Subject: RE: [Isms] TLA

From: "Fleischman, Eric" <[email-masked]>
Date: Mon, 2 May 2005 13:49:27 -0700

PFS is defined in Section 3.3 of RFC 2409 as:
"When used in the memo Perfect Forward Secrecy (PFS) refers to the
notion that compromise of a single key will permit access to only data
protected by a single key. For PFS to exist the key used to protect
transmission of data MUST NOT be used to derive any additional keys, and
if the key used to protect transmission of data was derived from some
other keying material, that material MUST NOT be used to derive any more
keys."

-----Original Message-----
From: Tom Petch [mailto:[email-masked]] 
Sent: Monday, May 02, 2005 12:25 PM
To: Khanh Nguyen (khanhvn); [email-masked]
Subject: [Isms] TLA

mmm there's something about Three Letter Acronyms at the back of my
mind.

I probably shouldn't be asking - since we are in the security area - but

PSK is Pre-Shared Key - yes?
PFS is what concept; perhaps you can point me at a seminal article on it
or a reference in a standard text.  (I may know it under another name).

Tom Petch

----- Original Message -----
From: "Khanh Nguyen (khanhvn)" <[email-masked]>
To: "Tom Petch" <[email-masked]>; <[email-masked]>
Sent: Monday, May 02, 2005 9:00 PM
Subject: RE: [Isms] SNMPv3 security; the fundamental flaw?

> From: Tom Petch
>
> My thinking ... is of a basic level consisting
> of a shared secret with the security server (not with the SNMP 
> manager-server) which is then used to generate session keys for a 
> small fixed set of userids; not as secure as USM but much more secure 
> than some systems I see.  This can be a one touch solution; the secret

> can be randomised by engine id (as Khanh suggested), to vary with box 
> type (switch, router,
> firewall) etc.

For proper credit, key localization is not my new idea :)
I borrowed the concept from USM.

The problem with USM is that it uses the localized authKey and privKey
statically: the same keys are used again and again for all messages.
This breaks perfect forward secrecy (PFS).

This is not really USM fault (it has a smart design).  Its lack of PFS
and replay attack protection is rather an inherent problem with any
sessionless secure transport.  All ISMS proposals we have seen here
solve that problem (conciously or not) by using some sorts of
sessions/connections.

Back to localization, my LPSK proposal retains that USM concept, but
instead of using the localized secrets statically, the secrets are used
in conjuction with a secure PSK auth protocol such as SRP to (1)
authenticate *both* the user/manager and the agent, and
(2) generate a dynamic key for encryp & data integrity session. LPSK can
be used in all 3 keying environments: OOBK (e.g. EAP-SRP, SASL-SRP), IBK
(SNMP implementation of SRP, which is quite simple), and EK (TLS-SRP,
etc.)

SRP (http://srp.stanford.edu) is just one of many PSK methods that can
be used to implement LPSK.  I picked it as an example because it's
secure, relatively simple, and widely used.

Khanh

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