On the Linear Capacity of Conditional Disclosure of Secrets

Conditional disclosure of secrets (CDS) is the problem of disclosing as efficiently as possible, one secret from Alice and Bob to Carol if and only if the inputs at Alice and Bob satisfy some function. The information theoretic capacity of CDS is the maximum number of bits of the secret that can be securely disclosed per bit of total communication from Alice and Bob to Carol. All CDS instances, where the capacity is the highest and is equal to 1/2, are recently characterized through a noise and signal alignment approach and are described using a graph representation of the function. In this work, we go beyond the best case scenarios and further develop the alignment approach to characterize the linear capacity of a class of CDS instances to be $(\rho -1)/(2\rho)$ , where $\rho $ is a newly introduced and highly specific covering parameter of the graph representation of the function.