GATE 2016: Consider that BB wants to send a message mm that is digitally signed to AA. Let the pair of private and public keys for AA and BB be denoted by Kx−Kx− and Kx+Kx+ for x=A,Bx=A,B, respectively. Let Kx(m)Kx(m) represent the operation of encrypting mm with a key KxKx and H(m)H(m) represent the message digest. Which one of the following indicates the CORRECT way of sending the message mm along with the digital signature to A?
a) {m,KB+(H(m))}
b) {m,KB−(H(m))}
c) {m,KA−(H(m))}
d) {m,KA+(H(m))}
Correct answer: B.
1. B is sending a message to A
2. So B will send the message m
3. In addition to the message, B will send the digital signature generated using the message and his private key
4. To generate Digital signature, B will take the Hashcode of the message ie H(m) and then encrpt with his private key. ie. Kb-, hence the answer is message (m) and Kb-(Hashcode of m)