The listing will not tell you this
App Store pages for photo vaults tend to say the same things: private, encrypted, on your iPhone. Some of those apps upload a copy. Some only hide a folder inside the Photos app. Some lock with the same Face ID that already unlocks the phone.
Before you import anything, check three things. The privacy policy and the first launch screen are enough for most of it.
Do the photos leave the phone?
Ask where the file goes after import.
If the app needs a network connection to show you your own pictures, the pictures are stored somewhere else. The same is true of "view on the web," syncing the images to a second device through the company's servers, or a support form that asks you to attach the photo. iCloud Photos and most private-cloud albums work this way. The lock can be real, and a server copy can still exist.
The other model keeps the file on the phone and encrypts it there. The company has no photo server to hand over. That is the check that matters for the pictures themselves.
"No servers" is a stricter claim, and plenty of on-device vaults still fail it. Analytics and subscription code often call home. Those calls are not your pictures. They are also not "nothing leaves the phone." A privacy policy that names what is uploaded (the photo, the filename, a thumbnail, or only a receipt and anonymous events) is the one you can use. "We take your privacy seriously" is not an answer.
Does it ask for an account?
An account is an identity next to the photos. Email, a phone number, or Sign in with Apple means the company can attach that vault to a person, reset access from their side, and keep the library available after you delete the app.
No account is the other tradeoff. There is nothing for them to reset, and nothing for you to recover if the phone is gone and you no longer have the backup password. For photos you do not want tied to a login, that is usually the better deal. It is a worse deal if you wanted a spare copy in the cloud.
The first screen tells you. If the library will not open until you create a login, the photos are attached to an account.
Is the lock the phone's lock, or a separate one?
The iPhone Hidden album is the common version of the first kind. Hiding a photo takes it out of the main grid. Opening Hidden still uses the Face ID or passcode that unlocks the rest of the phone. The file stays in the Photos library, and with iCloud Photos on, it syncs with that library. Fine for keeping a picture out of a casual scroll. It does not add a second secret. The longer version of that comparison is in why Hidden is not an encrypted vault.
A separate vault does two further things. The photo leaves the Photos app, so it is not in the camera roll and it does not follow iCloud Photos. Reading it takes a key that is not the phone passcode: its own PIN, or biometrics you can turn off. The encryption has to be on the file. A folder of ordinary pictures with a passcode in front of the list is only a lock screen.
How Privately scores on the same checks
Photos. Each photo you import is encrypted on the iPhone with AES-256-GCM. There is no photo upload, and we cannot open the files. Two other connections exist, and neither one carries a photo: anonymous usage events through Firebase (no advertising identifier, no filenames) and subscription status through RevenueCat, so the app can tell whether Pro is active. A phone that never opens a network connection is a different product.
Account. There isn't one. No email, no password, no profile. Lose the device without a backup whose password you still know, and the photos are gone. That is what "no account" costs.
Lock. An imported photo is no longer in the iOS Photos app, so it does not ride along with iCloud Photos. Locked Albums add a second check, Face ID, Touch ID, or a PIN, and stay out of the in-app library until that check passes. Album Lock is part of Pro. The free app covers import, viewing, and organizing up to 3 albums and 3 tags, plus basic search and Recently Deleted. Turning Face ID off so only the PIN works (Paranoid Mode) is also Pro. Encrypting the imported file is not behind the subscription. How that sits on top of the iPhone's own encryption is in the Secure Enclave piece.
If what you want is "not in my camera roll, and not behind the same Face ID as Messages," the separate lock is the part that does that, and it is the paid part.
Privately is free to download. Pro is a weekly or yearly subscription, with no free trial. The price for your region is on the subscription card in the app.