K6 Subtle Webcrypto Jwt Signing, this simple example is for ES256 (ECDSA with the P-256 Elliptic Curve), but the same technique should be applicable with different keys

signing-utils.test.ts

import {
  describe,
  expect,
  // @ts-expect-error importing from url
} from 'https://jslib.k6.io/k6chaijs/4.3.4.3/index.js';
import { crypto } from 'k6/experimental/webcrypto';
import {
  encodeJwt,
  printArrayBuffer,
  string2ArrayBuffer,
} from './signing-utils.js';

export default async function () {
  describe('can sign a jwt', async () => {
    const keyPair = await crypto.subtle.generateKey(
      {
        name: 'ECDSA',
        namedCurve: 'P-256',
      },
      true,
      ['sign', 'verify']
    );
    const result = await encodeJwt(
      { header: 'this is a header' },
      { claim: 'admin' },
      keyPair.privateKey
    );
    // Should return a string with jwt structure
    expect(result.split('.').length).to.equal(3);
  });

  describe('decodes a payload into the correct raw array', async () => {
    // example payload encoding from https://www.ietf.org/archive/id/draft-jones-json-web-signature-03.html#rfc.appendix.Appendix%20A.3.1
    const payload =
      'eyJhbGciOiJFUzI1NiJ9.eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ';

    const expected = [
      101, 121, 74, 104, 98, 71, 99, 105, 79, 105, 74, 70, 85, 122, 73, 49, 78,
      105, 74, 57, 46, 101, 121, 74, 112, 99, 51, 77, 105, 79, 105, 74, 113, 98,
      50, 85, 105, 76, 65, 48, 75, 73, 67, 74, 108, 101, 72, 65, 105, 79, 106,
      69, 122, 77, 68, 65, 52, 77, 84, 107, 122, 79, 68, 65, 115, 68, 81, 111,
      103, 73, 109, 104, 48, 100, 72, 65, 54, 76, 121, 57, 108, 101, 71, 70,
      116, 99, 71, 120, 108, 76, 109, 78, 118, 98, 83, 57, 112, 99, 49, 57, 121,
      98, 50, 57, 48, 73, 106, 112, 48, 99, 110, 86, 108, 102, 81,
    ];
    const result = printArrayBuffer(string2ArrayBuffer(payload));
    expect(JSON.stringify(expected) == JSON.stringify(result)).to.equal(true);
  });
}

signing-utils.ts

import { CryptoKey, crypto } from 'k6/experimental/webcrypto';
import encoding from 'k6/encoding';

export const string2ArrayBuffer = (str: string) => {
  const buf = new ArrayBuffer(str.length);
  const bufView = new Uint8Array(buf);
  for (let i = 0, strLen = str.length; i < strLen; i++) {
    bufView[i] = str.charCodeAt(i);
  }
  return buf;
};

export const printArrayBuffer = (buffer) => {
  const view = new Uint8Array(buffer);
  return Array.from(view);
};

export const encodeJwt = async (
  header: Object,
  payload: Object,
  key: CryptoKey
) => {
  const headerString = encoding.b64encode(JSON.stringify(header), 'rawurl');
  const payloadString = encoding.b64encode(JSON.stringify(payload), 'rawurl');

  const signatureBuffer = await crypto.subtle.sign(
    {
      name: 'ECDSA',
      hash: 'SHA-256',
    },
    key,
    string2ArrayBuffer([headerString, payloadString].join('.'))
  );

  const signature = encoding.b64encode(signatureBuffer, 'rawurl');

  return [headerString, payloadString, signature].join('.');
};