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Re: Shared Secret Recovery in RADIUS
From: adam () HOMEPORT ORG (Adam Shostack)
Date: Thu, 31 Jul 1997 10:23:57 -0400

Riku Meskanen wrote:
| On Tue, 29 Jul 1997, Thomas H. Ptacek wrote:
| > This attack was sent to Livingston and posted to the RADIUS discussion
| > list (I'm at a loss for the name of it) last year. I think it's worthwhile
| > to note that the attacks you're pointing out are actively being exploited,
| > and have been for awhile. "Global roaming" systems involving RADIUS
| > proxies will dramatically increase the implications of this attack.
| >
| Some work seems to be done by Dale Cook <cdm () hyperk com> of SCIENTECH to
| solve these issues, see
| http://www.livingston.com/Tech/Technotes/Security/RADIUS-RSA.shtml

        Some comments on this:

        1.  There may be speed issues; I can stop your radius server
by making more requests  for authentication than you can handle.  I
may even do this legitamately.

        2.  The use of RSA is incorrect; see Anderson's "Robustness
Principles" paper from Crypto 95.  You need to sign before encrypting,
not afterwards.  ("This public key is used to encrypt the entire
authentication packet along with a dummy secret key, the resulting
encrypted packet is signed with the private key of the server.")
Anderson's paper can be found at http://www.cl.cam.ac.uk/users/rja14/

        3.  Since the code uses RSAref, its probably vulnerable to a
timing attack.  (See Kocher's paper in Crypto 96;

        The use of signing an encrypted message leads me to worry
substantially about the implementation.  I haven't spent time looking
to see if there are other problems, but with one that large, I'd be
suprised if its the only one.


"It is seldom that liberty of any kind is lost all at once."

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