[Haskell-cafe] Digests

The data integrity checks is well-known problem. A common soluting is use of 'checksums'. Most of them , however, are built in quite obfuscated manner (like md5) that results in ugly and error-prone implementations (see reference implementation for same md5). So, the question is: is there a checksum, that is easy to implement over stream of bytes and may work as good checksum and is good in sence that creation of messages with same checksum that given message has is very hard problem (at least 2^128 tries) ? The reason is that I wish a good checksum to be implemented im my stream-oriented library.