Document hash
A document hash is a fixed-length string computed from a file's exact contents by a hash function such as SHA-256. It works like a fingerprint: the same file always gives the same hash, and any change gives a different one.
Updated November 5, 2026 · 1 min read
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How it works
SHA-256, defined in NIST's Secure Hash Standard, turns a file of any size into 256 bits, usually written as 64 hexadecimal characters. It's one-way, so the file can't be rebuilt from the hash, and finding two different files with the same hash is practically impossible. To check a copy, hash it again and compare: if the values match, the copy is identical byte for byte.
An example
A property manager sends a lease and keeps its hash. Months later, a tenant produces a copy showing a different rent. Hash that copy: if the value doesn't match the one on record, the document was changed after signing.
In SignMaster
SignMaster records a SHA-256 hash of each original file, checks the files against it before building the signed PDF, and prints each original hash on the certificate of completion. The signed PDF's own hash is recorded too, and the API and webhooks return it alongside the download link.
Questions? Answers.
How do I check a file's SHA-256 hash?
On a Mac, run shasum -a 256 and the file name in Terminal. On Windows, run certutil -hashfile, the file name and SHA256 in Command Prompt. Compare the result with the recorded value.
Sources
- NIST FIPS 180-4, Secure Hash Standard (SHA-256) · csrc.nist.gov