Vault apps and SD cards: why storing private photos on the card is risky

Memory cards made sense when phones had little storage, and many Android vault apps still offer to keep hidden photos on one to save space. It sounds harmless: the files are hidden, after all. But a card is a small object that leaves the phone easily, and once it’s in another device, the vault app’s lock isn’t there to stop anyone.

Why the card is different from the phone

Your phone’s internal storage is protected in two ways. Modern Android phones encrypt it, with keys tied to your screen lock, and apps are walled off from each other. Since Android 11, apps can’t reach files inside another app’s private folders at all.

A memory card used as portable storage has neither protection by default:

• It isn’t encrypted. Pop it out, put it in a laptop’s card reader, and every file on it can be opened.
• It’s easy to lose or take. A card can be removed in seconds, often without turning the phone off.
• It outlives the phone. Cards get moved to a camera, given away or sold with an old phone.

Android’s own developer guidance warns apps against putting sensitive data on external storage for these reasons, and notes that files on a removable card may disappear when it’s taken out.

What some vault apps actually do on the card

Vault apps that store on the card tend to fall into three groups:

1. Renaming. Photos get a new name or extension, or move into a folder whose name starts with a dot so galleries skip it. Any file manager or computer shows them.
2. Scrambling the start of the file. A few bytes are changed so the photo doesn’t open by double-click. That’s easy to reverse.
3. Real encryption. The file is encrypted with a key that never leaves the phone, or is derived from your PIN. Without the key, the file is noise.

Only the third protects you once the card leaves your phone. The difference between hiding and encrypting is explained in how photo vault apps work.

How to check your own vault

You can test it in a few minutes, without trusting the app’s description.

1. Add a test photo you don’t mind showing to the vault.
2. Turn off the phone and take out the card, or browse the card with a file manager.
3. Look for a folder named after the app, a dot-folder, or files with unusual names or sizes.
4. Try to open them on a computer. Rename one to end in .jpg and try again.

If your test photo opens, the vault is hiding, not encrypting. The checklist in are photo vault apps safe covers more tests like this, and calculator vault apps looks at a type of app that often relies on hiding.

Safer options

• Keep private files in internal storage, where Android’s encryption and app separation apply.
• If you need the card for space, use it for things that aren’t private: music, maps, ordinary photos.
• Adoptable storage, offered on some phones, formats the card as encrypted internal storage tied to that phone. It can’t then be read elsewhere, but also can’t be moved to another phone without formatting.
• Encrypted backups are fine on a card, because the card only ever holds ciphertext.

If you’ve been storing on the card

Move the files into an encrypting vault, check they open there, then delete the old copies from the card. Formatting the card afterwards removes the leftovers. If you sold or gave away a card in the past, assume its old contents could be read.

Silo, coming soon to iPhone and Android, keeps its vault in the app’s own internal storage, encrypted on the phone with XChaCha20-Poly1305 before anything is written. It doesn’t store files on a memory card, and its file format is published so anyone can check the claim.