Using public key cryptography, X adds a digital signature σ to message M, encrypts <M, σ>, 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