Bounds on Traceability Schemes

TL;DR

This paper establishes a new relationship between traceability schemes and cover-free families, leading to improved bounds and constructions for traceability schemes with enhanced anti-collusion properties.

Contribution

It reveals that a t-traceability scheme is a t^2-cover-free family, and derives new bounds and optimal constructions for traceability schemes using combinatorial structures.

Findings

A t-traceability scheme is a t^2-cover-free family.

New upper bounds for traceability schemes are established.

Infinite families of optimal traceability schemes are constructed.

Abstract

The Stinson-Wei traceability scheme (known as traceability scheme) was proposed for broadcast encryption as a generalization of the Chor-Fiat-Naor traceability scheme (known as traceability code). Cover-free family was introduced by Kautz and Singleton in the context of binary superimposed code. In this paper, we find a new relationship between a traceability scheme and a cover-free family, which strengthens the anti-collusion strength from $t$ to $t^2$, that is, a $t$-traceability scheme is a $t^2$-cover-free family. Based on this interesting discovery, we derive new upper bounds for traceability schemes. By using combinatorial structures, we construct several infinite families of optimal traceability schemes which attain our new upper bounds. We also provide a constructive lower bound for traceability schemes, the size of which has the same order with our general upper bound. Meanwhile, we consider parent-identifying set system, an anti-collusion key-distributing scheme requiring weaker conditions than traceability scheme but stronger conditions than cover-free family. A new upper bound is also given for parent-identifying set systems.