This is a cryptography class problem

5. If an encryption function ek is identical to the decryption function dk, then the key k is said to be involutory key. (a) Suppose k = (a,b) is a key in an Affine Cipher over Zn. Prove that k is an involutory key if and only if a2 = 1 and B.(a + 1) = 0 in Zn. (b) Use part(a) to determine all the involutory keys in the Affine Cipher over Z40. Avoid brute force search. 5. If an encryption function ek is identical to the decryption function dk, then the key k is said to be involutory key. (a) Suppose k = (a,b) is a key in an Affine Cipher over Zn. Prove that k is an involutory key if and only if a2 = 1 and B.(a + 1) = 0 in Zn. (b) Use part(a) to determine all the involutory keys in the Affine Cipher over Z40. Avoid brute force search