How to import an SSH ed25519 key to GPG?

I have an SSH ed25519 key which I would like to import to GPG as an Authentication Subkey. There doesn't appear to be any documentation available on how to do this.

Solution 1:

The first step is impossible.
But you can import SSH ed25519 private key to GPG.

Assume that you have already setup gpg-agent.

1. Setup SSH_AUTH_SOCK and GPG_TTY environment variables by export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket) and export GPG_TTY=$(tty).
2. Add your SSH private key to GPG by ssh-add ed25519-filename.
   Note that if you are asked for a new password, it is recommended that you use the same as your GPG master password.
3. Identify the keygrip of the private key that you added by step 2.
   1. Check the keygrips of your existing GPG private keys by gpg -K --with-keygrip.
   2. Check private key files by ls -l $(gpgconf --list-dirs homedir)/private-keys-v1.d.
   3. The filename of step 3-2 that is not in the keygrips of step 3-1 is the keygrip that you added by step 2.
4. Setup the key added by ssh-add to a subkey.
   1. Start GPG by `gpg --expert -edit-key "Your-existing-GPG-Key-ID".
   2. Enter addkey command.
   3. Select "Existing key". (Maybe "13")
   4. Enter the keygrip identified by step 3.
   5. Set the capability as you wish and Finish.
   6. Then, answer the questions appropriately.
      Note that you should answer Y to Really create? and Save changes? etc.
5. Verify that the import was successful by gpg -K --with-keygrip.
   You should be able to see the keygrip that you just added.