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Using public key cryptography, X adds a digital signature s to message M, encrypts <M, s>, 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 X's private key
4. Encryption: X's public key followed by Y's public key; Decryption: X's private key followed by Y's public key

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

Solution

The correct option involves encrypting the message with X's private key to create a digital signature, ensuring authenticity, and then using Y's public key to encrypt the entire package for confidentiality. During decryption, Y uses their private key to access the message and X's public key to verify the signature, confirming that the message originated from X.

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