Help please

[Noname1190]

Hi!
Im a programer for quite sometime now.. and I really need help with my next assignment, I need to.. Wait for it.. make a program that gets numbers and traslates it to letters with a secret principle that only I should know.
the numbers are aligned not at all randomly and it's basicly a "code language" to communicate privatly.

So in short, my question is how can I tell a program in C++, to see lots of numbers, know the principle and translate it to letters and words.
I'll give an example:

1431341541541234 = Hello

Thanks! :)

[Chervil]

The program needs to parse the input according to a set of rules which you supply and decrypt the values into text using a set of rules which you supply. Lastly it needs to output the resulting text.