AES-256 encryption explained for photo vault apps

What the name means

AES stands for Advanced Encryption Standard. It is a cipher: a fixed recipe that mixes data with a key so that only the same key can unmix it. NIST’s standard names three sizes, AES-128, AES-192 and AES-256, and says the cipher works on data in blocks of 128 bits. The number in the name is the length of the key.

The recipe is public on purpose. Anyone can read it, test it and write their own copy. The safety comes from the key, not from keeping the method quiet. That is a good sign in a vault: an app that won’t name its cipher gives you nothing to check.

What a 256-bit key means in practice

A 256-bit key is a number picked at random from a range so large that trying every value is out of reach for any computer that exists. In a photo vault, that means one thing: nobody gets your photos by guessing the key. Your vault key is a random 256-bit key. It is not made from your passcode.

They get them some other way. The rest of this page is about those other ways, because that is where vaults differ.

The mode matters as much as the cipher

AES on its own scrambles one block. To scramble a whole photo, an app runs it in a mode, and modes differ in one key point: does the result also prove that nobody changed it?

GCM, described by NIST, is an authenticated mode. Each encrypted file carries a short tag, and if a single byte is changed, the tag no longer matches and the app refuses the file. Older modes scramble without that check.

Sealed encrypts every file with AES-256-GCM before it is saved. Large files are encrypted in chunks, and Sealed refuses a file that has been changed, reordered or cut short.

So when a vault says “AES-256”, the follow-up question is “in which mode?” A vault that names an authenticated mode, such as GCM, has thought about tampering. One that names no mode leaves you guessing.

What AES-256 can’t do

A cipher protects bytes at rest. It does nothing about the following, and these are the real risks.

• Where the key is kept. If the key sits in a file next to the photos, anyone who copies the folder copies both. Ask where the key lives. In Sealed: Your vault key is kept in the iPhone Keychain, on this device only, and it can be read only while the phone is unlocked. Without Maximum Security, the app can read your vault key whenever the phone is unlocked, because the key sits in the Keychain.
• The passcode in front. Someone who knows your vault code opens the app the normal way, and the app decrypts for them. Your passcode is a gate in front of the app. It is not part of the key.
• Copies outside the vault. The original in your camera roll, a photo sent in a chat, a screenshot: none of these were ever encrypted by the vault. Until you delete them, the originals stay in Photos, outside the vault.
• What the app shows on screen. Once a photo is open, it is a picture on a display.

This is why a vault page that leads with the cipher’s name and says nothing about keys tells you little. The five checks for any vault cover the rest.

How to check a vault’s claim

1. Look for the mode. AES-256-GCM, or another authenticated mode, is a real answer. “Bank-level encryption” is not.
2. Look for one key per file. A fresh key for each file means a key taken from one file opens only that file. The vault key that seals those file keys still opens them all, so where that one key is kept matters most. Each file gets its own random 256-bit key, and that key is sealed with your vault key.
3. Ask if names and notes are encrypted too. File names and captions can say as much as the photo. Names, notes and the fields of a record are sealed with your vault key too, not just the photos.
4. Look for a written spec. A vault that publishes its formats lets others check the claim. Sealed’s is on the encryption specification page.
5. Check the backup. An encrypted vault with a plain backup is a plain vault. Files, names, notes and record fields are encrypted on your iPhone before they are uploaded, so iCloud receives only ciphertext.

Where Sealed helps, and where it doesn’t

It helps with an authenticated cipher and a key for each file. Sealed encrypts every file with AES-256-GCM before it is saved. Each file gets its own random 256-bit key, and that key is sealed with your vault key.

It helps with a written format that others can test. The details are on the specification page.

It doesn’t protect a photo once you export it, or the original you left in Photos. Until you delete them, the originals stay in Photos, outside the vault.

It doesn’t stop a person who knows your vault passcode. Your passcode is a gate in front of the app. It is not part of the key.