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Using public key cryptography, $X$ adds a digital signature σ to message $M$, encrypts $\langle M, \sigma \rangle$, 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: $Y’s$ 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
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Best answer
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41 votes

$X$ adds his digital signature$:$ In order to identify the authentic user, $X$ uses his Private Key to encrypt his signature.

$X$ then encrypts the whole message with the digital signature$:\ X$ uses $Y's$ Public Key to encrypt the message so that $Y$ can decipher it when it reaches to him using his private key.

Message then reaches $Y$.

$Y$ then uses his Private key to decrypt the message, and extracts the message and along with the signature.

But as the signature has been encrypted using $X's$ private key so$:$

$Y$ uses $X's$ Public Key to see the signature if it matches $X's$ actual signature (this step ensures that no one can fake as $X$ and sends a message to $Y$ ). 

Nobody can tamer the message as in order to do that he/she has to first know $Y's$ private key to decipher the message extract the signature and then change the signature and then recreate that using $X's$ private key which is not with him.

So, sequence of operations:

$X's$ Private Key$\Rightarrow$Y's public key$\Rightarrow$Y's Private key$\Rightarrow$X's public Key.

Answer is option (D).

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It is obvious that X can't encrypt using its own public key as it leads to security breach. X has to encrypt using its private key. So, option (c) is incorrect.

As X can only know about Y's public key (and not private key), this message will further be encrypted using Y's public key. So, option (a) is incorrect.

While decrypting, Y can decrypt the message (using its private key) only after deciphering the encrypted message using X's public key. Accordingly, the option (b) is also not correct.

 

 

therefore option D.
–2 votes
–2 votes
ans d)
Answer:

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