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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. For a simple example of using a hash value to avoid storing keys and passwords on servers, see zero-knowledge proof.

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. See blockchain, hash rate, HMAC, digital signature, MD5 and hash table.

Elon Musk and Bitcoin Hashing
So much hash processing takes place for mining Bitcoin worldwide that the electricity used in computing the hashes could power a small country. In early 2021, Tesla added Bitcoin as an alternate payment on its online order form, only to remove it shortly thereafter. The rumor is that Musk did not understand how much electricity is used, but when he found out, he removed the option, claiming if Bitcoin mining is ever changed to use less electricity, he would reinstate the option. Considering Musk is privy to everything that goes on in the technology world, the rumor is dubious.




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.