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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?

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There is an informational RFC for use of OpenPGP keys in SSL/TLS; as the RFC says:

The term "OpenPGP key" is used in this document as in the OpenPGP
specification [RFC4880].  We use the term "OpenPGP certificate" to
refer to OpenPGP keys that are enabled for authentication.

That's what these keys are for: usages as part of authentication protocols which may technically look like signatures, but are not meant to fulfil "non repudiation" requirements.

I never saw a working implementation of RFC 6091, though. I don't think it gets much used anywhere.

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