  Security aspects of the Index Coding with Side Information (ICSI) problem are
investigated. Building on the results of Bar-Yossef et al. (2006), the
properties of linear coding schemes for the ICSI problem are further explored.
The notion of weak security, considered by Bhattad and Narayanan (2005) in the
context of network coding, is generalized to block security. It is shown that
the coding scheme for the ICSI problem based on a linear code C of length n,
minimum distance d and dual distance d^\perp, is (d-1-t)-block secure (and
hence also weakly secure) if the adversary knows in advance t \le d - 2
messages, and is completely insecure if the adversary knows in advance more
than n - d^\perp messages.
