Tamper-Evident Seal

What is Tamper-Evident Seal?

A tamper-evident seal is a security feature that protects the integrity of a signed document. It works by creating a cryptographic fingerprint (hash) of the document at the time of signing. If anyone attempts to modify even a single character of the document after signing, the hash will no longer match, and the tampering will be immediately detectable.

The technology behind tamper-evident seals typically relies on hash functions — mathematical algorithms that convert any input into a fixed-length string of characters. Even a tiny change to the input produces a completely different hash value. By storing the original hash as part of the digital signature, any subsequent alteration to the document becomes evident when the hash is recalculated and compared.

Tamper-evident seals are distinct from tamper-proof seals. A tamper-evident seal does not prevent modification — it detects it. This distinction is important because in the digital world, it is impossible to truly prevent someone from modifying a file. Instead, the goal is to make any modification immediately apparent, thereby invalidating the associated signatures and alerting all parties.

In the context of electronic signatures, tamper-evident seals are a key component of advanced and qualified electronic signatures under eIDAS. They provide the "linked to the data signed in such a way that any subsequent change in the data is detectable" requirement. PDF documents, the most common format for signed documents, support built-in tamper-evident seals through the PAdES (PDF Advanced Electronic Signatures) standard.