---
title: "File checksum — Glossary | LLMind"
description: "A fixed-length digest (typically SHA-256) that uniquely identifies a file's byte contents."
url: https://llmind.org/glossary/file-checksum/
source_format: html
---
[← Glossary](https://llmind.org/glossary/)

# File checksum

**A fixed-length digest (typically SHA-256) that uniquely identifies a file's byte contents.**

A checksum is a fixed-size numerical or hexadecimal value computed from all the bytes of a file using a hash function. A single bit change in the file produces a completely different checksum, making it useful for detecting corruption or tampering. File checksums are analogous to fingerprints — they are unique and deterministic.

## Purpose

The primary use is integrity checking: download a file from the internet, compute its checksum, and compare it to the published value. If they match, the file has not been corrupted in transit or modified since the publisher created it. If they differ, something went wrong — network corruption, bit rot on disk, or deliberate tampering.

## In LRFS

The file-level SHA-256 checksum is stored inside the XMP packet as part of the provenance metadata. Crucially, the checksum is computed over the original file bytes \*before\* adding the XMP metadata, so the checksum remains valid when the XMP packet is added later. This allows LRFS to capture the original file's integrity fingerprint without creating a chicken-and-egg problem.

## Checksum vs. signing

A checksum detects accidental or malicious changes to a file if you trust the checksum source. A signed checksum goes further: it proves the claimed signer computed the checksum, adding authenticity to integrity. In LRFS, both are used — the file checksum is often included inside a signed HMAC-SHA256 or ed25519 signature.

## Related terms

-   [HMAC-SHA256](https://llmind.org/glossary/hmac-sha256/)
-   [Tamper-evident metadata](https://llmind.org/glossary/tamper-evident-metadata/)
-   [Signed semantic metadata](https://llmind.org/glossary/signed-semantic-metadata/)

## See also

-   [Signing scheme](https://llmind.org/spec/signing-scheme/)
-   [Learn](https://llmind.org/learn/)