Secure Frameproof Code Through Biclique Cover

Abstract: For a binary code $\Gamma$ of length $v$, a $v$-word $w$ produces by a set of codewords ${w^1,...,w^r} \subseteq \Gamma$ if for all $i=1,...,v$, we have $w_i\in {w_i^1,..., w_i^r}$. We call a code $r$-secure frameproof of size $t$ if $|\Gamma|=t$ and for any $v$-word that is produced by two sets $C_1$ and $C_2$ of size at most $r$ then the intersection of these sets is nonempty. A $d$-biclique cover of size $v$ of a graph $G$ is a collection of $v$-complete bipartite subgraphs of $G$ such that each edge of $G$ belongs to at least $d$ of these complete bipartite subgraphs. In this paper, we show that for $t\geq 2r$, an $r$-secure frameproof code of size $t$ and length $v$ exists if and only if there exists a 1-biclique cover of size $v$ for the Kneser graph ${\rm KG}(t,r)$ whose vertices are all $r$-subsets of a $t$-element set and two $r$-subsets are adjacent if their intersection is empty. Then we investigate some connection between the minimum size of $d$-biclique covers of Kneser graphs and cover-free families, where an $(r,w; d)$ cover-free family is a family of subsets of a finite set such that the intersection of any $r$ members of the family contains at least $d$ elements that are not in the union of any other $w$ members. Also, we present an upper bound for 1-biclique covering number of Kneser graphs.