SHA-256 checker: compare a file with its certificate
An exhibit produced by ChatExhibits prints the SHA-256 of the conversation archive it was made from. Drop that archive here and the two can be compared, character for character, without trusting anybody to have read them correctly.
Any file, of any size. It is read inside this browser and never sent anywhere, which is also why the whole page works with the network switched off after it has loaded.
What a matching digest proves
That the file in front of you is byte for byte the one the certificate was made from. Change a single character anywhere in it and the digest changes completely, so a match is not a resemblance: it is identity.
And what it does not
It says nothing about what happened before the exhibit was made. If messages were deleted in WhatsApp and the chat was exported afterwards, the digest faithfully certifies a shortened export. That is why the certificate also carries a declaration for a person to sign: the digest covers the technical half, and only a person can answer for the other.
A digest is also not a signature. Nothing here involves a certificate authority, and no document produced by ChatExhibits should be described as digitally signed.
Doing it yourself instead
You do not have to take this page’s word for it either. On macOS or Linux, shasum -a 256 yourfile.zip prints the same value; on Windows, certutil -hashfile yourfile.zip SHA256. Getting the same answer two ways is the whole idea.
More on how this works: SHA-256 and file integrity.