Glossary
What is a document fingerprint
A short number that changes if a single comma in the document changes.
Hash (fingerprint)
A short value calculated from a file's entire content, such that any change to the file — however small — produces a completely different value.
It is what lets you check a document has not changed without keeping a copy to compare against. Storing the fingerprint, a few dozen characters, is enough.
The three properties that make it useful
The third matters more than it seems: you can publish the fingerprint of a confidential contract without revealing anything about its content, and still prove later that it is the same contract.
Worth knowing
When verification says "does not match", it is almost never fraud: reprinting a PDF, re-saving it from another program or converting it changes the file. Change the file, change the fingerprint.
›Can two different documents share a fingerprint?
With the algorithms in use, not in practice.
›Does the fingerprint say what changed?
No, only that something changed. To know what, compare the two versions.
›Do I need to understand this to use it?
No. Verification tells you "matches" or "does not match".
A real case
The situation
Two parties want a record of a confidential agreement without showing it to anyone.
What you do
- Seal the document
- Share only the fingerprint with a third party
What you get
They can later prove the agreement is exactly that one, without having revealed its content.
The situation
Somebody forwards a document and it must be established it is the same one.
What you do
- Compares the received fingerprint with the registered one
What you get
The check does not require reading it through.
The situation
A comma is changed in an already sealed contract.
What you do
- Checks that the fingerprint no longer matches
What you get
Any change is detected, however small.
The situation
Two copies look identical and one has been altered.
What you do
- Computes the fingerprint of both
What you get
The original is identified without opening a dispute.
The situation
A document's existence must be proved without disclosing it.
What you do
- Shares only its fingerprint
What you get
The proof travels without the content.
The situation
A third party wants to verify a document that reached them another way.
What you do
- Tells them how to check the fingerprint themselves
What you get
Verification does not require trusting whoever sent it.
This article answers
- what is a file hash
- document digital fingerprint
- how to tell if a pdf was modified
- sha256 document