This guide was written and tested on Android 5, and according to the authors of used applications, should also work for Android 4.0.3+. Additionally, for Open Keychain, permissions will be requested on the go in Android 6+.
Manage my keys
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Confirm with fingerprint
Further information on Open Keychain is available in the
Help section of the app, available under the hamburger(upper left corner).
K-9 mail is a fork of the Android Mail with a long history. It was chosen for the guide thanks to it's excellent integration with Open Keychain that allows you to encrypt all emails in just 3 more clicks per email, plus some initial setup.
Three dots(lower right corner) >
When composing a new email, you will now see a lock next to your email address. The lock will change according to PGP is used:
The number of dots is also displayed next to each recipient separately.
You can press the lock to change the encryption mode. The default is
encrypt if possible and in it emails will be sent encrypted and unencrypted. You can also switch it to
Don't Encrypt or
Encrypt. In the last case, the email will fail to send if any recipient lacks a verified key in the keychain. That situation is indicated with a red lock with a white x and a singe red dot.
After pressing send, if the lock is green, Open Keychain will fire up to ask you for your PGP passphrase. Upon entering it, K-9 will send the encrypted message.
Pictures to the K-9 section will be added later.
PGP in Gmail using OpenKeychain encrypt file for PGP/MIME and Oversec for PGP/INLINE