Figure 1: Confidential communication
Alice wishes to send Bob a secret message – say, a bank transaction – over a potentially insecure communication channel. To do this, Alice and Bob must share a secret key – a long binary number. Alice can then encrypt her message into "cipher text" using the key in conjunction with an encryption algorithm, such as AES. The cipher text may then be transmitted using an ordinary data channel, as it will be unintelligible to an eavesdropper, and Bob can use the key to decrypt the message. In contrast to traditional methods of key distribution, such as a trusted courier, quantum cryptography guarantees the secrecy of the key. The key can also be frequently changed, thereby reducing the risk of it being stolen or of it being deduced by cryptanalysis – statistical analysis of the cipher text.