Cryptography/Hashfunction

Hash functions are used in Cryptography for many purposes; for example, verifying the validity of a public key (this is commonly known as the key's fingerprint). A hash function returns a representation of the bits of a string in the form of a hash value, which has a fixed size. This is done in such a manner that the probability of two different strings having the same hash value is infinitesimal. Changing just one character in string will result in a different hash value.

Practical example: The record companies came up with the idea of identifying MP3 files by using their hash value in order to be able to trace the files on the Internet and in various filesharing communities. However, by changing or adding just one bit to the file will defeat this system completely.