  The security of quantum communication using a weak coherent source requires
an accurate knowledge of the source's mean photon number. Finite calibration
precision or an active manipulation by an attacker may cause the actual emitted
photon number to deviate from the known value. We model effects of this
deviation on the security of three quantum communication protocols: the
Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol without
decoy states, Scarani-Acin-Ribordy-Gisin 2004 (SARG04) QKD protocol, and a
coin-tossing protocol. For QKD, we model both a strong attack using technology
possible in principle, and a realistic attack bounded by today's technology. To
maintain the mean photon number in two-way systems, such as plug-and-play and
relativistic quantum cryptography schemes, bright pulse energy incoming from
the communication channel must be monitored. Implementation of a monitoring
detector has largely been ignored so far, except for ID Quantique's commercial
QKD system Clavis2. We scrutinize this implementation for security problems,
and show that designing a hack-proof pulse-energy-measuring detector is far
from trivial. Indeed the first implementation has three serious flaws confirmed
experimentally, each of which may be exploited in a cleverly constructed
Trojan-horse attack. We discuss requirements for a loophole-free implementation
of the monitoring detector.
