Webhooks

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.HexFormat;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

public class Verify {
    /**
     * True when one of the header's v1 values matches the raw body and the header is at most five minutes old.
     * For 24 hours after a secret rotation the header carries two v1 values, so check them all.
     * Pass the body exactly as it arrived.
     */
    static boolean verify(String secret, String header, byte[] rawBody, long nowSeconds) throws Exception {
        String t = null;
        java.util.List<String> signatures = new java.util.ArrayList<>();
        for (String part : header == null ? new String[0] : header.split(",")) {
            String[] pair = part.trim().split("=", 2);
            if (pair.length == 2 && pair[0].equals("t")) t = pair[1];
            if (pair.length == 2 && pair[0].equals("v1")) signatures.add(pair[1]);
        }
        if (t == null) return false;
        long seconds;
        try {
            seconds = Long.parseLong(t);
        } catch (NumberFormatException e) {
            return false;
        }
        if (Math.abs(nowSeconds - seconds) > 300) return false;
        Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(new SecretKeySpec(secret.getBytes(StandardCharsets.UTF_8), "HmacSHA256"));
        mac.update((seconds + ".").getBytes(StandardCharsets.UTF_8));
        byte[] expected = mac.doFinal(rawBody);
        boolean ok = false;
        for (String signature : signatures) {
            if (signature.matches("[0-9a-f]{64}") && MessageDigest.isEqual(expected, HexFormat.of().parseHex(signature))) ok = true;
        }
        return ok;
    }

    // Check it against the sample on this page: java Verify.java
    public static void main(String[] args) throws Exception {
        String secret = "sample-secret";
        byte[] body = "{\"event_id\":\"evt_12345678\",\"sequence\":1}".getBytes(StandardCharsets.UTF_8);
        String header = "t=1790000000,v1=bd9a6a2d85d7e3706ff8209156fdc43b8291094a4c7af458ed31e7ae9174449e";
        System.out.println(verify(secret, header, body, 1790000100L)); // true
        System.out.println(verify(secret, header, body, 1790000400L)); // false: older than five minutes
        byte[] changed = "{\"event_id\":\"evt_12345678\",\"sequence\":2}".getBytes(StandardCharsets.UTF_8);
        System.out.println(verify(secret, header, changed, 1790000100L)); // false: body changed
    }
}

import { createHmac, timingSafeEqual } from 'node:crypto';
import { fileURLToPath } from 'node:url';

// True when one of the header's v1 values matches the raw body and the header is at most five minutes old.
// For 24 hours after a secret rotation the header carries two v1 values, so check them all.
// Pass the body exactly as it arrived, as bytes. A parsed and re-serialised copy will not match.
export function verify(secret, header, rawBody, nowSeconds = Math.floor(Date.now() / 1000)) {
  const parts = String(header ?? '').split(',').map((part) => part.trim().split('='));
  const seconds = Number(parts.find(([name]) => name === 't')?.[1]);
  const signatures = parts.filter(([name]) => name === 'v1').map(([, value]) => value);
  if (!Number.isInteger(seconds) || Math.abs(nowSeconds - seconds) > 300) return false;
  const expected = createHmac('sha256', secret).update(`${seconds}.`).update(rawBody).digest();
  let ok = false;
  for (const signature of signatures) {
    if (/^[0-9a-f]{64}$/.test(signature ?? '') && timingSafeEqual(Buffer.from(signature, 'hex'), expected)) ok = true;
  }
  return ok;
}

// Check it against the sample on this page: node verify.mjs
if (process.argv[1] === fileURLToPath(import.meta.url)) {
  const secret = 'sample-secret';
  const body = Buffer.from('{"event_id":"evt_12345678","sequence":1}');
  const header = 't=1790000000,v1=bd9a6a2d85d7e3706ff8209156fdc43b8291094a4c7af458ed31e7ae9174449e';
  console.log(verify(secret, header, body, 1790000100)); // true
  console.log(verify(secret, header, body, 1790000400)); // false: older than five minutes
  console.log(verify(secret, header, Buffer.from('{"event_id":"evt_12345678","sequence":2}'), 1790000100)); // false: body changed
}