1Choose the file you are issuing
A report, a photograph, a statement, a piece of evidence. This works out its fingerprint: a long code that changes completely if even one character or pixel in the file changes.
Method: SHA-256
Leave this alone unless someone has asked for a particular method.
2Keep the record
Save the record and keep it apart from the file: a separate email, a case folder, a printed copy. Anyone can check the file against it later.
The time shown is this device’s clock. It is not a trusted timestamp.
What this proves, and what it does not
It proves the file has not changed. Take the fingerprint of the same file again, and a match means it is identical, byte for byte, to the one you issued.
It does not prove who made the file, or that it was genuine to start with. A fingerprint is taken of whatever you give it, real or forged. Proving who something came from needs a digital signature or a trusted third party.
It is not a trusted timestamp. The time is your device’s clock. If a date has to stand up to a challenge, use a timestamping service as well.
For anyone technical: the fingerprint is a cryptographic hash, SHA-256 unless you change the method, and the record names it, so it can be checked with any other tool.
1Choose the file to check
The file as you have it now. Its fingerprint is worked out again here and compared with the one that was kept.
2Give the fingerprint that was kept
Load the saved record, or paste the fingerprint. The method is recognised from its length.
3The answer
What a match tells you, and what it does not
A match means the file is unchanged since that fingerprint was taken. Not one byte has moved.
It does not vouch for whoever gave you the fingerprint. If the fingerprint came with the file, a match only shows the two agree with each other. Get the fingerprint by a separate route (another email, a printed sheet, a call) when that matters.
No match means a different file. Either it has been changed, or it is not the file the fingerprint was taken from. A renamed file still matches: the name is not part of the fingerprint.