sign({
        Message: message,
        KeyId: keyid,
        SigningAlgorithm: 'ECDSA_SHA_256',
        MessageType: 'RAW'
    }).promise();

    console.log("Signature RAW", kmsResponse.Signature);
    console.log("Signature String", kmsResponse.Signature.toString());
    console.log("Signature base64", kmsResponse.Signature.toString('base64'));

    let response = parseBERSignature(kmsResponse.Signature);
    console.log("response", response);
    return jwtHeader + "." + jwtPayload + "." + response;

}

Any NodeJs Javascript implementation to convert the DER encoded ANS format to R || S format base64url encoded?

Answer

Any NodeJs Javascript implementation to convert the DER encoded ANS format to R || S format base64url encoded?

Yes, ecdsa-sig-formatter will help you convert signatures back and forth between the formats.