[38436] in Kerberos
Re: Kerberos n00b question.
daemon@ATHENA.MIT.EDU (Grant Taylor)
Tue Jan 8 22:55:12 2019
To: kerberos@mit.edu
From: Grant Taylor <gtaylor@tnetconsulting.net>
Message-ID: <66a3c7f0-912f-378b-043c-5c8c1ee070f4@spamtrap.tnetconsulting.net>
Date: Tue, 8 Jan 2019 20:55:05 -0700
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In-Reply-To: <87h8eia4g8.fsf@hope.eyrie.org>
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On 1/8/19 8:35 PM, Russ Allbery wrote:
> Yes. A lot of higher education institutions that have used Kerberos=20
> for many, many years have their KDCs directly on the Internet and allow=
=20
> clients to authenticate from anywhere.
Oh. Good!
> PKINIT is just a replacement preauth mechanism, instead of enc-timestam=
p.=20
> Basically, the client uses an X.509 authentication instead of a proof=20
> of key possession as the preauthentication step to establish a shared=20
> session secret that is used to encrypt the TGT. (This may not be 100% =
> accurate; it's been a while since I dug into the protocol.)
>=20
> FAST is a replacement for the whole preauth step. It uses some=20
> pre-existing shared session key between the KDC and the client to=20
> encrypt the whole preauthentication exchange. Inside of that, you can =
> use various preauthentication mechanisms.
>=20
> Where they usefully combine is in how to get that pre-existing shared=20
> session key to be able to start using FAST. This is a chicken-and-egg =
> problem with traditional Kerberos: you have to authenticate first in=20
> order to authenticate. You can, for instance, use the local host key=20
> (which is probably randomly generated and therefore safer to use in=20
> a direct exchange with the KDC) to get a session key to start FAST,=20
> and then do preauthentication with the (weaker) password-derived key.
>=20
> Anonymous PKINIT lets you out of that trap by letting the client=20
> "authenticate" with anonymous Diffie-Hellman to the KDC. This doesn't =
> establish any meaningful identity, but it *does* get you a shared=20
> session key, and with that you can start FAST, and use it to protect=20
> any subsequent preauthentication exchange.
>=20
> Note that you can enable anonymous PKINIT even if you don't otherwise=20
> use PKINIT and don't have any client certificates. (You ideally do hav=
e=20
> a KDC certificate, though, that the clients know about.)
Thank you for the concise responses. I will do more reading on FAST,=20
PKINIT, Anonymous PKINIT. But now I have a better idea how the pieces=20
fit together.
Plus, CA thrown in for good measure.
Isn't security fun and simple? - What ever happened to the days of=20
3Rot13. ;-)
--=20
Grant. . . .
unix || die
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