from finger remailer@soda.berkeley.edu:
This feature currently uses the DES cypher, which is considered non-secure. Not only that, but this system greatly facilitates a chosen plaintext attack, which is a strong attack. This is not a cryptographically optimal system in this respect. In other words, a determined adversary with suffecient resources could probably decrypt the response block, and thus break your anonymity, fairly easily. I will soon switch from DES to IDEA, which is thought to be a more secure cypher...
Which is why my remailer uses 3DES and adds random padding to the address before encrypting it. Random padding will thwart many chosen plaintext attacks, especially if you do some transpositions prior to encrypting it. Since the text to encrypt is so small, doing five or ten consecutive DES encryptions with different keys would not use up much CPU time, but could dramatically increase security. Also I compress the address slightly by stripping off the high bits so that 8 bytes fit into 7. Just a few suggestions to keep in mind... Overall, looks pretty good. BTW, what's the number that it prepends to your email address when you get a reply?