  Covert channels are stealthy communication channels that enable manifold
adversary and legitimate scenarios, ranging from malware communications to the
exchange of confidential information by journalists and censorship
circumvention. We introduce a new class of covert channels that we call history
covert channels. We further present a new paradigm: covert channel
amplification. All covert channels described until now need to craft seemingly
legitimate flows or need to modify third-party flows, mimicking unsuspicious
behavior. In contrast, history covert channels can communicate by pointing to
unaltered legitimate traffic created by regular network nodes. Only a
negligible fraction of the covert communication process requires the transfer
of covert information by the covert channel's sender. This information can be
sent through different protocols/channels. Our approach allows an amplification
of the covert channel's message size, i.e., minimizing the fraction of actually
transferred secret data by a covert channel's sender in relation to the overall
secret data being exchanged. Further, we extend the current taxonomy for covert
channels to show how history channels can be categorized. We describe multiple
scenarios in which history covert channels can be realized, analyze the
characteristics of these channels, and show how their configuration can be
optimized.
