Why previews and small thumbnails need encrypting too

Updated 3 November 2026 · By the FirstHour team

What a thumbnail is

When you scroll through a grid of pictures, your iPhone doesn't load every full-size photo. It shows small copies, called thumbnails or previews, and loads the big one when you tap. Those small copies have to be saved somewhere to be quick.

Think of a safe with a reinforced door, while a photocopy of its contents sits in a drawer beside it. The door is only as good as the drawer.

Why a small copy still gives things away

• You can still recognise it. A card, an ID, a face or a document is easy to spot even at small size.
• Large print can still be read, such as a name or a number at the top of a page.
• Names and dates tell a story too. A file called "Passport.pdf" or a note's first line says what's inside before anyone opens it.

What FirstHour seals

• Every preview, kept as its own sealed file with the same AES-256-GCM key as the picture. A picture's preview is cut down to about 600 pixels on its longest side, then sealed. A video's preview is sealed too.
• The list of items. Names, dates, kinds, labels and sizes sit in one sealed list, so the files outside don't say what's inside.
• The Vault's small settings, such as how you've sorted it, and whether a decoy exists.

A PDF's preview is made by FirstHour itself, then sealed. For other kinds of file, the tile is a plain icon showing the file type, not a picture of the file. That's on purpose: Apple's own thumbnail maker can keep copies outside the Vault, and FirstHour can't control that. Notes get a plain tile too.

While the Vault is open, previews are opened in memory to show you the grid. Search reads your notes only while you search, in memory, and saves no list.

Where previews can still leak

• The number of sealed files. Their number, sizes and dates aren't hidden from someone who copies them off your iPhone, even though their contents are.
• The original, if it's still in Photos. Moving a copy into the Vault doesn't change it, and anything Photos keeps for it stays too. The original goes only if you let iPhone remove it, so check Recently Deleted in Photos too.
• iPhone's Quick Look. To show some kinds of file, FirstHour hands a temporary unlocked copy to Apple's viewer. What that viewer keeps while it's showing the file is outside FirstHour's control.
• An open Vault. Anyone looking at your screen sees what you see.
• Copies you make. A screenshot, or a picture you save back to Photos or share, is no longer sealed.
• Other apps. Any app that keeps its own readable saved copy of thumbnails is a weak point. That's one thing to ask when you compare vault apps.

Questions