NullKey 
Hiding and encrypting are different jobs
A photo vault app may encrypt the files it holds or may only hide them, depending on its design. A hidden view can keep photos out of your gallery while the stored file stays readable to anyone who can reach that file. A properly designed vault scrambles the stored contents so that they cannot be read without the right keys, and you can ask how those keys are protected and how the vault is unlocked.
The word encrypted on its own does not settle the question. You need to look at how the app describes its storage, its unlock method and its recovery path. Use sample photos and follow the app's own instructions for each check.
Questions to put to any vault app
Start by asking whether the app names its encryption method. A phrase such as uses encryption tells you very little, while a named cipher and a named key derivation method, described in the app's own documentation, give you something to check. Next, look at where the files physically sit, because files kept in the app's private storage behave differently from files that were only moved into a hidden folder on shared storage.
Then check whether the unlock secret is separate from your phone's lock. If the vault opens with the same PIN that unlocks the phone, anyone who knows that PIN can usually open the vault too. Finally, find out what stays visible, because file names, sizes and folder structure can remain visible even when the contents are not.
Where copies can still exist
Encryption protects the copy inside the vault, not the copies made around it. A photo can still sit in your gallery, in a cloud backup, in a messaging app that received it, or in a readable export you made to view it elsewhere. Deleting an original does not ensure that every backup or trash folder lets go of it.
Ask also how backups work. Some vault apps create an encrypted archive you can store elsewhere, while others rely on your phone's general backup features. Find out which one you have, and whether a backup needs its own passphrase or recovery key to open. A backup you cannot open does not help you recover anything.
How NullKey handles this
NullKey encrypts the photos, files and notes it stores. AES-256-GCM protects the stored contents, and Argon2id derives a wrapping key from your passphrase using a random salt and 64 MiB of memory. Each item has its own random key, and the vault's master key is random too.
The limits matter as much as the design. Biometric and screen-lock unlock are optional, and anyone who knows your phone's PIN can open NullKey if screen-lock unlock is enabled. While the vault is unlocked, readable content is processed on the phone, and file counts, approximate sizes and filesystem times are not hidden. A readable export leaves the vault's protection.
A routine for checking any vault
Run these checks on the app you already use, not only on the one you are considering. Use sample photos for the tests so that nothing irreplaceable is at risk.
- Find the app's own description of its encryption method and read it fully.
- Confirm whether the unlock secret is separate from your phone's lock.
- Check the app's documentation for where files are stored and what stays visible. A file browser may not reach private app storage.
- Create a backup, then restore it into an empty test vault before relying on it.
- Export one sample item and treat that export as an unprotected copy from then on.
- Delete the test copies and any exports you no longer need.
Labels do not prove anything
Words such as strong, secure, advanced or bank-level appear in many app listings, and on their own they tell you little. A useful description names what is stored, how it unlocks, what a backup needs and what the app cannot stop. An app that explains its limits in plain language is usually easier to judge than one that only lists features.
Even a well-encrypted vault cannot protect a photo that is open on a compromised phone, or a copy that another app has already made. Treat the vault as one layer of your privacy, and keep the other habits in place.
