MD5 Algorithm
The MD5 algorithm is a secure hash function. It takes a string input, and produces a fixed size number - 128 bits. This number is a hash of the input - a small change in the input results in a substantial change in the output. The functions are thought to be secure, in the sense that it would require an enormous amount of computing power to find a string which hashes to a chosen value. In others words, there's no way to decrypt a secure hash. The uses of secure hashes include digital signatures and challenge hash authentication.
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