Tamper-evident seal

A tamper-evident seal is a cryptographic mark applied to a signed document, usually a PDF, that makes any later change detectable. Open a sealed PDF in a reader such as Adobe Acrobat and you may see a banner saying the document has not been modified since it was signed, or a warning that it has. That banner is the tamper-evident seal doing its job.

Technically, the process starts with a document hash of the finished file. That hash is then signed using a private key tied to a digital certificate, following PKI standards. When someone opens the file, their software recalculates the hash and checks it against the signed value using the public key. If anything in the covered content has changed, the check fails and the reader flags it.

Say a recruitment firm sends a signed placement agreement to a client. The client's finance team opens it months later to check the fee percentage. If the PDF carries a valid seal, their reader confirms the file is intact. If someone had edited the fee after signing, the seal would show as broken, and the team would know not to trust that copy.

A key misconception is that tamper-evident means tamper-proof. The seal does not stop anyone from editing a file; it just makes the edit visible. Another is that the seal proves who signed. It proves which organisation or certificate applied the seal and that the file is unchanged since, while evidence about individual signers still comes from the audit trail. Seals can also show warnings simply because a reader does not trust the certificate authority, not because of tampering.

Tamper-evident seals rely on the same building blocks as digital signatures and qualified electronic signatures under eIDAS, but a seal applied by a platform is not the same as a qualified signature by the signer. Many platforms apply their own seal to every completed file. Where a document hash is published separately, it can serve a similar purpose even when no seal is embedded, because any change to the file produces a different fingerprint that will not match the recorded value.

Our take: embedded PDF seals are useful, but they are not the only way to prove integrity. A published SHA-256 hash that anyone can check is simpler, easier to understand, and does not depend on which PDF reader someone happens to use.

Today eSignSimple provides integrity checking through a SHA-256 hash of the final file, printed on the certificate of completion and verifiable on our public /verify page. Certificate-based PDF seals that show up inside readers like Acrobat are on our roadmap.