A hash function is an algorithm that converts input data into a hash, also called a digest. For a given algorithm and identical input bytes, the result is always the same. Common blockchain hash functions produce a fixed-length result even when the input length varies.
What cryptographic strength means
A cryptographic hash function is designed to make finding an input from its digest, or finding different inputs with the same digest, computationally impractical. Collisions remain mathematically possible. Security depends on the algorithm and attack resources, not a claim that every input has a unique output.
How blockchains use hashes
Hashes help identify transactions, link block headers and summarize data in Merkle trees. SHA-256 produces 256 bits, commonly displayed as 64 hexadecimal characters. Changing even a small part of an input generally changes its digest substantially, allowing comparison against a trusted reference.
A digest does not conceal everything
Hashing is not reversible encryption and uses no decryption key. Predictable inputs can still be guessed and checked by hashing candidates. A matching digest establishes a relationship to the referenced data; it does not prove that the data is truthful or that its author is trustworthy.