Cryptography Reference
In-Depth Information
9.4.3 Satellite-to-Ground Quantum Key Distribution
Utilizing Entangled Photon Pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
9.4.4 Other Quantum Key Exchange Scenarios . . . . . . . . . . . . . . . . . . . . 200
9.5 Experimental Feasibility of Key Exchange to Space . . . . . . . . . . . . . . . . . 201
9.5.1 Link Budgets for the Various Systems . . . . . . . . . . . . . . . . . . . . . . . . 201
9.5.2 Feasibility of Faint Pulse Quantum Key
Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
9.5.3 Entangled State Quantum Cryptography: Feasibility . . . . . . . . 206
9.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
Abstract
This chapter describes the development of free-space quantum cryptography
apparatus used for the secure exchange of keys. Existing systems use weak
laser pulses to approximate single photons and polarization coding. Minia-
ture multilaser sources and compact receiver units have been developed.
These can be incorporated with lightweight portable telescopes to exchange
cryptographic key material over long free-space ranges. The record distance
to date has been 23.4 km between two mountain locations. Future experiments
should be able to exchange keys over a 150 km range, and the feasibility of
key exchange to a low Earth orbit satellite has been proven.
9.1 Introduction
With the exponential expansion of electronic commerce, the need for global
protection of data is paramount. Data are normally protected by encoding
them bit-wise using a large random binary number known as a key. An iden-
tical key is used to decode the data at the receiver. The secure distribution of
these keys thus becomes essential to secure communications and transactions
across the globe. At present electronic commerce generally exchanges keys
using public key methods [1]. These methods rely on computational com-
plexity, in particular the difficulty of factoring very large (publicly declared)
numbers, as proof against tampering and eavesdropping. Any confidential
information exchanged using such a key thus becomes insecure after a time
when the rapid improvements in computational power or algorithmic de-
velopment render the public key insecure. To guarantee long-term security,
the cryptographic key must be exchanged in an absolutely secure way. The
conventional method used for this for most of the last century has been the
trusted courier carrying a long random key from one location to the other.
Following the idea of Bennett and Brassard in 1984 [2], it is only recently that
absolutely secure key exchange between two sites has been demonstrated
over fiber [3-6] and free-space [7-13] optical links. This technique, known as
quantum cryptography, has security based on the laws of nature and is, in
