A free app for Android, by theandroidseb.

PO Contacts is an app for storing your phone number. In the future, it will also be able to save email addresses. When creating a new contact, a user has the option to add a phone number, an email address, or both. When adding an email address, the user can specify whether it is primary, secondary, or TLD-based.

The contacts are stored in a database that is encrypted with AES and the keys are stored in the Android KeyStore. The encryption ensures that the contents of the database cannot be accessed by an attacker even if he/she gains access to the device.