Symmetric integer to integer encryption

Question

I need some pointers or a practical example on how to encrypt an int to another int, and a secret key would be required to decrypt the value.

Something like:

Answer 1

It depends how cryptographically secure you want to be. For not-very-secure (in the crypto sense - probably fine for everyday use if you don't really expect serious attack) then XOR with a fixed secret key will work. Just be aware that it will be vulnerable to some fairly basic cryptanalysis.

If you want real encryption, you'll probably have to use a stream cipher like RC4. You can grab 32 bits of keystream and XOR it with your value to encrypt it. As long as you get a new 32 bits of keystream for each value you'll be fine.

RC4 has some caveats, however, so read up on it first.

Block ciphers will not be your friend in this case as they all have block sizes of 64 bits or more. This means you need to pad your 32 bit integer to 64 bits and you'll get 64 bits back out...but you can't choose which 32 to keep. You won't be able to decrypt it with only half the bits. If you're happy to move to longs then you can use 3DES or Blowfish.

It all depends on exactly what you are encrypting and why, so it's hard to give a definitive answer. I hope this gives an idea of where to start, at least.