Cloudflare Worker ECDSA ES256 Signing

cf-es256.js

import { Buffer } from "buffer";
import * as rs from 'jsrsasign';

addEventListener('fetch', event => {
    try {
        event.respondWith(generateSignature(event.request));
    } catch (e) {
        return event.respondWith(new Response('Error thrown ' + e.message));
    }
})

async function generateSignature(req) {
    const url = new URL(req.url);
    const privateKey = `-----BEGIN PRIVATE KEY----- 
    -----END PRIVATE KEY-----`;

    const key = rs.KEYUTIL.getKey(privateKey);
    const sig = new rs.KJUR.crypto.Signature({ "alg": "SHA256withECDSA" });
    sig.init(key);
    const signatureASN1Hex = sig.signString(url);
    const signatureBase64 = Buffer.from(rs.KJUR.crypto.ECDSA.asn1SigToConcatSig(signatureASN1Hex), 'hex').toString('base64');

    const signatureHeader = {
        headers: {
            "ES256": signatureBase64
        },
    };
    return await fetch(url, signatureHeader);
}