Using public key cryptography, X adds a digital signature ? to message M, encrypts <M, ?>, and sends it to Y, where it is decrypted. Which one of the following sequences of keys is used for the operations?

(A) Encryption: X?s private key followed by Y?s private key; Decryption: X?s public key followed by Y?s public key

(B) Encryption: X?s private key followed by Y?s public key; Decryption: X?s public key followed by Y?s private key

(C) Encryption: X?s public key followed by Y?s private key; Decryption: Y?s public key followed by X?s private key

(D) Encryption: X?s private key followed by Y?s public key; Decryption: Y?s private key followed by X?s public key