1. In an apparatus for communicating securely over an insecure communication channel of the type providing for a calling entity and a called entity, each having such apparatus, to establish a cryptovariable for encryption and decryption, the apparatus comprising means for retrieving or generating a first secret parameter in each apparatus; exchanging variables over said insecure channel including one-way transformations of said first secret parameters; independently computing in each apparatus a cryptovariable from said exchanged variables and said first secret parameters; transmitting from each apparatus over said insecure channel data encrypted according to said cryptovariable; and decrypting in each apparatus data received over said insecure channel according to said cryptovariable, an improvement characterized by means for independently computing in each apparatus an antispoof variable from said exchanged variables and said first secret parameters; and means in each apparatus for displaying said antispoof variables; whereby a user associated with said calling entity and a user associated with said called entity may conversationally exchange said antispoof variables, may conclude that a third entity is interposed between said called and calling entities if said antispoof variables do not agree, and may terminate the communication thereupon.