Discussion
A sender S sends a message m 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 m with a fraudulent message
II. A third party attacker can launch a birthday attack to replace m with a fraudulent message
III. R can launch a birthday attack to replace m with a fraudulent message
Which of the following are possible security violations ?
Answer: B
The possible violation is I only.
A birthday attack finds two different messages with the same hash. The signer S can prepare such a pair in advance, sign the harmless one, and later claim the signature was on the other, because the signature covers only the hash. S can do this because S holds the private key and chooses what to sign.
A third party or the receiver R does not control which message S signs and cannot sign a new message without S's private key, so finding a colliding pair does not let them substitute a message S signed. That rules out options including II or III.
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