Subject: RE: [Sip-security] Basic/digest over TLS -- denial of service

From: [email-masked]
Date: Tue, 23 Oct 2001 09:41:18 +0200

Hello,

o  in TLS, one party can request a CPU expensive operation from the
other party without authentification. There is a paper
describing the problem and describing a possible solution:
http://www.cs.rice.edu/~astubble/tls-usenix.pdf

The proposed solution is the 'client-puzzle'; the server sends
a 'puzzle' to the client and the solution of this puzzle is
used as a temporary authentification for requesting expensive
public-key operations.

I like the approach, but it might be not applicable in the SIP
environment. Client puzzles that are expensive enough for fast
workstations to prevent them from performing DoS attacks may
cause unacceptable delays for users of cheap embedded SIP devices.

o  What are the memory and CPU requirements of keeping ten thousands
of TLS connections in a large softswitch?

o  Using existing protocols is good thing, but it can lead to code
bloat. What does a SIP device need to have?

- SIP stuff (parser, state machine, encoder)
- a MIME parser /encoder
- S/MIME (that means ASN1 BER for CMS)?
- MIME/PGP ?
- TLS ?

Does that fit in my SIP-capable wrist watch?

--
Wolfgang Beck
T-Systems Nova GmbH

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