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

One more way for simple encryption, you can just substruct each number from 10.

For example initial numbers: 123456

10-1 = 9 10-2 = 8 10-3 = 7 etc.

and you will get 987654

You can combine it with XOR for more secure encryption.