Secret communication method and secret communication device thereof
This invention finds particular application in the field of quantum cryptography. A sender and receiver each determine an initial random number, X and Y respectively. These initial random numbers are to be used for encryption and decryption of data. The random numbers are correlated but not identical. The degree of mismatch between them is measured as an error ratio 204, from which an encoding ratio is deduced. Furthermore an estimate is made of the extent to which the initial random numbers can be compromised by eavesdropping 219. In order to counteract eavesdropping an encryption function for encrypting data to be transmitted over a public communication path 210 includes a confidentiality matrix C, such as a Toeplitz matrix comprising random numbers. The dimensions of the confidentiality matrix are determined by the expected extent of eavesdropping.
Preferably encryption 207 is performed using the matrix equation on page 83, where Z is the ciphertext, M is the cleartext, D is a further random number generated 205 by the sender and F takes account of an error correction code (see page 38). Preferably the message is decrypted 217 using the equation on page 54, where g is an error correction decoding function. Confidentiality increase matrices may be exchanged between sender and receiver (213, claim 8) or generated by both sender and receiver.