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I am studying & configuring IPSEC ikev1 and in between i am analysing the wireshark captures. I am using the linux kernel for TCP/IP stack and user-space i took ipsec-tools.

In the first two packet exchanges initiator is offering the proposals & responder will respond to the supported proposal.

Next two packet exchanges(packet 3[initiator] & 4[responder]) will perform DH key exchange.Here how the responder will came to know that which prime number the initiator uses & how it will be exchanged.

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It's in the SA payload of the initial packets.

It's attribute type 4 within the SA payload; in Main Mode of IKE (which is what you're looking at) the initiator will propose a list of groups, and the responder will pick one and send it back its selection in packet 2.

Now, what they exchange is a short integer value; both sides are expected to know the translation between the short values (current standard values are between 1 and 30) and the actual DH (or ECDH) group. See this page for pointers to the definitions of the current standard groups.

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  • $\begingroup$ Yeah i seen it in wireshark capture, In Transform IKE Attribute Type: Group Description 1024-bit MODP. There what it specifies? In this expression A=g(pow)a mod p. The g & p are generator and prime number respectively, how they choosen both same at initiator & responder. $\endgroup$ – Kumar2080 Mar 3 '17 at 10:22
  • $\begingroup$ @Kumar2080: if an IKE implementation supports group 2, it is expected to know that, for that group, $g=2$, and $p$ is the specific value listed in section 6.2 of RFC2409. $\endgroup$ – poncho Mar 10 '17 at 4:14

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