Help

What a stamp does

A stamp shows a file existed, exactly as it is, from the moment you stamp it. Later, anyone can check the file against its stamp record and see whether even one byte has changed.

A stamp does not show that what the file shows is true. And it can't reach back in time: it shows the file existed from when you stamped it, not before.

Stamping in three steps

1. On Stamp, choose the file. On a computer you can also drag it onto the page.
2. Tap Stamp it.
3. Keep two files together: the original file, and its stamp record (the file ending in .stamp.json).

Keep the original itself. A copy that has been re-saved, edited, or sent through a messaging app, email or social media is usually changed along the way, and won't match.

iPhone: stamping the original photo

When you choose a photo from your Photo Library, your iPhone gives the page the version it shows you, and for many photos that is a copy, not the file it keeps:

• Most iPhone photos are kept as HEIC files. Choosing one from the Photo Library gives the page a JPEG the iPhone makes on the spot. It's the same every time, but it isn't the file your iPhone keeps.
• For an edited or cropped photo, you get the edited version.
• For a RAW photo, you get a JPEG version, not the RAW file.

To stamp the original:

1. In Photos, open the photo and tap Share (the square with an arrow).
2. Tap Save to Files, pick a folder, and tap Save.
3. Come back to Stamp, tap to choose, and pick Choose File (not Photo Library). Find the photo in the folder you saved it to.

Before you stamp, look at what the page says you chose: the original shows as a phone photo (HEIC), with a name ending in .HEIC. Not yet tested: a photo you have edited (Save to Files may save the edited version), and the Share sheet's Options setting, which can change the format.

The same steps work for checking: choose the original from Files on the Check page.

A HEIC photo's stamp protects that exact file. Recognising the same picture in a different file works today for PNG pictures, and for JPEG photos whose picture wasn't re-compressed (for example, when only a detail like the capture time changed). A JPEG re-saved by an app is usually re-compressed, and won't match. We tested these steps on an iPhone with Safari in November 2026. Other phones, browsers and versions may behave differently.

Where is my stamp record?

After you stamp, the page saves the stamp record for you. Its name is your file's name followed by .stamp.json.

• iPhone: if Safari asks, tap Download. A small download arrow appears next to Safari's address bar; tap it to see the record. It's kept in the Files app, in Downloads. Or tap Save to Files or share… on the page after stamping, and pick the folder yourself.
• Android: in your Downloads.
• Computer: in your Downloads folder.

Missed it? The page has a Save the stamp record again button until you leave it.

Checking a stamp

On Check, choose both files: the file and its stamp record. Together or one at a time, in any order.

• Green: this is the exact file that was stamped, unchanged.
• Amber: not a full yes. Either the picture itself matches but this isn't the exact file that was stamped (for example, it was re-saved without changing the picture, or details stored with it, like a capture time, differ), or the page couldn't check something, such as whether L0GIC made the stamp. Read the sentence beside it.
• Red: it doesn't match, or the record isn't a genuine L0GIC stamp.

No stamp record for a file? You can stamp it now. A new stamp shows it existed from now on, not from before.

Choosing a file: what the page can see

When you tap to choose, your phone or computer opens its own chooser, not the page. The page receives only the file you pick, and nothing else from your photos or folders.

That file never leaves your device. It's fingerprinted right there, inside your browser.

What L0GIC receives

• Never your file.
• Its fingerprints: short codes worked out from the file. They can't be turned back into the file, though someone who already has the very same file could tell it's the one. L0GIC signs them together with the time.
• The file's name, only if you tick "Include the file's name". It's left out unless you do.
• To get a trusted time, L0GIC sends a fingerprint of the stamp (never your file) to independent time authorities.
• No account and no sign-in. No access log of visitors is kept. Your internet address is used to limit how many stamps and checks one address can make in an hour.

The full details are in the Privacy Policy, and the rules for using Stamp in the Terms of Use.

Report a problem

Email us. Nothing is collected automatically: only what you write is sent.

Email support@l0gic.ai

It helps to say which page (Stamp or Check), your device and browser, and what you saw. A screenshot of the page helps too. Please don't attach a private file; describe it instead.