Hash

A hash is a data fingerprint from a cryptographic function like SHA-256, used to bind electronic signatures and timestamps to exact content.

A hash is a fixed-length fingerprint of data, computed with a cryptographic hash function such as SHA-256: the same input always gives the same hash, any change gives a completely different one, and the input cannot be reconstructed from it.

Hashing is the workhorse of signing: an electronic signature does not encrypt the document but signs its hash, binding the signature to exactly that content. Because only the hash is needed for that binding, remote signing and timestamp services can be designed so that only the hash travels and the document itself never leaves the signer's side - though this is a design choice, not something hashing guarantees by itself, and some services do handle full documents. On its own, a hash only proves integrity, not authenticity or time: binding it to an identity needs a private key (as in a signature), and binding it to a moment needs a trusted timestamp. Validation can then show that a single changed byte breaks that binding.