<b>Note:</b> The developer marked this project as "fallen asleep" on 2017-09-10, so you should rather use it's succesor https://f-droid.org/packages/org.sufficientlysecure.keychain. It has evolved very much, a much more active and well managed development community. APG is a port of OpenPGP for Android. Use it to encrypt and decrypt files, and in conjunction with K-9 Mail, to seamlessly add support for encrypting and decrypting emails, as well as adding and verifying digital signatures.