Signing a document from a phone

You are standing at a site. A file arrives on your phone with a message saying please sign. You put your thumb on the screen, draw something that looks roughly like your name, and send it back.

What just happened, and is it worth anything six months from now?

The answer depends entirely on which of three quite different things the app did, and most people signing have never been told which one they are using.

Three things called a signature#

A photograph of a wet signature, pasted onto a page. Somebody once signed a blank sheet, it was scanned, and now that image is dropped onto documents. This proves nothing about this document. The image can be lifted off one page and placed on another by anyone with the file, and it carries no information about when it was applied or by whom.

A mark drawn on a touchscreen. You actually signed, at that moment, with your own hand. The mark is genuine as an act. Whether it is attached to anything depends on what the app recorded around it — who was logged in, when, from where, against which version of the file. Drawn on paper with a witness present, a signature draws its strength from context. On a phone the context has to be manufactured deliberately, and many apps do not bother. A drawn mark stored as a picture on a page is only slightly better than the pasted photograph.

A cryptographic signature. The document is reduced to a fixed value computed from its exact contents, and that value is signed with a key held by one person. Change one character of the document afterwards and the value no longer matches, so the signature no longer verifies. This is the only one of the three that binds a specific person to a specific set of bytes and detects any later change to them. The general principle is in tamper-evident documents.

What each of these counts for in a dispute is a question of law, and law differs by jurisdiction and changes. Nothing here is legal advice. If you are about to rely on an electronic signature for something that matters, verify the position in force where you are with your own advisor, and ask specifically which of the three your system produces — the vendor will usually answer that question honestly if you ask it precisely.

Signing something at site#

You will not always have a choice about the tool. You do have a choice about how you use it.

Read the whole document, not the page you were sent. A single page photographed and forwarded is not a document. It is an extract chosen by somebody else. Ask for the file.

Check the page count. If it says one of nine and you have one page, you have not read eight pages of a thing you are about to be bound by. This is the most common way people sign a scope they never saw.

Keep the version you signed. Not the sender's promise to send it afterwards — the actual file, downloaded to your own storage, on the day. If the only copy of the signed document lives in the other party's system, then the signed document is whatever their system says it is. That principle is the whole of keeping your own copy.

Note the date and how it reached you. The message that carried it, from whom, at what time. Keep the message. The covering note often says something the document does not — as discussed, rates unchanged — and that note is part of the record.

Do not sign a scope you have not read because the sender is waiting. Urgency is the standard technique for getting signatures onto documents that would not survive a slow reading. A work order deserves the attention described in what a work order should say whether it arrives on paper or as an attachment.

If you are the one asking for a signature#

The obligations run both ways, and a supplier who collects signatures from site staff carries most of them.

Send the whole file. Every page, in one document, not a page at a time as questions arise.

State plainly what is being signed. "Please sign" is not a statement. "This is the acknowledgement of quantity received today against your order, four pages" is. A person who signs without knowing what they signed will say so later, and they will be believed.

Make the signed copy available immediately. Both sides should hold the same file, from the same minute. A signed document that only one party has is a document the other party can plausibly dispute.

Never allow a signature to be applied to something that can afterwards change. This is the one that matters most and gets ignored most. If the document is a record in a database and the signature is a flag on that record, then editing the record after the signature leaves a signed document that nobody signed. The fix is not a policy telling people not to edit. The fix is that the signed thing is a frozen copy, and any change produces a new version requiring a new signature — with the old one still visible. Why the history has to survive is the argument in what an audit trail is for.

Do not let one login stand for two people. Where a document needs two approvals, the system must know that two distinct people signed it, at two times, from two accounts. A shared login collapses that into one act, and two signatures on money explains why the second signature exists in the first place.

How ours works#

Since we ask people to sign from phones, here is what we do.

A document on a task goes to the people on it and to named outside vendors, each with an unguessable link — a vendor's arrives on WhatsApp — signed with a finger on their own phone. A saved mark can only be applied by a session signed in as that person: not by an administrator, not by a helpful colleague. That refusal is what makes storing one safe.

Marks are drawn first and every page stamped afterwards, so the fingerprint each page carries belongs to the signed document, not the blank one. A round can be stopped mid-flight with a reason, killing every outstanding link; a completed one cannot. Asking again on a sealed document makes a fresh copy rather than overwriting what was vouched for.

Verifying, from paper#

Most of this still ends on paper. Somebody prints the signed document and files it, and the printed copy is what gets produced in a meeting.

A printed page carries no proof of anything by itself. What makes it checkable is a reference on the page that can be looked up independently — a document number and a short verification code, or a code that resolves to the stored version. Then the copy in a file can be held against the copy in the system, and if they differ, that fact becomes visible instead of being argued about.

Without something of that kind, a printed electronic signature is just an ink pattern on paper, and the person disputing it has only to say the file was different when they saw it.

The short version#

Three quite different things are called signing on a phone: a pasted image, a drawn mark, and a cryptographic signature that binds a person to exact bytes. Only the third detects a later change, and what any of them counts for legally is something to verify where you are, with your own advisor.

Read the whole file, check the page count, keep your own copy of what you signed, and never let a signature sit on a document that can quietly change afterwards.