String compression/encryption

[chemicalNova]

Howdy all,

I'm working on trying to encrypt a string. Most of the examples I've seen and things I've attempted, end up making the result of the encrypted string, alot bigger in size than the original string. Example:
Trying to encrypt the string "hello world" would end up being something like "hdu87327634husdfsd7823434" which is obviously longer than the original.

Anyone know some algorithms to lower the sie of the string but still encrypt it (and be able to decrypt it)?

Thanks in advance,

[edit]
Also, these strings are going to be saved to a file, so if that helps anyone

Phreak