Quantum Cryptography

Classical digital cryptography is concerned with the problem of providing secure and secret communication over a medium which guarantees neither of these functions. This provision is achieved through the encryption of data at the sender, and the decryption of that cyphertext at the receiver (an encrypted message is known in cryptography as the cyphertext of the message). There are further considerations concerning cryptography such as fair exchange and non-repudiation, but these notions are separate to the security issue which we will be discussing here. The majority of cryptographic techniques in common use today rely on the notion of computational complexity to ensure the difficulty (but not impossibility) of decrypting cyphertexts. Such methods make use of cryptographic keys to encrypt and decrypt data, and depending on the encryption chosen can be extremely secure. The problem with this system is that the receiver must know which encryption was used by the sender in order to decrypt the cyphertext correctly. Obviously, this means that the decryption instructions (the decryption key) also need to be communicated from one party to another. Herein lies the quandary that quantum cryptography is looking to solve: the Key Distribution Problem.