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...