What is PAdES — and why PDF signatures 'break' in Acrobat

A PDF is a pile of objects. PAdES (PDF Advanced Electronic Signatures, ETSI) inserts signature bytes covering (parts of) the document: a hash, encrypted with a private key, plus a certificate chain. Open the file later and software can recompute the hash and see if it matches.

That is integrity. It is not automatically BankID-level identity. The key may belong to a platform CA. Then the stamp says 'this file is untouched since we signed it', not 'Kari from the registry stood here'.

Without PAdES you can still have SES: a picture of a squiggle and a log in the cloud. Swap a page and the pictures keep lying. The log may tell the truth if it still exists. The file itself stays quiet.

A visible signature field is not the proof. It is the sign. The proof is the bytes behind it. People photograph the sign and think they archived.

Acrobat keeps a trust list. Qualified and commercial CAs paper and pay their way in. A house CA in a small product does not. Reader then says identity cannot be verified, or the signature is unknown.

People translate that to 'broken' or 'void contract'. That is Adobe UX, not contract law. The same file can be verified against the issuer's public certificate and a SHA-256 you got on the receipt.

Other reasons for red: someone opened the file in an app that saved again and smashed the bytes; someone added comments after signing; someone printed and scanned. Then the maths is actually broken. That is a different story from a yellow CA.

Different readers show different icons. Preview, Chrome, Acrobat, a phone. Agreement on a hash beats agreement on a colour.

The squiggle is for humans. It can be SES alone. The crypto is for programs. You can have both. You can have crypto with no ink look. You can have ink with no crypto — the usual scanned sheet.

Platforms that offer 'CRYPTO' beside 'SES' often mean: we bake PAdES on top of the person drawing. The level against the person is still email, unless they wired in eID.

Do not call the crypto QES. QES needs a qualified certificate at a supervised issuer. A platform key with your name in a field is not that.

Do not repair a red PAdES by pasting in a picture of the old squiggle. Then you built SES theatre on top of a crypto break.

Email gateways that 'optimise' PDFs. Print to PDF again. Phone preview that saves a copy. Dropbox generating a preview file. Anything that creates new bytes kills a strict PAdES check.

The fix is to send the original, and to say 'do not open-save in Word'. It is boring and it works. An evidence page with a hash on the side is a safety net for when someone inevitably does it anyway.

Timestamps (TSA) help with 'the certificate was valid then'. They do not fix a compressor. Layers are layers.

Download the signed PDF. Compute SHA-256. Compare to the hash on the receipt or a verification wall. Open certificate details if you can: issuer, fingerprint, time.

If the vendor has a public verify link, use it as a second opinion — not as the only one, and not if you already suspect the vendor. Two independent checks beat one icon.

Sign This, Dammit! is blunt about this: not QES, not BankID, cryptographic integrity against their CA, ads on the wall. That is a checkable trail, not an Adobe membership.

When both sides only need a yes in a log and the file is three pages about a job next week. Cloud SES can be enough. Say where the log lives.

When you need QES, you care about eID and issuer first. PAdES may come along, but it is not what you are shopping for in the BankID queue.

When the other side is going to print anyway. Then crypto in the PDF is theatre. The law sits in the fact they signed. Paper is their problem. Your problem is sending one true file first.

A PDF can carry several signatures one after another. Each new signature covers what was already there, including the previous stamp. That is why order in the file and order in the envelope are not always the same story. Read the evidence pack for the humans, PAdES for the bytes.

If someone flattens the PDF between two signatures, the chain dies. Print-to-PDF is a flatten. Say that to people who just wanted to make it smaller.

Long-term validation with timestamps and revocation is a separate hole. Small teams pretend they have LTV when they have a file in Dropbox. They have a file in Dropbox. Say so.