  A wireless network of full-duplex nodes/users, using anti-eavesdropping
channel estimation (ANECE) based on collaborative pilots, can yield a positive
secure degree-of-freedom (SDoF) regardless of the number of antennas an
eavesdropper may have. This paper presents novel results on SDoF of ANECE by
analyzing secret-key capacity (SKC) of each pair of nodes in a network of
multiple collaborative nodes per channel coherence period. Each transmission
session of ANECE has two phases: phase 1 is used for pilots, and phase 2 is
used for random symbols. This results in two parts of SDoF of ANECE. Both lower
and upper bounds on the SDoF of ANECE for any number of users are shown, and
the conditions for the two bounds to meet are given. This leads to important
discoveries, including: a) The phase-1 SDoF is the same for both multi-user
ANECE and pair-wise ANECE while the former may require only a fraction of the
number of time slots needed by the latter; b) For a three-user network, the
phase-2 SDoF of all-user ANECE is generally larger than that of pair-wise
ANECE; c) For a two-user network, a modified ANECE deploying square-shaped
nonsingular pilot matrices yields a higher total SDoF than the original ANECE.
The multi-user ANECE and the modified two-user ANECE shown in this paper appear
to be the best full-duplex schemes known today in terms of SDoF subject to each
node using a given number of antennas for both transmitting and receiving.
