A message is encrypted using an affine cryptosystem in which plaintext uses the 26 letters A through Z (all blanks are omitted), the letters are identified with the residue classes of integers (mod 26) in the natural way, and the encryption and decryption functions are defined by x -> f(x) = ax + b (mod 26) and y -> g(y) = cx + d (mod 26) respectively, where a; b; c; d are certain constants (unknown to you) and g(f(x)) is the identity function. Suppose you intercept the ciphertext UWWPUWKPPFWPWKPJWW: Assuming that E and then T are the two most commonly occurring letters in English text, find the decryption function g(y) and decrypt the message.
This is my homework question. I really dont know how to solve it . Could anyone give me a help ? Thanks in advance.