Conditional Disclosure of Secrets: A Noise and Signal Alignment Approach

In the conditional disclosure of secrets (CDS) problem, Alice and Bob (each holds an input and a common secret) wish to disclose, as efficiently as possible, the secret to Carol if and only if their inputs satisfy some function. The capacity of CDS is the maximum number of bits of the secret that can be securely disclosed per bit of total communication. We characterize the necessary and sufficient condition for the extreme case where the capacity of CDS is the highest and is equal to
$1/2$
. For the simplest instance where the capacity is smaller than
$1/2$
, we show that the linear capacity is
$2/5$
.