Using public key cryptography, X adds a digital signature σ to message M, encrypts
1
Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key
2
Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key
3
Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key
4
Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’s public key