Re: SNMPv3 security

On Friday 08 August 2003 01:09 pm, Bruno Dias wrote:
> 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"

No, not a complete joke.

> (yes, there's a lot more about security on computer
> networks than cryptoanalisis!).

Yes. Still, cryptography is the heart of network security.

> I'd love to read the article where this was written...

Consider "Report on the Development of the AES" by NIST.
Not quite the same language, but similar idea.

> Eric was criticized by his apparent lack of cryptography background,

No, by the apparent lack of (a) cryptographic, (b) security, and
(c) SNMP background.

> but on a generic discussion about symmetric cripytography his
> toughts are a safer approach than we all normally tend to assume...

Safety can be overblown - it is then called paranoia (no personal
offence made or implied to anybody here).


> 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.

Of course it is. What you should have said is "key strength is
not the same as key length".

> ....... is believed that the larger the amount
> of data analised the better are the chances to compromise the key.

Of course (a great discovery, though not exactly new). Ultimately,
when one collects an entire codebook - no key is necessary any more.



> 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.

This really depends on the algorithm in question. For DES it is:
you need about 2^40 known plaintext pairs and must perform
about 2^45 operations - smaller than 2^55 operations for
exhaustive key search (though in the last case you
need only one known plaintext pair,, not 2^40).

For AES there is no such known weakness/attack - so one can keep
collecting traffic. 

> 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)......

This is very true. And to defend adequately from this kind of
threat, keys have to possess PFS property. In short, this is
where you involve Diffie-Hellman algorithm.

> Not such a bad (or new) idea... even if not directly applicable
> on current SNMP standards,

It requires (a) adding one to four MIB objects to  usmUserTable (or
augmenting the last one), (b) new algorithm (or Textual Convention), 
and that's about it. Oh, and an implementation...


> 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...

Not quite - SNMP is inherently asymmetric, so most of the intelligence
lives in the NMS. I doubt this paradigm is ready to be discarded.