GATE CS 2017 Set 1 — Question 15
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Computer Networks → Application Layer → Digital Signatures & Certificates
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Question
A sender S sends a message to receiver R, which is digitally signed by S with its private key. In this scenario, one or more of the following security violations can take place.(I) S can launch a birthday attack to replace with a fraudulent message.
(II) A third party attacker can launch a birthday attack to replace with a fraudulent message.
(III) R can launch a birthday attack to replace with a fraudulent message.Which of the following are possible security violations?
(II) A third party attacker can launch a birthday attack to replace with a fraudulent message.
(III) R can launch a birthday attack to replace with a fraudulent message.Which of the following are possible security violations?
Correct answer
(B) (I) only
Solution
A birthday attack allows finding two distinct messages and such that .
- (I) is possible: The sender S can generate a collision pair , sign , and later claim to have signed (since the signature on the hash is valid for both). This allows S to repudiate the original message.
- (II) and (III) are not possible via birthday attack: For a third party or R to replace with , they would need to find an that hashes to the same value as the specific already chosen and signed by S. This requires breaking second preimage resistance, which is computationally much harder than a birthday attack (finding any collision).
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