ISRO-DEC2017-29

Question

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 ?

• 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: ${X}'s$ private key followed by ${Y}'s$ public key; Decryption:${Y}'s$ private key followed by ${X}'s$ public key.  
• D. Encryption: ${X}'s$ public key followed by ${Y}'s$ private key; Decryption:${Y}'s$ public key followed by ${X}'s$ private key.

Comments

https://gateoverflow.in/1435/gate-cse-2013-question-13

Answer 1

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 C 

Comments

Please make ISRO Dec 2017 full length exam in exam section of gateoverflow.

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Up now.

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Thank you Arjun Sir :)

Answer 2

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

Comments

In decryption: the encrypted signed message has to decrypted first using private key of receiver and then decrypt the signed message using sender's public key.