How about ROT13? Ideally applied twice for twice the encryption.

ROT13 is cheap enough that you can afford to apply it many more times. I use one million iterations to store passwords securely.

640k oughtta be enough for anybody.

Md5 encryption would be far superior.

There are performance concerns with base64. Hardware-assisted null-key encryption offers security that's a non-strict superset of base64 encryption and with superior performance.

null-key encryption is write-once, read-never, so you don't have to cache it.