As per Nicolas Williams -->
The key is that eavesdroppers cannot easily compute g^ir (mod p).
The initiator computes g^ir = (g^r)^i mod p, while the responder
computes g^ir = (g^i)^r mod p. The initiator knows i and the responder
knows r. The attacker doesn't know i, nor r, because those are not
sent.
The attacker cannot easily compute them from g^i mod p nor g^r mod p.
Nor can the attacker easily compute g^ir mod p from g^i and g^r mod p.
The relevant number theory topic is known as the "Computational
Diffie-Hellman Problem" (and the related Decisional Diffie-Hellman
Problem).
Syed Ajim -->
Attacker can know who is initiator , who is responder , by the First
IKE_INIT_SA Message , by Checking Responder Cookie Zero ,
Initiator will send --> g^I , in KE payload with DH Group no. in
IKE_SA_INIT Message
Responder will send --> g^R , in KE payload with DH Group no.
IKE_SA_INIT Message
So, in SKEYSEED = prf(Ni | Nr, g^ir), nothing is secret , if some attacker
can capture IKE packets.
So he can derive the Key also.
With Regards
Syed Ajim
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________________________________
-----Original Message-----
From: Nicolas Williams [mailto:[email protected]]
Sent: Monday, November 22, 2010 11:45 AM
To: Syed Ajim Hussain
Subject: Re: [IPsec] Generating Keying Material for the IKE_SA (IKEv2)
On Mon, Nov 22, 2010 at 11:25:26AM +0530, Syed Ajim Hussain wrote:
> If attacker using some tools capturing all the IKE Packets from
network,
> he can easily generates the Keys. Although attacker can not establish
a
> SA without proper configuration information, but still he can easily
get
> the Keys, and he will be able to decrypt all the IKE Encrypted and
> IPSEC Encrypted packets.
>
> Don't you think this is a big Security Risk? In IKEv1 Pre-shared key
> auth, PSK was taken as part of key
>
> Calculation with is a secret to generate Key and provides some level of
> Security.
>
> IKE Key generation process:
>
> SKEYSEED = prf(Ni | Nr, g^ir)
^^^^
The key is that eavesdroppers cannot easily compute g^ir (mod p).
The initiator computes g^ir = (g^r)^i mod p, while the responder
computes g^ir = (g^i)^r mod p. The initiator knows i and the responder
knows r. The attacker doesn't know i, nor r, because those are not
sent.
The attacker cannot easily compute them from g^i mod p nor g^r mod p.
Nor can the attacker easily compute g^ir mod p from g^i and g^r mod p.
The relevant number theory topic is known as the "Computational
Diffie-Hellman Problem" (and the related Decisional Diffie-Hellman
Problem).
Nico
--
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