Encrypting a short string?

[erikcw]

Hi,

I'm trying to devise a scheme to encrypt/obfuscate a short string that
basically contains the user's username and record number from the
database.  I'm using this encrypted string to identify emails from a
user. (the string will be in the subject line of the email).

I'm trying to figure out which approach I should use to encrypt the
data.  The string will be less than 20 characters long, and I'd like
the encrypted version to be about the same size.

I tried DES in the Crypto module, but the cipher text was to long to
be usable in this case.

Any suggestions?

Thanks!