Quantum Information Group, Quantum Key Distribution
Quantum cryptography provides a secure means for distributing secret keys between two parties (usually referred to as Alice and Bob) on an optical network. A unique feature of the technique is that the secrecy of the keys is independent of the resources available to a hacker. In particular, their secrecy does not rely upon a difficult mathematical problem that could be solved, or a clever algorithm that could be cracked or even some ingenious hardware that might one day be reverse engineered.
The keys distributed using quantum cryptography can be used along with an algorithm to encrypt data or messages sent between the users, thereby ensuring their confidentiality. Only users in possession of the secret key can decrypt the data to recover the original message. The figure shows an example of how quantum cryptography can be used to send an encrypted message (in this case a photo) across the network. This example uses an encryption algorithm, called the 'one-time pad', which simply makes binary additions of the bits of the picture with those of the key, to form the encrypted picture. A similar addition of the encrypted picture and the key at the receiver's side recovers the original picture.
The one-time pad requires that the encryption key is the same length as the data to be sent. Quantum cryptography provides a method to distribute sufficient key material to make the one time pad viable for short, but highly confidential, messages. Data sent using the one-time pad can be guaranteed secret with certain probability, provided the key is used only once. Larger volumes of data can be encrypted using algorithms such as DES (Data Encryption Standard), 3DES (triple-DES) or AES (Advanced Encryption Standard). These use keys of fixed length, for example 112 bits for 3DES and thus allow a large volume of data to be sent with one short key.
Other important uses for the keys distributed by quantum cryptography are to authenticate messages sent across the network, i.e. to identify their origin and integrity, and to identify users.
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