importing keys via jwk format

[msabbouh]

My AI assistant is telling me:
"StarlingMonkey's crypto.subtle silently produces wrong, non-deterministic HMAC signatures when importing keys via jwk format — caught only by
  cross-checking against Node's crypto, since sign+verify self-roundtrips falsely passed. Fixed by always converting to raw import internally.

The ask is to fix this

[andreiltd]

The ask is to fix this

Fix what exactly?

[msabbouh]

The bug: in this project's StarlingMonkey/Spin build, native crypto.subtle.importKey('jwk', ...) for HMAC produces a different, wrong signature every call
— and verify() shares the same bug, so a sign→verify roundtrip falsely reports success (it looked "correct" until cross-checked against Node
byte-for-byte). importKey('raw', ...) with the identical key material is byte-exact correct.

The workaround today— decode the JWK to raw bytes yourself, never hand 'jwk' to native HMAC:

function b64urlToUint8Array(b64url) {
const b64 = b64url.replace(/-/g, '+').replace(/_/g, '/').padEnd(
b64url.length + (4 - (b64url.length % 4)) % 4, '=');
const bin = atob(b64);
const bytes = new Uint8Array(bin.length);
for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i);
return bytes;
}

async function importKey(format, keyData, algorithm, extractable, usages) {
const name = algName(algorithm); // e.g. 'HMAC'

if (name === 'HMAC') {
  // Native jwk import is broken -- always go through raw.
  const rawBytes = format === 'jwk' ? b64urlToUint8Array(keyData.k) : keyData;
  return nativeSubtle.importKey('raw', rawBytes, algorithm, false, usages);
}

// ...other algorithms pass straight through to native, since
// RSASSA-PKCS1-v1_5 and ECDSA importKey('jwk', ...) are confirmed correct.
return nativeSubtle.importKey(format, keyData, algorithm, false, usages);

}

So a caller can still pass a JWK-format HMAC key ({ kty: 'oct', k: '', ... }) and get correct sign/verify behavior — the shim just intercepts
it, pulls keyData.k, base64url-decodes it to raw bytes, and calls native importKey('raw', ...) under the hood instead of trusting native's 'jwk' path.