Thread: inversion

Ok, i have an encryption algorithm, and am working on the decryption algorithm. I need to reverse some stuff, and I'm having trouble trying to invert this line:

b = i % strlen(in) - 1;

How can I invert b so that say b = 0, i change it so b = strlen(in) - 1, or if b = 1, change it to b = strlen(in) - 2

Any help would be great, thanks

I don't know an exact answer to your question but I know the modulus function is hard to reverse. Have a look at RSA encryption scheme, that reverses a mod and you might get some ideas from that. Your problem seems more like maths than programming, figure out how to do it on paper before trying to code it.