In May's Inbox, Thomas Paul Karrmann, commenting on "Your E-Mail Is Obsolete" (February), and author Michael Nadeau agreed on the difficulty of encrypting messages for multiple recipients.
Pretty Good Privacy (PGP) allows you to do this simply-with minimal increase in the message size. PGP uses the public-/private-key protocol only for key management and digital-signature verification. The message itself is encrypted using the International Data Encryption Algorithm (IDEA). PGP creates a random key for the "session" (i.e., the message) and uses the public-key protocol to encrypt that session key for each recipient. This collection of encrypted keys forms a control block for the whole message. Recipients use their private keys to decrypt their portion of the control block, which contains a copy of the session key. For additional details about this increasin
gly vital field, see Applied Cryptography, Second Edition by Bruce Schneier (John Wiley &
Sons, 1996).
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Wrong Key, Part II
