Exact-byte continuity
Given the same algorithm and the expected digest, a matching digest provides strong evidence that the compared byte sequence is unchanged.
SOURCE REVIEW
A hash proves bytes, not truth.
Integrity is a boundary, not a verdict. Preserve the exact artifact—and refuse to pretend exactness proves what the artifact says.
QUICK ANSWER
In one sentence
What SHA-256 and preservation fixity can establish, what they cannot establish, and why provenance must remain separate from truth.
THE DIGEST
A HASH CAN TELL YOU THE FILE CHANGED. IT CANNOT TELL YOU THE FILE WAS TRUE.
Sixty-four hexadecimal characters can look like a verdict. They are not. SHA-256 is extraordinarily useful because it lets us compare exact digital content with a compact digest. That is a claim about bytes. The mistake begins when integrity is promoted into authorship, accuracy, morality, provenance, review, or truth.
TRUTH BOUNDARY
This article describes cryptographic hash and digital-preservation practice at a high level. It does not claim that hashes alone authenticate authorship, establish legal admissibility, or prove that preserved content is factually correct.
01 — WHAT THE STANDARD ACTUALLY SAYS
Hash algorithms generate message digests that can be used to detect change.
NIST's Secure Hash Standard specifies algorithms used to generate digests of messages. The digest gives you a compact value that can later be compared with another computation over the same bytes.
The Library of Congress uses the same idea in preservation practice: fixity metadata and cryptographic hash values are recorded and compared to monitor whether stored digital content has changed. PREMIS preservation guidance describes the same pattern—calculate a digest earlier, calculate it later, compare the two.
That is powerful. It is also bounded.
02 — WHAT A MATCH SUPPORTS
Transfer checking
A digest can help detect corruption or unintended modification during copying, storage, packaging, or distribution.
Version identity
Two files with different digests are not byte-for-byte identical under that algorithm.
Preservation receipts
Recorded digests make later fixity checks auditable when the manifest itself has trustworthy provenance.
03 — WHAT IT DOES NOT PROVE
Truth
A false report can be hashed perfectly.
Authorship
A digest does not tell you who created the file.
Originality
The first file you received may already be altered, incomplete, or misattributed.
Context
A preserved screenshot can still omit what happened before or after it.
Review
A checksum does not mean a qualified human evaluated the content.
Authority
Exact bytes do not turn a document into a controlling source merely because the copy is stable.
04 — THE TRUST CHAIN
Integrity becomes useful when it is connected to provenance.
For consequential archival work, keep several questions separate:
What are the bytes?
Filename, byte size, media type, digest.
Where did they come from?
Source URL, custodian, transfer route, retrieval date.
What do they claim?
Title, author, date, edition, scope.
Who reviewed the claim?
Reviewer, method, competing evidence, limitations.
What changed later?
Correction, supersession, withdrawal, replacement digest.
What remains unknown?
Missing custody, uncertain authorship, absent context, unresolved contradiction.
THE RECEIPT
Hash the artifact. Preserve the source. Name the authority. Record the review. Then keep those claims separate.
COMPETING INTERPRETATIONS
Hashes can participate in much stronger systems. Signed manifests, trusted timestamping, controlled repositories, content-addressed storage, and independently witnessed provenance can combine byte integrity with evidence about custody and identity. The point is not that hashes are weak. The point is that each layer proves a different thing.
Cryptographic standards also evolve. NIST has announced plans to revise FIPS 180-4. Long-lived archives therefore need algorithm policy and migration planning rather than treating one digest format as immortal.
What remains unknown
No digest can reconstruct missing context by itself. When custody before the first known hash is uncertain, say so. When the authority changed the file and published a new digest, preserve both states rather than pretending one timeless artifact existed.
SOURCE TRAIL
- MFTAC-SRC-0021 NIST — Secure Hash Standard, FIPS 180-4
- MFTAC-SRC-0022 NIST CSRC — FIPS 180-4 final record and revision note
- MFTAC-SRC-0023 Library of Congress — Data Integrity Management
- MFTAC-SRC-0024 Library of Congress / PREMIS — Preservation Metadata Data Dictionary v3.0
CORRECTION RECEIPT
Initial publication. Review when NIST replaces or materially revises FIPS 180-4. Submit corrections through Corrections.
THE CONSEQUENCE
PRESERVE THE BYTES. DO NOT WORSHIP THE DIGEST.
A hash is a witness to exactness. It is not a witness to meaning.