EAP-TLS, session resumption and OCSP

Stefan Winter stefan.winter at restena.lu
Fri Mar 3 10:34:29 CET 2017


we are curently testing a large-scale rollout of EAP-TLS for eduroam

One of the things we observed is that if an authentication uses TLS
session resumption, the server does not re-check the OCSP state of the
client cert that was used to login initially.

So, in a scenario where

- cert is valid, user authenticates with a full EAP-TLS handshake
- cert gets revoked
- user re-authenticates with session resumption

then the authentication does NOT fail but succeed.

Looking at the TLS-Client-Cert-* attributes which get restored from
session cache, it looks like it could be easy to do that though - the
serial number of the cert is saved; and the OCSP responder URL maybe
isn't but could be. And with both pieces of information, another OCSP
check can be run even on a resumed session.

A related question: how is the cache lifetime determined? When config
sets it to 24 hours, is that 24 hours after the initial, full,
authentication, or is that lifetime refreshed with every re-auth,
meaning the cache expires after 24 hours of non-use?

If the latter, a revoked user could perpetually prolong his account
lifetime even if OCSP wouldn't want to let him.

Also, something we didn't check but that just now comes to my mind: does
the server check the expiry time of the cert on a resumed session?


Stefan Winter

Ingenieur de Recherche
Fondation RESTENA - Réseau Téléinformatique de l'Education Nationale et
de la Recherche
2, avenue de l'Université
L-4365 Esch-sur-Alzette

Tel: +352 424409 1
Fax: +352 422473

PGP key updated to 4096 Bit RSA - I will encrypt all mails if the
recipient's key is known to me

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