PAdES explained: the PDF signature profile, level by level

A PDF is not a flat byte stream: content is stored as revision layers, and a signature must state exactly which byte range it covers. PAdES fixes how a cryptographic signature is placed inside that structure so that independent validators agree on what is signed and how to check it. The current edition of the standard, ETSI EN 319 142-1, is V1.2.1 from January 2024.

The standard defines four: B-B (basic signature — the signer's certificate over the document digest), B-T (adds a trusted timestamp for the signature), B-LT (adds a Document Security Store embedding the validation data), and B-LTA (adds periodic archival timestamps so the proof can be renewed before hash algorithms age out). Each level addresses all the requirements of the levels below it, which is why they are a staircase rather than four separate options.

The signer's own software writes a claimed signing time from its own clock — routinely wrong on unsynchronized laptops. B-T adds a detached timestamp token from an independent time-stamp authority under RFC 3161: the authority binds a hash of the signature to a creation time (genTime, in UTC) and never sees the document itself, only that digest. The token is mechanical: the imprint must be returned exactly as received.

Under eIDAS article 42, a qualified electronic time stamp is one that binds data in a way that reasonably precludes undetectable change and is based on an accurate time source linked to UTC; article 41(2) attaches the presumption of accuracy and integrity to the qualified stamp. Non-qualified stamps remain cryptographic evidence, assessed under general rules.

Years after signing, the systems that answered 'is this certificate still valid?' — OCSP responders and CRL distributors — may no longer exist. A B-T file proves a lot on signing day and thinner every year after. B-LT addresses this with the Document Security Store: the certificate chain and the revocation information are embedded inside the PDF itself, so validation does not depend on outside services.

B-LTA goes further: before the embedded validation data's algorithms age out, the document is re-sealed with fresh archival timestamps. If evidence must survive a decade — litigation, audits, retention duties — this is the level to ask for.

Any PAdES-aware validator recomputes the facts from the file: Adobe Reader's signature panel, the European Commission's open-source DSS validator, or a command-line OpenSSL call. A validator that states the level — and does so honestly, at the level actually reached — is the one to trust; a seal that claims a level it did not reach is a claim, not evidence.