How it works
When your product generates a picture, SignUnder marks it as made with AI and as yours, in three ways at once. If the picture travels the internet and loses one mark, the others still lead back to you, and anyone can check it on your public page.
Why it matters
A picture made in your app gets shared, saved, forwarded and screenshotted. A few steps later nobody can tell it was made with AI, or by whom. Laws now say the company that offers the generator has to fix that, and the platforms label what they can detect.
A customer asks for an image and your product generates it, on your own model or a hosted one.
Posted, downloaded, re-uploaded, compressed, cropped and screenshotted along the way.
Without a mark, there is no answer. With one, there is, and it names you.
Since 2 Aug 2026
EU law requires AI-generated images to be marked in a way machines can detect. Generators already on the market before then have until 2 December 2026. The law, read closely
It falls on you
The duty sits with the company that offers the generator under its own name, not with the model or the hosting platform. Ten of the eleven platforms we read mark nothing at all. The eleven platforms
Platforms are reading
TikTok, YouTube and Meta read Content Credentials when an image is uploaded and show their own AI label on it. What each platform does
What SignUnder does
Each mark covers a weakness in the others. Together they make a picture hard to separate from where it came from, without changing how it looks.
Like a signed delivery note attached to the file.
It says the picture was made with AI, which company published it and when, and it is signed so nobody can change it without it showing. This is the industry standard called Content Credentials (C2PA). TikTok, YouTube and Meta read it when an image is uploaded and show their own AI label.
Its limit: It lives alongside the picture, not in it, so it can be stripped off.
Like the watermark in a banknote, but invisible.
A short code is woven into the pixels themselves. It does not change how the picture looks, and it points back to the label, so a copy with the label stripped off can still be matched to it.
Its limit: It is a code, not a description: it needs our check to read it.
Like a description of a face, kept on file.
We record a compact summary of what the picture looks like. If a copy turns up with no label and no readable watermark, a near-identical picture can still be matched to your record.
Its limit: Cropping or screenshotting changes the look enough to defeat it.
Try it
Pick what happens to the picture and see which marks are still there afterwards. The watermark is the one that keeps going.
What happens to the picture?
All three marks are in place. Anyone with a Content Credentials viewer can read the label directly.
The watermark and fingerprint results are our own test of 13 October 2026 on four photographs: a small sample, and we will publish more as it grows. What each platform does to the label is still being measured; the platform pages say what is known so far.
Checking a picture
Every picture you sign is kept on record, without storing the picture itself. Your public verify page answers one question for anyone who asks: is this one of yours?
A journalist, a moderator, a customer, or a platform.
We look for the label first, then the watermark, then the fingerprint.
Yes, with who published it, when, and whether it was changed. Or: not one of ours.
Yes, this is one of ours
Found by its invisible watermark
The Content Credentials were stripped, but the invisible watermark in the pixels names the record.
In your name
A label is only as useful as the name on it. If your image supplier signs your pictures, viewers show their name, and your customers see someone else’s brand on your product.
SignUnder names your company in every signature, once you have proved you own your web domain. And if you want the certificate itself in your name, we take you through the industry’s conformance programme to get one.
Signed by your supplier
True, but it tells your customers who you buy from, not who published the picture.
Signed with SignUnder
Your company, your domain, checked by us before your name goes on anything.
Where we are today: SignUnder is going through the C2PA conformance programme. Until it is listed, signatures are valid but public Content Credentials viewers show them as “unrecognised”. Why the name matters
Which rules apply
We keep a rulebook for each place that has AI labelling law, and for the big platforms’ labelling policies. Every rule quotes its source and the date we read it, and we check the sources every week. The free checker tells you what applies to you.
Getting started
Three steps, and only the last one touches your code.
Sign in with your email, name your company and get an API key.
Add one DNS record so we know acme.com is yours. We approve your company name by hand.
Your app sends each picture to SignUnder and serves what comes back. On fal or Replicate, you can route their API through us instead.
SignUnder is being built now, in the open. Leave your email and we will tell you when the free checker and the signing API are live, and nothing else.
One address, used only to tell you when SignUnder is ready. No newsletter, no sharing with anyone, and you can have it deleted at any time by asking. Stored in Frankfurt; see the privacy notice.