Using the English alphabet (i.e., mod 26 arithmetic) let plaintext = {p1, p2,... , pn} and corresponding ciphertext = {c1, c2,... , cn}. Suppose the encryption function is ci = pi + 15 (mod 26). If you receive the ciphertext message YPHDCRPBEQTAA, decrypt to recover the plaintext. What is the decryption function, and the recovered plaintext? What type of cipher is this?