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

For example you can add digits of your number with digits some const (214354178963...whatever) and apply "~" operator (reverse all bits) this is not safely but ensure you can decrypt your number allways.