Hash

A cryptographic hash function takes any input, of any size, and produces a fixed-length output called a hash or digest. The process is deterministic (the same input always gives the same hash), one-way (you cannot practically reverse a hash back to its input), and highly sensitive (changing a single character produces a completely different hash). Ethereum and many other systems use the Keccak/SHA-3 family for this.

Why it matters

Hashing is everywhere in blockchains. A transaction is identified by its hash, which is how you look it up on a block explorer. Blocks are chained together by including the hash of the previous block, so altering old data would change every subsequent hash and be immediately detectable. Addresses are derived by hashing a public key. This makes hashes the backbone of integrity and tamper-evidence.

Common misunderstanding

A hash is not encryption. Encryption is designed to be reversed by someone with the key; hashing is deliberately one-way and has no key to undo it. Another misconception is that hashes hide data securely on their own. Because the same input always yields the same hash, a value from a small or predictable set can sometimes be guessed by trying inputs. Hashing proves that data has not changed and links records together; it is not a substitute for keeping secrets secret. In blockchains its job is integrity and linkage: proving that records have not been altered and tying each block to the one before it.

Related terms

• Transaction
• Public key
• Block explorer
• Smart contract