Birthday Attack
What is the significance of a birthday attack on a hash function?
Attempt to find a pair of messages that hash to the same value.
Outline how such a birthday attack would proceed.
Say sender A is preparing to generate the hash of a message
Opponent generates 2n/2 variations of the message that convey the same meaning (where n is the size of the hash; e.g. 160 bits)
Opponent generates an equal number of variations of a desired fraudulent message
50% chance of two messages being found that hash to the same value
A is presented with the appropriate valid message for signing