What is AES-256? Encryption explained for phone users
AES-256 is a published standard for scrambling data with a 256-bit key. GCM is a mode of using it that also detects tampering. The cipher is not the weak point of a vault; the key is. What decides the strength in practice is your credential, how it is stretched, and where the key is kept.
The three parts of the name
- AES is the Advanced Encryption Standard, a cipher: a recipe that turns readable data into scrambled data and back, given a secret key. It is public. Anyone can read how it works, and that openness is part of why people trust it.
- 256 is the size of the key, in bits. A 256-bit key is a number so large that trying every possibility is not a realistic plan. We do not put numbers on it because the number is not the interesting part.
- GCM is the mode. A cipher by itself works on small blocks of data; a mode says how to apply it to a whole photo, and what else to check. GCM both encrypts and attaches a tag. If anyone changes even one bit of the stored data, the tag no longer matches and the data is rejected instead of opened.
That last point matters for a vault. Without it, someone who cannot read your files could still damage them in ways you would not notice. With it, a change is detected.
Who uses it
AES-256 is not exotic. Apple's documentation says the iPhone's Data Protection creates a new 256-bit key for every file on the data volume, and that a dedicated AES engine on the path between flash storage and memory does the encryption. StrongRoom uses the encryption built into iOS (Apple's CryptoKit framework) for its own layer on top of that: AES-256-GCM for every item, its thumbnail and the index that lists them.
We use a published standard because nobody should have to trust an invented one. We do not use grade labels. Phrases borrowed from defence or finance say nothing about how an app is built.
Why the key matters more than the cipher
Think of a safe: the metal matters less than who has the combination. Attacks tend to go after the key rather than the cipher, and there are three places a weak design leaks it.
- Where the key comes from. If it is made from something short, such as a four-digit number, an attacker never needs to break AES; they try all the short possibilities. Your credential has to be hard to guess.
- How the credential becomes a key. A slow function makes each guess cost real time. StrongRoom stretches your pattern or passphrase with PBKDF2 and then mixes the result, with HKDF, with a Device Secret from the Secure Enclave.
- Where the key is kept. A key stored next to the data is no protection. StrongRoom's Master Key is random and never stored on its own, only wrapped by a key made from your credential and the Device Secret. Each item then has its own key, kept in the encrypted index.
So "AES-256" tells you which cipher a vault uses, not whether the vault is sound. A vault could use AES-256 and still be weak if its key comes from a four-digit code or sits beside the file. When an app says only "AES-256" without saying where the key comes from, you have learned the cipher and nothing else.
AES-256 and your phone passcode
They are different layers. The iPhone's own protection is Apple's, and it is what Apple's pages above describe. AES-256-GCM in StrongRoom is a second layer inside the app, with keys that come from your Vault credential. Your phone passcode does not open a Vault, and a Vault credential does not unlock the phone.
What it cannot do
Encryption protects stored data. It does not help while a Vault is open on screen, against spyware on the phone, or for an item you export as an ordinary file. See the limits page.
Keep reading
- Does a photo vault really encrypt your files?
- What is the iPhone Secure Enclave?
- How StrongRoom encrypts, feature page
- Is a vault app really encrypted?
- What StrongRoom does not protect against
Frequently asked questions
Is AES-256 secure?
It is a published, widely used standard and is not the weak point of a well-built system. How secure a particular app is depends on its key, its credential and where the key is kept.
What does GCM add?
It detects tampering. If stored data is changed, it is rejected rather than opened.
Is AES-256 the same as end-to-end encryption?
No. AES-256 is a cipher. End-to-end encryption describes who holds the keys when data moves between people or devices.
Does StrongRoom use AES-256?
Yes, AES-256-GCM, for every item, thumbnail and the index, using the encryption built into iOS.
Sources
- Data Protection overview (Apple Support), checked on 2026-10-30.
- The Secure Enclave (Apple Support), checked on 2026-10-30.