TCP/IP Tutorial and Technical Overview - IBM Redbooks

KE This is the key exchange payload that carries the
public Diffie-Hellman value chosen by Host-A, g qm x .
There is also a field called Group that indicates the
prime number and generator used in the
Diffie-Hellman exchange.
ID payload Specifies the endpoints for this SA.
IKE phase 2, message 2
After Host-B receives message 1 from Host-A and successfully authenticates it
using HASH_1, it constructs a reply, message 2, to be sent back to Host-A. The
Message ID of the reply is the same one that Host-A used in message 1.
Host-B chooses new values for the following:
Hash The hash payload now carries the value HASH_2,
which is defined as:
HASH_2 = prf(SKEYID_a, N qmi , M-ID, SA, N qmr , KE, ID qmi , ID qmr )
Security Association The Security Association payload only describes the
single chosen proposal and its associated transforms,
not all of the protection suites offered by Host-A.
Host-B also chooses an SPI value for the selected
protocol. Host-B's SPI does not depend in any way on
the SPI that Host-A assigned to that protocol when it
offered the proposal. That is, it is not necessary that
SPI A be the same as SPI B ; it is only necessary that
they each be non-zero and that they each be randomly
chosen.
Nonce Nonce payload now carries N r, a random value chosen
by Host-B.
KE Key exchange payload now carries Host-B's public
Diffie-Hellman value, g qm y .
At this point, Host-A and Host-B have exchanged
nonces and public Diffie-Hellman values. Each one
can use this in conjunction with other information to
derive a pair of keys, one for each direction of
transmission.
Chapter 22. TCP/IP security 843