RE: SSL/TLS passive sniffing

2004-12-01 Thread Ben Nagy
OK, Ian and I are, rightly or wrongly, on the same page here. Obviously my
choice of the word certificate has caused confusion.

[David Wagner]
 This sounds very confused.  Certs are public.  How would 
 knowing a copy
 of the server cert help me to decrypt SSL traffic that I have 
 intercepted?

Yes, sorry, what I _meant_ was the whole certificate file, PFX style, also
containing private keys. I assure you, I'm not confused, just perhaps guilty
of verbal shortcuts. I should, perhaps, have not characterised myself as
'bumbling enthusiast', to avoid the confusion with 'idiot'. :/

[...]
 Ian Grigg writes:
 I note that disctinction well!  Certificate based systems
 are totally vulnerable to a passive sniffing attack if the
 attacker can get the key.  Whereas Diffie Hellman is not,
 on the face of it.  Very curious...
 
 No, that is not accurate.  Diffie-Hellman is also insecure if 
 the private
 key is revealed to the adversary.  The private key for 
 Diffie-Hellman
 is the private exponent.

No, I'm not talking about escrowing DH exponents. I'm talking about modes
like in IPSec-IKE where there is a signed DH exchange using ephemeral DH
exponents - this continues to resist passive sniffing if the _signing_ keys
have somehow been compromised, unless I have somehow fallen on my head and
missed something.

 Perhaps the distinction you had in mind is forward secrecy.

Yes and no. Forward secrecy is certainly at the root of my question, with
regards to the RSA modes not providing it and certain of the DH modes doing
so. :)

Thanks!

ben
  


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SSL/TLS passive sniffing

2004-11-30 Thread Ben Nagy
Hi all,

I'm a bumbling crypto enthusiast as a sideline to my other, real, areas of
security expertise. Recently a discussion came up on firewall-wizards about
passively sniffing SSL traffic by a third party, using a copy of the server
cert (for, eg, IDS purposes).

There was some question about whether this is possible for connections that
use client-certs, since it looks to me from the spec that those connections
should be using one of the Diffie Hellman cipher suites, which is obviously
not vulnerable to a passive sniffing 'attack'. Active 'attacks' will
obviously still work. Bear in mind that we're talking about deliberate
undermining of the SSL connection by organisations, usually against their
website users (without talking about the goodness, badness or legality of
that), so how do they get the private keys isn't relevant.

However, I was wondering why the implementors chose the construction used
with the RSA suites, where the client PMS is encrypted with the server's
public key and sent along - it seems to make this kind of escrowed passive
sniffing very easy. I can't think why they didn't use something based on DH
- sure you only authenticate one side of the connection, but who cares? Was
it simply to save one setup packet?

Anyone know?

Cheers,

ben


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