The GATE Grind

GATE 2016 CS – Question 62

Computer Networks · Application Layer: DNS and HTTP · 2 marks · Multiple choice

Consider that B wants to send a message $m$ that is digitally signed to A. Let the pair of private and public keys for A and B be denoted by $K_x^-$ and $K_x^+$ for $x = A, B$, respectively. Let $K_x(m)$ represent the operation of encrypting $m$ with a key $K_x$ and $H(m)$ represent the message digest. Which one of the following indicates the CORRECT way of sending the message $m$ along with the digital signature to A?

  1. $\{m, K_B^+(H(m))\}$
  2. $\{m, K_B^-(H(m))\}$
  3. $\{m, K_A^-(H(m))\}$
  4. $\{m, K_A^+(m)\}$

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Correct answer: (B) $\{m, K_B^-(H(m))\}$

Explanation

A digital signature is made by the sender encrypting the message digest with the sender's own private key, so that anyone can check it with the sender's public key. B is the sender, so the signature is $K_B^-(H(m))$, sent together with $m$.