Question 1
(I) and (II) only
(I) only
(II) only
(II) and (III) only
       Computer-Networks       Security       Gate 2017 set-01
Question 1 Explanation: 
Birthday attack Problem is when sender replaces original message with fraud message having same message digest as the original message, along with the digital signature of the original message.
(I) Can the sender replace the message with a fraudulent message? Yes definitely because the sender will encrypt the message with its private key. It can encrypt another message also with its private key.
(II) Can the third party send a fraudulent message? No, because the third party doesn't know about the private key of the sender.
(III) Can receiver send the fraudulent message?
No, the receiver also doesn't know about the Private key of the sender, so receiver also cannot send the fraudulent message.
Question 2
       Computer-Networks       Security       2016 set-01
Question 2 Explanation: 
Digital signatures are electronic signatures which ensure the integrity, non-repudiation and authenticity of message.
Message digest is a hash value generated by applying a function on it. Message digest is encrypted using private key of sender, so it can only be decrypted by public key of sender.
This ensures that the message was sent by the known sender. Message digest is sent with the original message to the receiving end, where hash function is used on the original message and the value generated by that is matched with the message digest.
This ensures the integrity and thus, that the message was not altered. Digital signature uses private key of the sender to sign message digest.
Question 3
Anarkali digitally signs a message and sends it to Salim. Verification of the signature by Salim requires
Anarkali’s public key.
Salim’s public key.
Salim’s private key.
Anarkali’s private key.
       Computer-Networks       Security       GATE 2016 set-2
Question 3 Explanation: 
In digital signature generation process, Sender uses its own private key to digitally sign a document. Receiver uses sender's public key to verify signature.
Question 4
P and R only
Q and R only
Q and S only
R and S only
       Computer-Networks       Security       GATE 2014(Set-01)
Question 4 Explanation: 
RSA and DES are for Encryption where MD5 and SHA – 1 are used to generate Message Digest.
Question 5
Only I1 and I2
Only I1
Only I2 and I3
Only I3 and I4
       Computer-Networks       Security       Gate 2014 Set -02
Question 5 Explanation: 
An Intruder can’t learn [I1] through sniffing at R2 because URLs and Download are functioned at Application layer of OSI Model.
An Intruder can learn [I2] through sniffing at R2 because Port Numbers are encapsulated in the payload field of IP Datagram.
An Intruder can learn [I3] through sniffing at R2 because IP Addresses and Routers are functioned at network layer of OSI Model.
An Intruder can’t learn [I4] through sniffing at R2 because it is related to Data Link Layer of OSI Model.
There are 5 questions to complete.