Question

I need to find a way to (preferably asymmetrically)[1] encrypt/decrypt a single Int32, with the resulting encrypted value containing only valid characters[2] and being under eleven characters in length.

How would I go about doing this?

[1] Given the answers so far, symmetric encryption is fine

[2] alphanumerics [0-9a-zA-Z], the special characters "$-_.+!*'()

Answer 1

as another answer says, an asymmetric solution is difficult and your best, standard solution is to use a symmetric cipher with a short block size. your restriction to ten characters implies 60 bits with base 64, but you could use a custom encoding to get 64 bits (with a block cipher you need to send an exact number of blocks).

if you use a 64 bit block size you will have to fix the IV. in other words, if you send the same number twice, it will be encoded the same way both times [not true! – see neat idea in comments below]. but with a 32 bit block cipher you could use a random IV (padding to hide repeated values). it looks like skip32 would be a good choice – see Which "good" block encryption algorithm has the shortest output?

another, ad-hoc idea, that i just pulled out my ass, and which may therefore be insecure is to split your message into two: 32bits and the rest. fill the rest with a random value, which you use as a (zero-padded, if necessary) seed to any stream cipher (http://en.wikipedia.org/wiki/Stream_cipher) then xor the value you want to encrypt with the first 32 bits of that (so your final message is the 32bits xor result, plus the random seed).

finally, i have no idea why everyone is convinced that these messages will/should/must be insecure. the security does not depend on the message size (it is the key size that is important). as far as i know, small blocks are only weak against long messages. here you do not have a long message (there’s an important difference between able to guess a single message in 2^32 and knowing all messages after a similar “small” number of guesses).