Encrypt a number to another number of the same length

Question

I need a way to take a 12 digit number and encrypt it to a different 12 digit number (no characters other than 0123456789). Then at a later point I need to be able to decryp

Answer 1

anyone with reflector or ildasm will be able to hack such an encryption algorithm.

I don't know what is your business requirement but you have to know that.