If you actually want to encrypt a string (as opposed to merely obfuscate it by shifting or something) I recommend the Cryptix free encryption package.

Without a real algorithm, encryption is wishful thinking and you might as well not bother.

There are lots of options:

• Simple alphabet-shift (a=b, b=c, c=d, etc.)
  
• Base64 Encoding
  
• XOR encoding (xor each character with a character from a password, or some other data)
  
• etc...
  

-chris