A birthday attack is a type of brute force attack. The name comes from the birthday paradox which states that, as few as 23 people brought together double the probability that two of the people share the same birthday. This may fly in the face of common sense; however it is not possible to argue with the math86.
The high probability of duplicates within a relatively small number of samples means that it is possible to create a mathematically identical message digest (practically-unique identifier, discussed in the Cryptography chapter) for a different message or document. Statistically speaking, youll usually find a duplicate value before youve tried even half the possible combinations.
Fortunately, the message that produces the duplicate will (in all probability) obviously be invalid (probably a string of gibberish characters rather than coherent words or whatever else the original message contained), and the recipient of such a message would be suspect of it even if the message digest were the same.
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