E-signatures · Glossary
What is PAdES?
PAdES, short for PDF Advanced Electronic Signatures, is the ETSI standard EN 319 142 for embedding a cryptographic signature inside a PDF so any reader can verify it. It defines how the signature is stored in the file, what part of the document it covers, and how a timestamp and revocation data are attached to it.
A PAdES signature is not a picture of a signature. It is a block of cryptography written into the PDF itself, covering the bytes of the document, so that any change made afterwards can be detected by the reader rather than argued about later.
Nuwan Madhusanka · Co-founder
4 min read · Published
| Level | What it adds | When you need it |
|---|---|---|
| B-B | The signature itself, covering the document bytes | The baseline: proves the file has not changed since signing |
| B-T | A trusted timestamp over the signature | Proving when it was signed, not just that it was |
| B-LT | Certificate and revocation data stored in the file | Verifying years later, when the certificate may have expired |
| B-LTA | An archive timestamp over all of it | Long term archives, where the crypto itself may age out |
What is actually inside the PDF
Four things. A CMS signature block, which is the cryptography defined in RFC 5652. The signing certificate, so a reader knows who the key belongs to. A hash of the document bytes the signature covers. And, when one is used, an RFC 3161 timestamp token attached to the signature as an unsigned attribute. In a PDF the whole block sits under a signature dictionary with the filter Adobe.PPKLite and the sub-filter adbe.pkcs7.detached, which is the name you will see if you inspect the file.
Certified against approval signatures
An approval signature says this person signed. A certified signature also says what anyone is allowed to do afterwards, through a permission called DocMDP. At permission level one, no changes are allowed at all: fill in a form field and the signature breaks. That is the level to use for a finished agreement, and it is why the certification step has to be the last thing that touches the file. Anything appended after it, including an audit certificate, has to be appended first.
How to tell whether there is a timestamp
The recorded algorithm string says so. PAdES-SHA256-DocMDP means a certified signature with no timestamp. PAdES-SHA256-DocMDP-RFC3161 means a timestamp token was attached. That difference matters, because without a timestamp the only evidence of when the document was signed is the signer's own clock, which is not evidence at all in a dispute.
Where it sits against eIDAS
PAdES is a format, not a legal status. The eIDAS Regulation defines advanced and qualified electronic signatures in law, and a PAdES signature is the usual way an advanced one is carried in a PDF. To be qualified it also needs a qualified certificate from a trust service provider on the EU trusted list, which is a procurement decision rather than a file format decision.
Questions people ask
Is PAdES the same as a digital signature?
PAdES is one way of carrying a digital signature, specifically inside a PDF. Digital signature is the general cryptographic idea; PAdES is the standard that says where in the file it goes, what it covers and how timestamps attach. Other document types use sibling standards: XAdES for XML and CAdES for arbitrary data.
Is a PAdES signature legally qualified?
Not by itself. Qualified is a status defined by eIDAS that depends on the certificate, which must come from a qualified trust service provider on an EU trusted list, and on the device that holds the key. A PAdES signature made with an ordinary certificate is an advanced electronic signature, which is enough for most commercial agreements.
Does PAdES need a timestamp?
The baseline level does not. Add one and you move from B-B to B-T, which is what gives you independent evidence of when the signing happened rather than relying on the signer's computer clock. If the date could ever matter in a dispute, a timestamp is the cheapest insurance in the whole process.
Can a Word document use PAdES?
No. PAdES is defined for PDF files. Office documents use their own signature formats, and XML uses XAdES. In practice the usual answer is to export to PDF and sign that, which is also what makes the signed artefact something anybody can open and verify without the original application.
What is B-LTA for?
Long term archiving. It adds an archive timestamp over the signature, the certificates and the revocation data together, so the whole package can still be validated in ten or twenty years, after certificates have expired and possibly after the original hash algorithm has weakened. For a two year commercial contract it is more than you need.
Why does my reader say the signer is unknown?
Because the certificate is not chained to a root your reader trusts. The signature can still be cryptographically valid and the document still unmodified; what the reader cannot do is vouch for who the key belongs to. Adding the issuer to the trusted list, or using a certificate from a provider already on it, resolves it.
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Send a document for signingRelated questions
- What is an RFC 3161 timestamp?An RFC 3161 timestamp is a signed statement from an independent authority that a document existed at a moment. How it attaches to a PDF signature.
- Electronic signature against digital signatureAn electronic signature is any mark made with intent to sign. A digital signature is cryptography. One is a legal category, the other is a technique.
- What is signing order?Signing order sends the document to one person at a time, in sequence. When sequential signing matters and when it just slows everything down.
Step by step in the builder: Send a PDF for e-signature.
Written and checked by the OneCraft team. Last checked .