Quantum Information Group, Applications of Cryptography
Cryptography is best known as a way of keeping the contents of a message secret. Confidentiality of network communications, for example, is of great importance for e-commerce and other network applications. However, the applications of cryptography go far beyond simple confidentiality. In particular, cryptography allows the network business and customer to verify the authenticity and integrity of their transactions. If the trend to a global electronic marketplace continues, better cryptographic techniques will have to be developed to protect business transactions.
Sensitive information sent over an open network may be scrambled into a form that cannot be understood by a hacker or eavesdropper. This is done using a mathematical formula, known as an encryption algorithm, which transforms the bits of the message into an unintelligible form. The intended recipient has a decryption algorithm for extracting the original message. There are many examples of information on open networks, which need to be protected in this way, for instance, bank account details, credit card transactions, or confidential health or tax records.
In order to allow different users to use the same algorithm, the algorithm is used in conjunction with a secret key, a long sequence of binary numbers, as shown in the illustration, which is known only by the legitimate users. Only users sharing the same key will be able to decrypt each other's encrypted messages. Since the key allows access to the encrypted information, it is of paramount importance that it is kept secret and is frequently changed.
Before two parties can send information securely, they must first exchange a secret key. This however presents a dilemma, sometimes called the ‘Catch 22 of Cryptography’ — how can the two parties exchange a key secretly before they can communicate in secret? Even if the sender and receiver found a channel that they believed to be secure, in the past there has been no way to test the secrecy of each key. Quantum cryptography solves this problem. It allows the sender and receiver to test and guarantee the secrecy of each individual key.