If you've read anything about blockchains, timestamping, or passwords, you've seen the word hash. It sounds technical, but the idea is simple — and it's the engine behind proving a file existed and hasn't changed.
A hash is a fingerprint
A hash function takes any input — a word, a document, a 4GB video — and produces a short, fixed-length string of characters. SHA-256 always produces a 64-character (256-bit) result. For example, hashing the word "hello" with SHA-256 always gives the same long string, every time, on every computer.
The properties that make it useful
- Deterministic — the same input always gives the same hash.
- Fixed length — a tweet and an entire movie both produce a 64-character SHA-256 hash.
- Avalanche effect — change a single character and the hash changes completely. There's no "close" — it's entirely different.
- One-way — you can't reverse a hash back into the original file. It reveals nothing about the contents.
Why this proves a file is unchanged
Because any change — even one byte — produces a totally different hash, a hash is a perfect integrity check. If you record a file's hash today and the file's hash still matches next year, the file is provably identical. If even a comma changed, the hashes wouldn't match.
That's exactly why hashing is the foundation of trusted timestamping: you don't store the file anywhere public — you store its hash. Prove the hash existed at a time, and you've proven the exact file existed then, while keeping the contents private.
Where you've seen it
SHA-256 secures Bitcoin, verifies software downloads, and stores passwords safely. BlockchainSign uses it too: your file is hashed with SHA-256 in your browser, and only that fingerprint is recorded on the Ethereum blockchain — giving you tamper-proof, dated proof without ever exposing your file.