Using public key cryptography, X adds a digital signature σ to a message M, encrypts $$\langle M, \sigma \rangle$$ and sends it to Y, where it is decrypted. Which one of the following sequence of keys is used for 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 private key followed by Y’s public key; Decryption : Y’s private key followed by X’s public key.

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