Keys must be changed frequently. 20.4 Digital Signatures Messages are decrypted, but message is also retained in the encrypted version. Used to prove that a message was sent and by whom. Enables the creator of a message to attach a code that acts as a signture. The signature guarantees the source and integrity of the message.
Digital signature is one of the great innovations in the history of cryptography.
The recipient of a signed message knows that the message is in fact from the alleged source and that the message hasn't been altered in any way after it was signed. Public-key encryption is used to authenticate the public key. Impersonation 1 and 2 are the worst problems. Intruders can put rogue code onto a Web server by overrunning a software buffer. Intruders can embed a Unix command into a Web request so that the Web server is tricked into passing that command on to the operating system.