Using public key cryptography, $X$ adds a digital signature $\sigma$ 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.