It's impossible to tell what the problem could be as there are too many unknowns. Break your code up so that you can treat each bit individually. Start by making a program that sends plain characters with the RF module and ensure that you can reconstruct it at the other end correctly. Then put this into a function that takes an arbitrary array of bytes, again test with simple data to ensure it works. When you have that function created and working, your encryption and sending become independent of each other, for example if the output is incorrect then it must be the encryption, you will know where to look.