  In network coding, information transmission often encounters wiretapping
attacks. Secure network coding is introduced to prevent information from being
leaked to adversaries. For secure linear network codes (SLNCs), the required
field size is a very important index, because it largely determines the
computational and space complexities of a SLNC, and it is also very important
for the process of secure network coding from theoretical research to practical
application. In this letter, we further discuss the required field size of
SLNCs, and obtain a new lower bound. This bound shows that the field size of
SLNCs can be reduced further, and much smaller than the known results for
almost all cases.
