A KuduDoc seal makes a finished PDF tamper-evident: it records a SHA-256 fingerprint of the exact bytes, a certification signature and an OpenTimestamps anchor, so anyone can compare a later copy — free, without an account.
We confirm the date of sealing and that this file is unchanged since. We cannot confirm what the document says is true.
A document fingerprint is the SHA-256 hash of the file’s exact bytes: a 64-character code that changes if even one byte does. An edit, a redaction or a fresh export changes it, even when the pages look the same.
It tells you the bytes differ, not why.
A certification signature is applied to the completed PDF with KuduDoc’s certificate, issued by a certificate authority on Adobe’s Approved Trust List. Adobe Acrobat shows the document as certified by KuduDoc and flags any change made after sealing. To compare a copy with the sealing record, use the free checker.
Signed somewhere else? Check the signatures in any PDF.
Acrobat shows
“certified by KuduDoc”
The seal timestamps a document through its fingerprint. The fingerprint — never the document — goes to the OpenTimestamps calendars and is later confirmed in a Bitcoin block. Every check shows which state the anchor is in.
Keep the OpenTimestamps proof file if you want to inspect it yourself with open-source tools.
Timestamp any file, free →Tamper-evident means a seal can’t stop anyone editing a copy of a sealed PDF, but the edit shows: the copy’s fingerprint no longer matches. A signature can look identical on two versions of the same page. The fingerprint shows whether the bytes still match.
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Holding a PDF that was never sealed? Here’s how to check whether a PDF was changed.
Every check gives three separate lines: the file comparison, where the record came from, and the timestamp. There is no overall pass or fail.
A match means the file’s bytes are unchanged since sealing. It doesn’t tell you whether what the document says is true, who signed it or who owns anything.
An account seal and a free-tool stamp are different sources. Anyone can stamp a file with the free tool, so a stamp doesn’t say who stamped it. A timestamp of any file is different again: it records when, not whose — timestamp a document online.
Check your sealed PDF →The checker can’t tell a file that was never sealed from a changed copy.
Sent, link opened, completed, sealed: each event in a document’s activity record is chained to the event before it, so a changed entry breaks the chain. It stays beside the document in your Library.
A sealed document can be issued as a Controlled Original. The registry records exactly one current controller, and control moves only when a handover is offered and accepted, or surrendered.
Sending someone a PDF copy to inspect doesn’t hand over control. Copies issued from a Controlled Original carry a live status link and are marked as copies, so their bytes differ from the sealed file. And holding the original doesn’t make anyone the owner of the property or rights it describes.
Why we built it this way →Live status link → the current registry record
The kdid is printed on the certificate page and kept in the file’s metadata. A QR code or check link takes a recipient straight to the checker.
Check the file itself, not the printed code: a seal id can be copied onto any document, and only comparing the PDF shows whether it is unchanged.
Send the other side a file they can check →The owner can export a forensic ZIP with the audit rows and signer details.
Not necessarily. An intended edit, a redaction or a fresh export changes the bytes too. The fingerprint tells you the files differ, not why.
No. It is tamper-evident: a seal cannot stop anyone editing a copy, but any change to the bytes means that copy no longer matches its seal record.
No. Signing records who signed and when. Sealing gives the finished file a fingerprint that later copies can be compared with. A signed document is sealed the moment the last signature lands.
No. A redacted or edited copy is a new file with its own fingerprint, so it won’t match the original’s seal record. How redaction works →
No. Sealing a scan gives that digital file a reference to check against. Keep the paper original where you can find it.
OpenTimestamps is a free, open protocol that shows a fingerprint existed by a certain time, by committing it to the Bitcoin blockchain through public calendar servers. In a KuduDoc seal, the OpenTimestamps anchor sits beside the SHA-256 fingerprint and the certification signature, and its state — pending or confirmed — is reported separately.
Yes — that it existed by a certain time, once its timestamp is confirmed. The seal sends the file’s SHA-256 fingerprint — never the document — to OpenTimestamps. When the anchor is confirmed in a Bitcoin block, it is a record that this exact file existed by that time. Until then, every check shows the anchor as not yet confirmed.
Yes. Timestamp any file free with your KuduDoc account, on every plan: only its fingerprint leaves your device. A timestamp records that the exact file existed by the anchor time; a seal also adds a certification signature and a record anyone can check a copy against. Timestamp a file →
No. Checking is free and needs no account. Drop the PDF itself on the checker; a kdid lookup on its own shows the record’s details but doesn’t compare your file.
Free, no account. Drop the PDF on the checker and read the three lines.