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Using public key cryptography, X adds a digital signature σ to message M, encrypts <M, σ>, and sends it to Y, where it is decrypted. Which one of the following sequences of keys is used for the operations?

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: X's public key followed by Y'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

Correct answer: Encryption: X's private key followed by Y's public key; Decryption: Y's private key followed by X's public key

Solution

The correct option involves X using their private key to create a digital signature, ensuring authenticity, and then encrypting the message with Y's public key, which ensures that only Y can decrypt it with their private key. This sequence maintains both the integrity of the message and the confidentiality of its content.

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