Before exploring quantum key distribution, it is important to understand the state

of modern cryptography and how quantum cryptography may address current

digital cryptography limitations. Since public key cryptography involves complex

calculations that are relatively slow, they are employed to exchange keys rather

than for the encryption of voluminous amounts of date. For example, widely

deployed solutions, such as the RSA and the Diffie-Hellman key negotiation

schemes, are typically used to distribute symmetric keys among remote parties.

However, because asymmetric encryption is significantly slower than symmetric

encryption, a hybrid approach is preferred by many institutions to take advantage

of the speed of a shared key system and the security of a public key system for

the initial exchange of the symmetric key. Thus, this approach exploits the speed

and performance of a symmetric key system while leveraging the scalability of a

public key infrastructure.

However, public key cryptosystems such as RSA and Diffie-Hellman are not

based on concrete mathematical proofs. Rather, these algorithms are

considered to be reasonably secure based on years of public scrutiny over the

fundamental process of factoring large integers into their primes, which is said to

be “intractable”. In other words, by the time the encryption algorithm could be

defeated, the information being protected would have already lost all of its value.

Thus, the power of these algorithms is based on the fact that there is no known

mathematical operation for quickly factoring very large numbers given today’s

computer processing power.

Secondly, there is uncertainty whether a theorem may be developed in the future

or perhaps already available that can factor large numbers into their primes in a

timely manner. At present, there is no existing proof stating that it is impossible

to develop such a factoring theorem. As a result, public key systems are thus

vulnerable to the uncertainty regarding the future creation of such a theorem,

which would have a significant affect on the algorithm being mathematical

intractable. This uncertainty provides potential risk to areas of national security

and intellectual property which require perfect security.

In sum, modern cryptography is vulnerable to both technological progress of

computing power and evolution in mathematics to quickly reverse one way

functions such as that of factoring large integers. If a factoring theorem were

publicized or computing became powerful enough to defeat public cryptography,

then business, governments, militaries and other affected institutions would have

to spend significant resources to research the risk of damage and potentially

deploy a new and costly cryptography system quickly.