Subject: [Isms] What are people's objections to TLSM

From: Sam Hartman <[email-masked]>
Date: Sun, 06 Mar 2005 10:09:22 -0500

Hi.  Reading the comparison document, I notice that TLSM seems to not
be considered very seriously.  In addition, the presentation for TLSM
at IETF 61 said that even the presenters didn't consider it seriously.

Asking as an individual, not as an AD, why is that the case?  Could
someone help me understand what the disadvantages of TLSM are?

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From: "David B Harrington" <[email-masked]>
Date: Mon, 7 Mar 2005 20:50:23 -0500

Hi,

TLSM only solves some of the problems that need to be solved, and may
be difficult to solve the ones it does solve.

Transport level security (TLS or other protocols) is useful for
securing the message, but many transport layer security protocols
authenticate against a host, not a user.

It is important to use security models that will provide
user-authentication for use with SNMP authorization (e.g. VACM). 

Second, the user identity need sto be mapped to an authorization
policy, e.g. VACM groups. Transport layer security protocols generally
don't provide any mechanism for identifying an authorization policy
associated with the authenticated user. So even if the message was
secured by TLSM, you might still need to have a RADIUS or other
approach to provide the mapping between authetication and
authorization.

Part of my reasons for not strongly pushing TLSM at the last meeting
was simply that all three approaches appeared viable, and I suggested
dropping TLSM to winnow the field. That said, I have given more
thought to TLSM and like the leveraging of existing infrastructure to
secure messages that TLS, SASL, or SSH provide.

David Harrington
[email-masked]

> -----Original Message-----
> From: [email-masked] 
> [mailto:[email-masked]] On Behalf Of Sam Hartman
> Sent: Sunday, March 06, 2005 10:09 AM
> To: [email-masked]
> Subject: [Isms] What are people's objections to TLSM
> 
> 
>

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From: Juergen Schoenwaelder <[email-masked]>
Date: Tue, 8 Mar 2005 07:48:30 +0100

> someone help me understand what the disadvantages of TLSM are?

TLSM breaks with one of the many holy myths of SNMP, namely that SNMP
must not rely on other services to be functional when the network burns.
I personally do not believe in this myths as I have never met someone
who really uses SNMP to bring a network back into operational state
when the network burns. (But perhaps I am just talking to the wrong
people. ;-) My fear is basically that pushing for TLSM may yield 
another instance of this "you are not allowed to break SNMP myths"
discussions which I have experienced before and I prefer to stay 
away from. [For those not familiar with the SNMP history, you may
want to read the "Towards Useful Management" published almost nine(!) 
years ago in the July 1996 issue of the Simple Times:
http://www.simple-times.org/pub/simple-times/issues/4-3.html]

In practical terms, the only security mechanism having a chance to be 
deployed in environments I am familiar with is one that uses a security
protocol and authentication mechanism that is already well understood
and deployed.

/js

-- 
Juergen Schoenwaelder		    International University Bremen
<http://www.eecs.iu-bremen.de/>	    P.O. Box 750 561, 28725 Bremen, Germany

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From: Juergen Schoenwaelder <[email-masked]>
Date: Tue, 8 Mar 2005 07:56:20 +0100

> authenticate against a host, not a user.

I am not sure this statement is generally true. I do authenticate as a 
user using ssh. I do authenticate as a user using TLS, typically in 
combination with SASL (which is considered to be broken but I guess 
fixing this will happen anyways).

The more important point with all these alternatives is hat they are 
all stream based and assume TCP as a transport. Running SNMP over
TCP touches another myths. I am not sure DTLS is such an appealing
alternative - someone needs to carefully analyze the number of round
trips needed and the amount of state information needed for the DTLS
session in an SNMP context. My fuzzy reading of the specs made me 
believe that things might become close to running SNMP over TCP,
but I can't claim that I do understand the details.

As a code writer, I love to have a solution which allows me to pick
an already available and debugged library which I can simply link
against to get a viable solution.

/js

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From: Eric Rescorla <[email-masked]>
Date: Tue, 08 Mar 2005 05:32:04 -0800

Juergen Schoenwaelder <[email-masked]> writes:
> trips needed
Two (or three if you want the anti-DoS protection). This is pretty
much the standard number for this kind of protocol.

> and the amount of state information needed for the DTLS
> session in an SNMP context.
4 session keys and whatever identity information you hang off them,
pretty much the same as with any protocol of this kind.

> against to get a viable solution.
We're working on this, but there's already a working version
checked into reSIProcate:

http://list.sipfoundry.org/archive/resiprocate-devel/msg01674.html

-Ekr

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From: Ken Hornstein <[email-masked]>
Date: Tue, 08 Mar 2005 10:33:26 -0500

>fixing this will happen anyways).

I think what is really happening is that you're encrypting with TLS,
but you're authenticating with SASL.  While this isn't as perfect as
crypto weenies like me would like, it has the advantage of supporting
a bunch of mechanisms (enough that I think it would meet the needs
of most everbody) and it's well understood.

--Ken

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From: Juergen Schoenwaelder <[email-masked]>
Date: Tue, 8 Mar 2005 16:30:06 +0100

On Tue, Mar 08, 2005 at 05:32:04AM -0800, Eric Rescorla wrote:

> > I am not sure DTLS is such an appealing

Another question: How much overhead will DTLS consume in terms of number
of bytes in a UDP packet? How much space will be left for SNMP payload?

/js

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From: Eric Rescorla <[email-masked]>
Date: Tue, 08 Mar 2005 11:39:39 -0800

> of bytes in a UDP packet? How much space will be left for SNMP payload?

With AES, order 50 bytes.

This really isn't an issue of DTLS vs. ISMS vs. USM.

Basically, if you want to provide security for datagram packets, 
there's kind of a minimum amount of overhead for any given set
of algorithms. All of these protocols are about equally
efficient.

-Ekr

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From: Juergen Schoenwaelder <[email-masked]>
Date: Tue, 8 Mar 2005 20:44:33 +0100

> of algorithms. All of these protocols are about equally

Well, I know what USM does and I just want to understand what DTLS
would do (or any other protocol running over UDP). I am really just
trying to learn (and I apologize that it is about three months ago
that I last looked into the DTLS draft and that my memory is rather
short term).

/js