sbool getAuthorOfSignedText_debug;

sS getAuthorOfSignedText(S signed) {

  ret pairB(getAuthorOfSignedText2(signed));

}

// returns pair(purported author, verified author)

static PairS getAuthorOfSignedText2(S signed) {

  try {

    L<S> lines = toLines(signed);

    S authorLine = nextToLast(lines);

    S sigLine = last(lines);

    if (getAuthorOfSignedText_debug) print("authorLine: " + authorLine);

    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 (key: ..." + publicKey + ")");

      print("Sig: " + sig);

      print("Text:");

      printIndent(">> ", text);

      ret pair(author, null);

    }

    ret pair(author, author);

  } catch (RuntimeException e) {

    printShortException(e);

    null;

  }

}