You can implement a protocol to know whether it was alice who gave this code to bob. Using public key encryption, alice can have a private key, which she uses

You can implement a protocol to know whether it was alice who gave this code to bob. Using public key encryption, alice can have a private key, which she uses to sign the code she gives to bob. Then anybody can use the public key to check the signature on that code, to very that the owner of the private key (ie. alice) did sign it. As for "too long", it's relative. For example: $ echo 'code given to bob by Pascal Bourguignon: 42'|gpg --armor --clearsign - -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 code given to bob by Pascal Bourguignon: 42 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJU6JnbAAoJEMqG5FsaozHfUMAIAJh+6MLorX8vOCzKve/ia4nv cloO1Ctp1xEF19Pou72HuMyinYl5S0Xh94JGclquUMxzA1RwlXY3kH65tZ/WgmpY xEK+f5VYNeX3uoPQ/3TnV1Tp1vP7IxAmtctqYImGprvdwmCQRkBCDXIX2q/A2WVV ZXwKHqdgSU5TVtLblAbLEX3ZdWsQbV8uekMU9bJy87lWMOFM96sD/TGpmnGxo6a4 S1C9uEtYtvGTLjQCWhSn+TelhNluDyphwv+eDSmLNYAeelgYeUOKKgqpBFglVgSj HZQrjg/C7sPFJt77KpImhJaHzcLf8azX7IcSv74Uawd2EYhL97imQlNwEoL+v7Q= =m2xA -----END PGP SIGNATURE----- $ echo 'code given to bob by Pascal Bourguignon: 42'|gpg --armor --clearsign -|gpg --verify gpg: Signature made Sat Feb 21 15:45:35 2015 CET using RSA key ID 1AA331DF gpg: Good signature from "Pascal J. Bourguignon (Maven) <pjb@informatimago.com>"