static S getAuthorOfSignedText(S signed) {
  try {
    L<S> lines = toLines(signed);
    S authorLine = nextToLast(lines);
    S sigLine = last(lines);
    S text = fromLines(dropLast(lines));
    //print("Verifying text: " + l(text) + " - " + quote(text));
    if (!startsWith(authorLine, "-")) null;
    S author = dropPrefix("-", authorLine);
    author = beforeComma(author); // drop date etc.
    if (!isComputerID(author)) fail("Not a computer ID: " + author);
    //print("Possible author: " + author);
    S publicKey = getPublicKeyOfComputer(author);
    if (publicKey == null)
      fail("Computer " + author + " has to publish a public key.");
    if (!startsWith(sigLine, "sig: ")) null;
    S sig = dropPrefix("sig: ", sigLine);
    if (!PKI.verifySignature(PKI.publicKeyFromString(publicKey), PKI.signatureFromString(sig), toUtf8(text))) {
      print("Signature doesn't verify");
      null;
    }
    ret author;
  } catch (RuntimeException e) {
    ret null;
  }
}