Q.

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?

[A] Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key

[B] Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key

[C] Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key

[D] Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’s public key

Answer & Explanation
Answer: Option [D]

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

Encryption is done using X's private key followed by Y's public key to prevent unauthorized users in accessing the message by converting it to ciphertext.

Decryption is done to retrieve the original message from the ciphertext by using Y’s private key followed by X’s public key.

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