Reversible number generator/encrypter/compressor?

Does anyone know a good reversible number generator?

By this i mean that i would input a seed into a number generator (in my case the seed would only ever be composed of 1s and 2s, for example 121111111112111)and the output would be a much smaller number.

And, if later on i want to find the seed using the output number, i could just enter that output number into a separate function that does the opposite of the first, and the output of that would be the original seed.

to make it clearer, if i entered the seed of 1221121112, i would get a number X. Number X could include letters/symbols, not just numbers. i could then input this number X into another function, and receive 1221121112.

Sorry if this question is a bit vague, but i'm really clueless at this point since i can't find anything good on my own.