This app provides an easy way to keep your notes protected. You can individually choose for every single note to either lock it by password fingerprint or to keep it unlocked.
The app saves your notes in unreadable form on your smartphone.
Once you open the note by authenticating yourself the app converts the note back into readable text. You can then view and edit its content again.
You also have the option to automatically synchronize your notes with your Dropbox account.
To use the fingerprint feature you must pay a one time fee.

I have developed the encryption algorithm used by the app myself. It has therefore not been tested for absolute security. The notes are NOT stored in plain text so you can't read the content of a note just by having access to the note file. It is still not impossible that someone who has access to the file of a protected note could bypass the password protection somehow.