"authentication" key flag

[Atom 'Smasher']

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can anyone tell me how/what gpg authentication may be used for? rfc 
2440bis-12 doesn't mention any details other than mentioning (in 
"5.2.3.21. Key Flags") that such capability exists.

any use that i can think of for such a key seems like it could be handled 
by a ~regular~ signing key...


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