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What problems does compression before encryption have? It at least seems to work for PGP.

Most compression schemes put a header/index on the front of the compressed data. This makes recognizing the correct decryption very simple.

Call it a limited "known plaintext" situation.

Using a random IV also limits the effectiveness of using known headers for "known plaintext" attacks. Also note that a good block cipher isn't that vulnerable even to "known plaintext" attacks. -- David F. Ogren ogren@concentric.net (alternate address: dfogren@msn.com) PGP Key ID: 0xC626E311 PGP Key Fingerprint: 24 23 CD 15 BF 8D D1 DE 81 71 84 C8 2C E0 4B 01 (public key available via server or by sending a message to ogren@concentric.net with a subject of GETPGPKEY)