What is PAdES? Electronic signatures in PDF files

PAdES (PDF Advanced Electronic Signatures) is the ETSI EN 319 142 standard for signing PDFs. We explain the B-B, B-T, B-LT, B-LTA profiles and how PAdES compares with XAdES and CAdES.

PAdES (acronym for PDF Advanced Electronic Signatures) is the set of profiles defined in the ETSI EN 319 142 standard for embedding advanced electronic signatures inside a PDF file. It is the format produced by FirmaEC desktop, Adobe Sign, DocuSign and firmar.ec.

Why PAdES and not something else?

Historically, signing a PDF “by hand” meant printing it, signing with a pen, scanning, and pasting an image. That approach has zero cryptographic validity: the image of a handwritten stroke proves neither authorship nor integrity.

PAdES solves this by embedding inside the PDF:

  1. A cryptographic signature (CMS / PKCS#7 SignedData) over the entire document content.
  2. The signer’s certificate and the chain up to the root CA.
  3. (Optional) A TSA timestamp proving the exact date of signing.
  4. (Optional) Revocation information (OCSP/CRL) embedded for long-term validation.

Any subsequent modification of the PDF — changing a word, a figure, a visual stamp — invalidates the cryptographic signature, and PDF viewers display a red warning to the reader.

The 4 PAdES profiles

ETSI defines four “Baseline” profiles in ascending order of robustness:

ProfileWhat it includesWhen to use it
B-B (Basic)Signature + cert + chainSimple use cases; validates while the cert is valid and the CA is online. This is what firmar.ec v1 produces.
B-T (Timestamp)Above + TSA timestampNeeded when it matters to prove when signing occurred. firmar.ec v1.1 (roadmap).
B-LT (Long-Term)Above + embedded revocationAllows verifying the signature years later even if the CA no longer exists or the cert has expired. firmar.ec v1.2 (roadmap).
B-LTA (Long-Term Archive)Above + periodic timestampsFor legal archives that must be kept for decades.

For most everyday use cases — contracts, invoices, letters, declarations — B-B is sufficient. It validates with any modern PDF viewer (Adobe Reader, Foxit, FirmaEC, Minka, etc.) while your certificate is valid.

How does PAdES differ from XAdES and CAdES?

Both are siblings of PAdES within the ETSI family:

  • XAdES (XML): signs XML directly. This is the format required by the SRI for electronic receipts.
  • CAdES (CMS detached): signs any file and leaves a separate .p7s file.
  • PAdES (PDF): signature embedded within the PDF itself.

Which one to use?

  • PDF → PAdES.
  • SRI XML → XAdES.
  • Any arbitrary file where you want to keep the original intact → CAdES.

What does a PAdES signature look like?

There are two components:

Cryptographic component (always invisible to humans)

A /Sig dictionary inside the PDF with fields /Filter, /SubFilter, /Contents (the CMS DER signature), /ByteRange (which bytes are covered), /Cert, /M (signing time), optional /TS (timestamp).

Visible stamp (optional)

A “stamp” on a specific page that shows the human reader: signer’s name, role, date, reason (“Approved”, “Reviewed”), and optionally a QR code linking to a verifier. firmar.ec generates the QR pointing to app.firmar.ec/verificar?h=<hash>.

Algorithms: what crypto does PAdES use?

ETSI TS 119 312 maintains the list of current algorithms. Today the following are used:

  • Hash: SHA-256 (minimum), SHA-384 or SHA-512.
  • Asymmetric signature: RSA with key ≥ 2048 bits, or ECDSA P-256/P-384.
  • Padding: RSASSA-PKCS1-v1_5 (legacy interoperable) or RSA-PSS (modern preferred).

firmar.ec explicitly rejects SHA-1, MD5, and RSA<2048: if your certificate still uses those, it was issued against policy and your ECI should have renewed it.

Standards and references

  • ETSI EN 319 142-1 — PAdES Baseline (text)
  • ETSI EN 319 102-1 — AdES validation procedures
  • ETSI TS 119 312 — Current cryptographic suites
  • RFC 5652 — CMS Cryptographic Message Syntax
  • RFC 7292 — PKCS#12 Personal Information Exchange
  • ISO 32000-2 — PDF 2.0 (signatures section)

Verification: how do I know a signed PDF is valid?

You can:

  1. Open the PDF in Adobe Reader or Foxit Reader — they show a signature panel with signer details.
  2. Upload it to firmar.ec/verificar (our top recommendation: free, in your browser, validates the chain up to ARCOTEL roots + OCSP).
  3. Use Minka by MINTEL (official validator).
  4. The SRI validator for XAdES (not PAdES).

All these tools produce the same result for a valid signature: they run the same set of checks because they all follow ETSI EN 319 102.