Covert Channels in One-Time Passwords Based on Hash Chains

We present a covert channel between two network devices where one authenticates itself with Lamport's one-time passwords based on a cryptographic hash function. Our channel enables plausible deniability. We also present countermeasures to detect the presence of such a covert channel, which are non-trivial because hash values are randomly looking binary strings, so that deviations are not likely to be detected.