Re: SNMPv3 security

[The first time the message seemed trashed, at least on my mail reader...]
[I'll email it again... Sorry for the duplicate.]

At 21:58 07-08-2003, Bill Strahm wrote:
>Ok, I have to ask...
>
>What security protocol defines session key timeout values as a part of the
>standard.  I certainly don't know of any including IPsec and SSL/TLS.  All
>things associated with session key timeouts resides within administrative
>control as a policy on top of the protocol for example IKE negotiates
>session key timeouts in the IPsec protocol, IPsec has nothing to do with it,
>now where does IKE get the value to negotiate for well that is a system policy
>set by the adminstrator, there isn't ANYTHING in the IKE RFCs that determine
>what value to negotiate.  That said there are a HOST of research done on
>the appropriate ammount of data to send under a key of given strength to
>give an appropriate amount of security.  With AES 128 my understanding is that
>it provides about 10000 years of protection given the current ramp of hardware
>and a governmental sized budget... The numbers grow exponentially to AES 256.
>Now there is quite a bit of research showing that AES may only be about 1000th
>the stength that it might be (known hacks at 9-10 rounds given a rediculous
>amount of memory vs. the 16 rounds in the algorithm) so lets say AES 128 is
>good to protect your management traffic for 10 years (1 year if they have an
>obscene budget)... is your management traffic THAT valuable after 1 year ?

I just like the way the we're talking about these security protocols...
Statements like "about 10000 years of protection given the current ramp of 
hardware
and a governmental sized budget..." is quite funny for a non expert like me 
and a complete joke
for a "network computer security professional" (yes, there's a lot more 
about security on computer
networks than cryptoanalisis!). I'd love to read the article where this was 
written...

Eric was criticized by his apparent lack of cryptography background, but on 
a generic
discussion about symmetric cripytography his toughts are a safer approach 
than we
all normally tend to assume...

The "amount of data to send under a key of given strength to
give an appropriate amount of security", as I understand it, is not exactly 
the same
as the time of "protection" a key can assure. In fact, these are two different
types of security strengths. The first is about security strength to 
cryptoanalisis attacks and the second
is about security strength to brute force attacks. While the later is 
"only" dependent on the key size vs
computer power (you need to increase key size to figth the computer power 
evolution), the first one
is much more hard to evaluate but it is believed that the larger the amount 
of data analised the
better are the chances to compromise the key. Well, on a communication 
between two (or more)
network entities the amount of data transmited, and prone to be 
cryptoanalised, increases as time
passes, so is the chance of the key to be compromised. Also, in general, 
the amount of
time need to crack a key by cryptoanalisis is much smaller than the brute 
force attack when the
some weekenesses are discovered on the crypto algorithm or on the key itself.
But the real intention beyond limited key time life spans has to do about 
compromised keys
due to bad key management (week key values, management applications and 
operating systems
security weekenesses, unsecure key storage, etc) than compromised keys from 
brute force or cryptoanalisis
attacks.
That's where a protocol imposing a limited key time life span (or key 
sessions) woud help to
reduce the amount of this data.
Not such a bad (or new) idea... even if not directly applicable on current 
SNMP standards, but
the regular changing of passphrases by some sort of application could be 
usefull
(as someone as already discovered *;o), although it brings some "third 
party trust" security problems
that makes this key distribution a not so simple matter...

Regards,
--
Bruno Dias


>I have no idea why you don't think a simple application can't be written that
>will update SNMP passphrases as often as you want to on your network.  Yes it
>will take protocol overhead time, but then so does renegotiating IPsec
>session keys in IPsec.  For that matter you can make it a simple policy on
>either end of the control that automatically initiates a keychange when a
>certain crypto quantia happens (up to after every other message, once for the
>key exchange, once to get data).  This is simple software that has nothing to
>do with the underlying protocol - other than the underlying protocol allows
>keys to be negotiated over it.
>
>Now I want to know who you are talking to that says proprietary algorithms
>are trusted more than publicly available ones.  I know of no serious crypto
>researcher that believes this.  That said, you might be using other supplied
>crypto for your custommers, if so, just plug the algorythm into your SNMPv3
>implementations and you are off to the races (I plugged in the AES candidates
>many years ago for fun, guess what it worked w/out an RFC, and the eventual
>AES RFC looks just like my implementation of AES did, of course I had to
>throw out the other candidates)
>
>Time to end my discussion for now...
>
>Bill Strahm

Bruno Dias
Departamento de Informática - Universidade do Minho
Campus de Gualtar, 4710-057 Braga, PORTUGAL
E-Mail<[email protected]> Telef<+351 919 312 312> Fax<+351 253 604 471>