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