Encrypting a message requires the public key(s) of the recipient(s). Signing a message requires your private key.
Private keys are stored by Android and can be imported via the Android advanced security settings.
Private keys are stored by Android and can be imported via the Android advanced security settings
(Encryption & credentials, Install a certificate, VPN & app user certificate).
There is a shortcut (button) for this in the encryption settings for Android version 10 and before.
Android will ask you to set a PIN, pattern, or password if you didn't before.
If you have a Nokia device with Android 9, please [read this first](https://nokiamob.net/2019/08/10/a-bug-prevents-nokia-1-owners-from-unlocking-their-screen-even-with-right-pin-pattern/).