Digest::MD5 isn't a suitable encryption technique

There are ways to use a hash function as a cipher. IIRC, it involves using the digest as an output-feedback stream cipher. The security of the resulting cipher is based on the security of the hash function (in other words, don't use MD5 for this). Also, keep in mind that the creaters a cryptographic hash function generally don't have in mind its use as a cipher.

The only practical use I've been able to find for this is a situation where a government has banned strong encryption software, but not digest functions. In which case a coder can grab an existing digest function and make themselves a strong encryption program. Other than that, you're better off using a real cipher.