The OpenPGP specification (RFC 4880) describes, page 34, the different capabilities that an OpenPGP key may have; including:
0x20 - This key may be used for authentication.
In GnuPG, one of the most popular implementations of OpenPGP, it is possible to generate [sub]keys with the authenticate capability (using the
--expert flag). Okay, great, I know have a key which has the four capabilities – Encrypt, Certify, Sign, Authenticate (ECSA). Now what?
I am curious about that Authenticate capability. Is there any real-world use for authentication OpenPGP [sub]keys, other than not using them and export them as SSH keys instead?