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A hash function takes a group of characters (called a key) and maps it to a value of a certain length (called a hash value or hash). The hash value is representative of the original string of characters, but is normally smaller than the original.
Hashing is done for indexing and locating items in databases because it is easier to find the shorter hash value than the longer string. Hashing is also used in encryption.
This term is also known as a hashing algorithm or message digest function.
Hashing is used with a database to enable items to be retrieved more quickly. Hashing can also be used in the encryption and decryption of digital signatures. The hash function transforms the digital signature, then both the hash value and signature are sent to the receiver. The receiver uses the same hash function to generate the hash value and then compares it to that received with the message. If the hash values are the same, it is likely that the message was transmitted without errors.
One example of a hash function is called folding. This takes an original value, divides it into several parts, then adds the parts and uses the last four remaining digits as the hashed value or key.
Another example is called digit rearrangement. This takes the digits in certain positions of the original value, such as the third and sixth numbers, and reverses their order. It then uses the number left over as the hashed value.
It is nearly impossible to determine the original number based on a hashed value, unless the algorithm that was used is known.