A free app for Android, by Taymay.io.

Calculator Vault Gallery Lock is an app that lets you create a secure vault to hide your photos and videos on your Android phone.

The app lets you import your private pictures, videos, audio, and files into the vault, and it provides an underground interface to help you lock photos and videos and manage files with a secret calculator.

If you need to restore deleted files, just go to the recycle bin and restore them.

The app also supports viewing files in all formats, including JPEG, GIF, PNG, SVG, MP4, MKV, RAW, etc.