Subject: RE: Kerberos-revisions nits and comments

From: "John Brezak" <[email-masked]>
Date: Wed, 31 Oct 2001 22:21:04 -0800

If replay detection is different (or non existent) on UDP, wouldn't an
attacker choose to use UDP over TCP in searching for weakness?

-----Original Message-----
From: Derek Atkins [mailto:[email-masked]] 
Sent: Wednesday, October 31, 2001 5:37 PM
To: John Brezak
Cc: Ken Raeburn; [email-masked]
Subject: Re: Kerberos-revisions nits and comments

"John Brezak" <[email-masked]> writes:

> I'm saying it looks like a tradeoff to me -- theoretical security 
> versus practical concerns -- and that MIT's code, at least lately, has

> not performed the replay detection as specified in the draft alone; 
> either none at all, or an extra layer of checks comes first.  But we 
> should specify it at least enough so that the required behavior of 
> both client and KDC under UDP is clear.
> 
> [Jbrezak] We need to handle UDP and TCP the same - IMHO.

I'm not convinced of this.  We already have to treat UDP and TCP
different in terms of retransmissions; treating them differently in
terms of replay cache is, IMHO, a related distinction.

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       [email-masked]                        PGP key available

---

From: Derek Atkins <[email-masked]>
Date: 01 Nov 2001 10:13:41 -0500

Perhaps..  But there are other tools than just the KDC itself to
notice and prevent such attacks.  For example, if I wanted to
I could just keep sending random packets to a KDC to flood the
network.  Or I can make many random TCP connections in order to
flood it.  The attacks are similar, but need to be tuned to the
transport.

Since the transports provide different retransmission guarantees, I
think we need to live with the consequences of it.  For example, you
KNOW that with TCP you WILL NOT get a duplicate request, at least
within the same TCP session.  You KNOW that the network will not send
a duplicate packet.  So, what exactly does replay protection buy you
here?

Step back and ask yourself: what attacks are you trying to protect
yourself against with replay protection on the KDC?  While you are
answering this, keep in mind that there _are_ legitimate reasons for a
UDP client to retransmit a request to the KDC, and there are even
reasons for a TCP client to retransmit (but not as many ;)

-derek

"John Brezak" <[email-masked]> writes:

> From: Derek Atkins [mailto:[email-masked]]=20
> > I'm saying it looks like a tradeoff to me -- theoretical security=20
> > not performed the replay detection as specified in the draft alone;=20
> > either none at all, or an extra layer of checks comes first.  But we=20
> > should specify it at least enough so that the required behavior of=20
> >=20
> --=20