Re: Is IP address spoofing possible with ESP transport mode?

Travis Watson writes:
 > Enabling Perfect Forward Secrecy (PFS) will encrypt the entire
 > packet in ESP--inculding the header.

Enabling PFS has no impact on whether a IP header is encrypted or not;
The outer header IP header is never encrypted in either transport or
tunnel mode ESP (otherwise you couldn't route the packet) and the
inner header is always encrypted in tunnel mode ESP regardless of
whether PFS was used or not.

 > A spoofed IP address in this case doesn't really do an attacker any
 > good because the handshake would fail unless the guy knows the
 > pre-share (or has a copy of the certificate),

That's true, however as I understand Son's description the attacker
*does* know the pre-share or certificate and has already established a
valid transport mode ESP connection to the server S.  The intent of
the attacker is then to pass off traffic on this connection as coming
from someone else (Alice in Son's description) rather than the attacker.