TCP/IP Tutorial and Technical Overview - IBM Redbooks

Hash A Hash payload must immediately follow the ISAKMP
header. HASH_1 uses the keyed pseudo-random
function that was negotiated during the phase 1
exchanges, and is derived from the following
information:
842 TCP/IP Tutorial and Technical Overview
HASH_1 = prf(SKEYID_a, M-ID, SA, N qmi , KE, ID qmi , ID qmr )
SKEYID_a was derived from the phase 1
exchanges.
M-ID is the message ID of this message.
SA is the Security Association payload carried in this
message, including all proposals that were offered.
Nonce is a new value different from the one used in
phase 1.
KE is the public Diffie-Hellman value carried in this
message. This quantity is chosen by Host-A, and is
denoted as g qm x . Note that this is not the same
quantity as g x that was used in the phase 1
exchanges.
IDs, which can identify either the endpoints of the
phase 1 exchange or endpoints on whose behalf the
protocol SA should be negotiated (proxy IDs when
IKE is used in client mode). These can subsequently
be different from the IDs used in phase 1.
Note: The use of KE and ID is optional, depending if PFS is used.
Security Association Indicates IP as the Domain of Interpretation.
Proposal, Transform Pairs
There can be one or more of these pairs in this
message. The first proposal payload is numbered 1,
identifies an IPSec protocol to be used, and includes
an SPI value that is randomly chosen by Host-A for
use with that protocol. The proposal payload is
followed by a single transform payload that indicates
the cryptographic algorithm to be used with that
protocol. The second proposal payload is numbered 2,
and so on.
Nonce payload This contains the nonce N qmi that was chosen
randomly by Host-A.