Glossary

SHA-256

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SHA-256 is a cryptographic hash function. A cryptographic hash function has a few key properties. It takes an input, called a preimage, and produces an output of a fixed length—all SHA-256 outputs are 256 bits long. This process is deterministic, meaning a given input will produce the exact same output every time. Hash functions are also unpredictable: the slightest change to an input yields an entirely different output, such that it is infeasible to craft a desired output or calculate an input based on the outputs. SHA-256 is a member of a family of hash functions called Secure Hashing Algorithm (SHA) functions.

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The Bitcoin protocol uses SHA-256 to derive transaction IDs (txids), block hashes, addresses, and Merkle trees. Occasionally, SHA-256 is applied twice, as in the case of txids.

Related Terms
Hash Function
A cryptographic hash function is a one-way, random mathematical function. Hash functions underlie Bitcoin's security model in several ways.
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Cryptography
The study of hashing algorithms, encryption and decryption, and public and private keys all fall in the realm of cryptography. Bitcoin leverages cryptography to achieve many of its central properties.
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Proof-of-Work (PoW)
Bitcoin miners must submit Proof-of-Work in the form of a valid hash in order to have their block be considered valid.
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Mining
Mining is the process of building the blockchain by adding new blocks one at a time. Most of mining involves finding a valid Proof-of-Work, which is an energy-intensive guessing game.
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Transaction ID (txid)
A txid or Transaction ID is a string of letters and numbers that identify a specific transaction. It can be used to look up a transaction on a node or block explorer.
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