gpg-preset-passphrase: caching passphrase failed: Not supported

Question

I am building a script that is using msmtp to send myself regular system reports by email. I have used gpg to encrypt my email password and used password-eval in msmtp.conf.

I sent a test email successfully but during the process, gpg asked me for the passphrase I used to create keys when I gpg --full-generate-key.

Given that the script will be running unattended, I need to cache the passphrase using gpg-preset-passphrase. However when I run /usr/lib/gnupg2/gpg-preset-passphrase --preset <keygrip> where keygrip is obtained using gpg --list-keys --with-keygrip, I get the error message: gpg-preset-passphrase: caching passphrase failed: Not supported.

For reference, I am using gpg 2.2.12 on a Raspberry Pi 2 Model B running Raspbian Buster.

Kindly assist.

Answer 1

This feature must first be enabled in ~/.gnupg/gpg-agent.conf using the option:

allow-preset-passphrase

After editing the config it might be enough to gpg-connect-agent reloadagent /bye, or you might need to restart the agent completely.

Note that the preset passphrase is only stored in memory – it will not persist across reboots. (How would gpg-agent store it securely, anyway?)