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?

Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key

Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key

Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key

Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’s public key