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 thesis describes how to calculate the parameters and how to use the results for fast estimation of confusion and diffusion for any cipher instance within the defined cipher space.