A
practical trojan horse for Bell-inequality-based quantum cryptography
(pp434-442)
J. Larsson
doi:
https://doi.org/10.26421/QIC2.6-2
Abstracts:
Quantum Cryptography, or more accurately, Quantum Key
Distribution (QKD) is based on using an unconditionally secure ``quantum
channel'' to share a secret key among two users. A manufacturer of QKD
devices could, intentionally or not, use a (semi-)classical channel
instead of the quantum channel, which would remove the supposedly
unconditional security. One example is the BB84 protocol, where the
quantum channel can be implemented in polarization of single photons.
Here, use of several photons instead of one to encode each bit of the
key provides a similar but insecure system. For protocols based on
violation of a Bell inequality (e.g., the Ekert protocol) the situation
is somewhat different. While the possibility is mentioned by some
authors, it is generally thought that an implementation of a
(semi-)classical channel will differ significantly from that of a
quantum channel. Here, a counterexample will be given using an identical
physical setup as is used in photon-polarization Ekert QKD. Since the
physical implementation is identical, a manufacturer may include this
modification as a Trojan Horse in manufactured systems, to be activated
at will by an eavesdropper. Thus, the old truth of cryptography still
holds: you have to trust the manufacturer of your cryptographic device.
Even when you do violate the Bell inequality.
Key words: quantum
cryptography, Trojan horse, Ekert protocol, Bell iequality |