Signing a document, and what that really proves

A signature on a PDF is not a picture of your handwriting pasted onto the last page. It is a seal, inside the file itself, that the reader checks on its own — and it answers two questions nobody could answer before: who sent this, and has anything been changed since?

A scanned signature proves nothing at all

Anybody can lift a picture of a signature off one document and drop it onto another. Nothing in the file says who did it, and nothing says whether an amount was altered afterwards. Whoever opens it sees a picture, and a picture is all it is.

An electronic signature is a different thing entirely. It is worked out from every single byte of the document. Change a comma, a date or a figure, and the seal no longer matches: the reader says so in a banner across the top of the page, before anyone has read a word.

The two things a signed document proves

• Who sent it. The seal names the company or the person behind the document, and somebody else checked that name before handing the seal out.
• That nobody has touched it. Not one byte has moved since it was sealed: not the amount, not the date, not the small print at the bottom.

That is what makes a signed quotation, contract or invoice worth sending. The person receiving it does not have to take your word for it, nor trust the email it arrived in.

European law says as much. Under the eIDAS regulation of 23 July 2014, a signature may not be turned down in court merely for being electronic, and the strongest kind carries the same weight as a signature written by hand.

What a certificate is, and why it is yours and not ours

A certificate is your identity written in a form a machine can check. You get one from a body whose trade is verifying who you are: the same idea as an identity card, issued by somebody trusted rather than printed at home. It comes with a private key — the one thing that makes a seal yours, and the one thing you must never hand to anybody.

So the certificate stays where it is. The engine never asks for it, never reads it, and never sends it anywhere. At the moment of sealing it hands the document to your own code, and writes back whatever comes out. Your key does not travel, because nothing here is able to make it travel.

The fiddly part, and what the engine takes off your hands

Sealing a PDF is awkward in a way that has nothing to do with your business. The seal has to sit inside the very file it covers, so the file must state, byte by byte, which parts of itself the seal accounts for and which single part it does not: the hole the seal itself goes into. One byte out either way and every reader in the world rejects the document, with a message nobody can do anything about.

The engine does that part. It reserves the room, works out the exact extents, hands your code the whole file bar that hole, and writes the answer back in place without moving a single byte. What it looks after:

• A signature area placed on the page like any other box, with a frame around it — or no area at all, when the seal is meant to leave no mark.
• Either of the two names a seal may go under in the PDF standard, including the one European rules are written around — the one a public body has to accept.
• Sealing a PDF that already exists, even one written by somebody else's software: the original file is kept exactly as it was, and the seal is added after it.
• A second signature after the first, so two people can approve the same document without the first seal being disturbed.
• A seal that also states what may still be changed afterwards: nothing at all, filling in the boxes of a form, or adding notes.

What it does not do, and why that is the right answer

The engine holds no key, issues no certificate, calls no authority and reaches no network — it cannot: there is nothing inside it that opens a connection. It does not add a trusted timestamp, and it does not gather the proof that a certificate was still valid on the day. Those belong to whoever runs the sealing, and that is on purpose: the moment a library asks for your private key, your key is somewhere you did not put it.

Which means one thing has to be said plainly. The engine prepares the document and writes the name of the format your seal claims; the seal itself is made by your own code. Whether the finished document satisfies the European format down to its last requirement depends on what that code produces, and that is not ours to promise.

Where this fits

Sealing means holding your key, so it belongs where you run the engine yourself: inside your own program with the library, or on your own machines with the server. Nothing leaves your walls, and no key is ever copied anywhere.

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