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’spublic key
Answer : (D)
Explanation :
The encryption & decryption logic in the options are related to Public Key encryption. Public keys are universally available but private keys are kept with the key owners.
When X needs to send message to Y, X first needs to encrypt it with their own private key and Y's public key and Y needs to apply decrypt the message using the keys in reverse order i.e., first with its own private key and then X's public key.
Important point to note here is that decryption needs to be done in the reverse order.
Clearly D is the answer.
Elimination approach
(A) Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key : This is clearly wrong because X does not access to Y's private key.
(B) Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key : This is clearly wrong answer as well, because Y is not decrypting in the reverse order. Y is decrypting the message with X's private key which is encrypted with Y's public key.
(C) Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key : This is wrong answer as well as X does not access to Y's private key.
(D) Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’spublic key : This is the correct answer as the encryption and decryption are done with correct keys and decryption was also done in reverse order.
References :
(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’spublic key
Answer : (D)
Explanation :
When X needs to send message to Y, X first needs to encrypt it with their own private key and Y's public key and Y needs to apply decrypt the message using the keys in reverse order i.e., first with its own private key and then X's public key.
Important point to note here is that decryption needs to be done in the reverse order.
Clearly D is the answer.
Elimination approach
(A) Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key : This is clearly wrong because X does not access to Y's private key.
(B) Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key : This is clearly wrong answer as well, because Y is not decrypting in the reverse order. Y is decrypting the message with X's private key which is encrypted with Y's public key.
(C) Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key : This is wrong answer as well as X does not access to Y's private key.
(D) Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’spublic key : This is the correct answer as the encryption and decryption are done with correct keys and decryption was also done in reverse order.
References :
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