An Affine Cipher has encryption function E(m) = 7m + 4 mod 26. Find the decryption function and use it to decrypt the ciphertext uyyz cytw.

ii. If you are choosing a key (a, b) for an Affine Cipher with encryption function E(m) = am+b mod 26 what constraints are there on a and b? How many possible keys are there?

iii. Explain what a substitution cipher is. Explain how to attack it with a frequency attack, given an encrypted English text.