A method is proposed to offer privacy in computer communications, using symmetric product block ciphers. The security protocol involves a cipher negotiation stage in which two communicating parties select privately a cipher from a public cipher space. The cipher negotiation process includes an on-line cipher evaluation stage in which the cryptographic strength of the proposed cipher is estimated. Cryptographic strength is measured by confusion and diffusion. A method is proposed to describe quantitatively these two properties. A number of parameters are defined, such as the confusion and diffusion matrices and the marginal diffusion. These parameters involve computationally intensive calculations that are performed off-line, before any communication takes place. Once they are calculated, they are used to obtain estimation equations, which are used for on-line, fast evaluation of the confusion and diffusion of the negotiated cipher. A technique proposed in this... Это и многое другое вы найдете в книге Symmetric cryptography in computer communications (Vasilios Katos)