Quantum Crypto.

["Joseph Reagle Jr." <reagle@gl.umbc.edu>]

[Here is the conclusion to my QC paper, unfortunately I can't get the 
whole file into a PS format because of the faulty file translators in the 
Mac applications.]

Conclusion
	Quantum cryptography has proven to be an interesting and novel 
application of quantum physics.   It does posses some severe limitations 
that I have considered.
	Optimistic predictions of it�s affective area is still far below 
100 km.  This may of course change depending on technological 
development.  It has been suggested to me that one could have secure 
stations where interception and reception of the message would be 
allowed. [10]  This is possible, but weakens the �absoluteness� that is 
the appeal of quantum cryptography.  
	A basic assumption is made previous to the research mentioned: 
that Eve will not interfere on the public channel.  It could be very 
possible that Eve would set herself up between Alice and Bob on the 
quantum and private channels, and act as a relay station that I mentioned 
in the first point.  She would have to impersonate both Alice and Bob, 
who in reality might not even be on the same public and quantum channels, 
but merely think they are.  Public key methods could be used for 
authentication, but this destroys the motivation for the use of quantum 
cryptography.
		I feel the solution here is in the definition of 
�public�.  Meaning a random and public switching of public channels, 
phone numbers and such.  Even this may be subverted by a very powerful 
Eve who may also control the phone company�s switching circuits.  Perhaps 
further thought can resolve this issue, but the problem of identification 
and authentication on the public channel is severe.
	Further, quantum cryptography is subject to a denial of service 
attack.  If Eve wishes, she may destroy the unique and expensive quantum 
channel, or merely observe everything that goes by, not caring to read 
the information, just making it unsuitable for use by Alice and Bob.  
Ekert�s concept of keeping shared EPR pairs in permanent storage (perhaps 
using a superconductor to warehouse keys when the quantum channel is 
open) is not yet feasible, and it will be necessary to keep these keys 
somewhere , but the security of keys is not a problem unique to quantum 
cryptography.  
		I look forward to the resolution of these issues and the 
further  development of the technology that will allow quantum 
cryptography to become a �practical� security mechanism.


1.	C. Bennett. Science.. vol. 257, p. 752 (August, 1992).
2,	C. Bennett, G. Brassard, and A. Ekert.  Scientific American. p. 	
	50 (Oct., 1992)
3.	A. Ekert, Phys. Rev. Lett. vol. 67, p. 661 (1991)
4.	C. Bennet, and G. Brassard, Phys. Rev. Lett. vol. 68, p. 557 	(1992)
5.	A. Ekert, J. Rarity, P. Tapster, and G. Palma, Phys. Rev. Lett. 
	vol. 	69, p. 1293, (1993).
6.	A. Muller, J. Breguet, and N. Gisin. Europhs. Lett., vol. 23 (6), 
p. 	383 (1993).
7. 	S. Barnett, and S. Phoenix. Phys. Rev. A, vol 48 (1), p. R5, 
	(July, 	1993).
8.	C. Bennett. Phys. Rev. Lett. vol 68 (21), p. 3121 (1992)
9.	D. Denning. Cryptography and Data Security.
10.     Personal e-mail as a follow-up to a posting to sci.crypt.  I have 	
	unfortunately lost the person�s name.