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Definition: cryptographic hash function

An algorithm that transforms a given amount of data (the "message") into a fixed number of digits, known as the "hash" or "digest." Hash functions are a fundamental component in digital signatures, password security, random number generation, message authentication and blockchains.

One-Way Processing
Also called a "one-way hash function" because it is nearly impossible to turn the digest back into the original data. It is also exceedingly rare that two different inputs can result in the same output.

This one-way processing is what guarantees the integrity of a blockchain-based currency or contract. All the transactions in a block are hashed into the subsequent block creating a chained linkage. Any alteration to an existing block breaks the chain. Around the world, there is so much hash processing taking place that the electricity used could power a small country. See blockchain, hash rate, HMAC, digital signature, MD5 and hash table.

Hashes Are Fixed in Size
The hash value guarantees only that it is mathematically equal to the data it has hashed. If the data are changed in any way, that same hash cannot be generated. No matter how large or small the input, the hash output is fixed; for example, Bitcoin hashes are 256 bits long. See SHA.