Birthday attacks are a class of brute-force techniques used in an attempt to solve a class of cryptographic hash function problems. These methods take advantage of functions which, when supplied with a random input, return one of equally likely values. By repeatedly evaluating the function for different inputs, the same output is expected to be obtained after about evaluations.
See also Birthday Problem
References
RSA Laboratories. ``Question 95. What is a Birthday Attack'' and ``Question 96. How Does the Length of a Hash Value Affect Security?''
http://www.rsa.com/rsalabs/faq/.
van Oorschot, P. and Wiener, M. ``A Known Plaintext Attack on Two-Key Triple Encryption.'' In
Advances in Cryptology--Eurocrypt '90. New York: Springer-Verlag, pp. 366-377, 1991.
Yuval, G. ``How to Swindle Rabin.'' Cryptologia 3, 187-189, Jul. 1979.