PUBKEY_USAGE_AUTH

[NIIBE Yutaka]

Hello,

I'm currently considering using WebPG for authentication in web
application.

With Gnuk Token, I have been using a subkey for authentication, that
is, a subkey with PUBKEY_USAGE_AUTH flag.  But I only use it through
gpg-agent for SSH-agent service and Scute for X.509 client certificate
authentication.

It seems for me that there is no way by gpg frontend or GPGME to use
authentication subkey.

Does it make sense to add an option like --auth to enable using
authkey for --sign or --clearsign?  Or some flag to enable
gpgme_op_sign using authkey?

I know that we can use gpg-connect-agent and PKSIGN.  I want somewhat
public API for authentication subkey.
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