OAuth2 authentication for offline email clients

More than a year ago, in my first post on this blog, I described my head in the cloud, feet on the ground strategy for offline email access. At that time, my solution (based on offlineimap) required me to store my email password in my Gnome keyring. This is far from satisfactory because as I explained in my blog post on using encryption, I do not like to keep important passwords in the Gnome Keyring. For that, I use a KeePass password file in an encrypted file system. This means that I need three passwords to get to access my important passwords: first to login to the computer, second to mount my encrypted volume and third to open the password manager. On the other hand, I need only my login password to get to the less important passwords sitting in the Gnome Keyring. In my view, email passwords are among the most important ones, and it unfortunate that to use offlineimap, I had to store this critical stuff in the Gnome Keyring.

The solution is to use OAuth2. In the last year or so, offlineimap has acquired the capability to use OAuth2, and now I have completed my migration to this method. As part of this process, I sat down and read the official document on OAuth 2.0 Threat Model and Security Considerations. That made me uncomfortable with the suggested approach in offlineimap (and many other software as well) of storing the OAuth2 refresh token in plain text in the configuration file. It might be acceptable if the home partition is encrypted, but as I explained in my Using Encryption post, that is not how my laptop is set up. I therefore came up with the idea of storing the refresh token in the Gnome Keyring. Since it is possible to use arbitrary python code for almost all settings in the offlineimap configuration file, this is easy.
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