TCP/IP Tutorial and Technical Overview - IBM Redbooks

IKE phase 1, message 6
After receiving message 5 from Host-A, Host-B verifies the identity of Host-A by
validating the hash.
If digital signatures were used for authentication, the signature of this hash are
verified by Host-B.
If this is successful, Host-B sends message 6 to Host-A to allow Host-A to verify
the identity of Host-B.
The structure of message 6 is the same as that of message 5, with the obvious
changes that the identity payload and the certificate payload now pertain to
Host-B:
HASH_R = prf(SKEYID, g y , g x , CookieB, CookieA, SA p , ID B )
Notice that the order in which Diffie-Hellman public values and the cookies
appear has been changed, and the final term now is the identity payload that
Host-B has included in message 6.
If digital signatures were used for authentication, this hash is also signed by
Host-B, which is different from the one previously signed by Host-A.
When Host-A receives message 6 and verifies the hash or digital signature, the
phase 1 exchanges are then complete. At this point, each participant has
authenticated itself to its peer. Both have agreed on the characteristics of the
ISAKMP Security Associations, and both have derived the same set of keys (or
keying material).
Miscellaneous phase 1 facts
There are several miscellaneous facts worth noting:
► Regardless of the specific authentication mechanism that is used, there will
be six messages exchanged for the Oakley Main Mode. However, the content
of the individual messages differs, depending on the authentication method.
► Although Oakley exchanges make use of both encryption and authentication,
they do not use either IPSec's ESP or AH protocol. ISAKMP exchanges are
protected with application-layer security mechanisms, not with network-layer
security mechanisms.
► ISAKMP messages are sent using UDP. There is no guaranteed delivery for
them.
► The only way to identify that an ISAKMP message is part of a phase 1 flow
rather than a phase 2 flow is to check the Message ID field in the ISAKMP
header. For phase 1 flows, it must be 0, and (although not explicitly stated in
the ISAKMP documents) for phase 2 flows, it must be non-zero.
Chapter 22. TCP/IP security 839