static bool isSignedWithKey_verbose;

static boolean isSignedWithKey(S signed, S key) {
  try {
    L<S> lines = toLines(signed);
    if (swic(last(lines), "signer:")) popLast(lines);
    S sigLine = last(lines);
    S text = fromLines(dropLast(lines));
    S sig = dropPrefixMandatory("sig: ", sigLine);
    ret PKI.verifySignature(PKI.publicKeyFromString(key), PKI.signatureFromString(sig), toUtf8(text));
  } catch (RuntimeException e) {
    if (isSignedWithKey_verbose)
      printStackTrace(e);
    ret false;
  }
}

Began life as a copy of #1001567