A hash is a fingerprint of data. Feed any input into a hash function and you get a fixed-length string of characters back. The same input always produces the same hash. Change one character in the input and the hash changes completely. That property makes hashes useful for verification. You can check that a file has not been altered by comparing its hash to a known good value. Bitcoin uses SHA-256. Ethereum uses Keccak-256. Every block contains the hash of the previous block, which links the chain together.

Hashes are one-way. You cannot work backward from a hash to the original input. That is why they are used for passwords and proofs. Miners compete to find a hash below a certain target. They change a nonce, hash the block header, and check the result. The process is guess-and-check at massive scale. Finding a valid hash proves that work was done. Verifying it takes almost no effort. That asymmetry is the foundation of proof-of-work. Hash functions are also used to generate wallet addresses and verify transaction integrity.

Hash function properties

• Deterministic — same input always gives same output
• Fast to compute — verification is cheap
• Preimage resistant — cannot reverse the hash
• Avalanche effect — small input change, completely different output

A hash is not encryption. It is a one-way digest. There is no key and no way to decrypt.