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Using public key cryptography, $X$ adds a digital signature $\sigma$ to a message $M,$ encrypts $\langle M,\sigma \rangle$ and sends it to $Y,$ where it is decrypted. Which one of the following sequence of keys is used for 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: ${X}'s$ private key followed by ${Y}'s$ public key; Decryption:${Y}'s$ private key followed by ${X}'s$ public key.  
  4. Encryption: ${X}'s$ public key followed by ${Y}'s$ private key; Decryption:${Y}'s$ public key followed by ${X}'s$ private key.
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Image result for digital signature block diagram

Summary of digital signature:

  1. Sender computes hash value. Encrypt this hash value using its private key to make digital signature.
  2. Certificate of sender contains public key of sender. Digital signature and certificate are added to message and sent
  3. Receiver extracts public key from certificate. Using public key, decrypt the signature to get hash.
  4. Receiver also computes hash of the message and compares with received hash.

Answer is

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Digital signature is signed(encrypted) using private key of sender and decrypted using public key of sender(known to all)

Message encryption using public key cryptography: Encrypted using receiver's public key and decrypted using receiver's private key.

option C

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