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MD5
MD5 is a hash-function which maps any message to a fixed number of 128 bits called the hash. This is done in such a manner that the following two problems are hard:

* finding a collision: two messages with the same hash
* finding a pre-image: for a given hash find a message that maps to that hash

Because of these properties MD5 is used commonly for the following purposes:

* Integrity checking: to verify that a file did not change or was transmitted correctly
* Digital signatures: a message is signed by signing its hash