A slightly reduced strength bulk encryption. In exchange for the reduced strength, you get the ability to rsync the encrypted files, so that local changes in the plaintext file will result in (relatively) local changes to the cyphertext file.
Cheap Cracks - Of dictionaries and rainbows - The H Security: News and Features
Modern cryptological attacks can crack mobile phone calls, as well as debit and credit card systems, in seconds. The trick is to find a practical compromise between computing time and memory space with the help of precomputed tables. Probably no algorithm is immune to such an approach, but special techniques can thwart such attacks.