Process and device for making secure a telephone link connecting two subscriber sets

Process for making a secure telephone link connecting first and second subscriber sets, said telephone link including first and second network terminals to which the first and second subscriber sets are respectively connected, and first and second subscriber lines respectively linking the first and second network terminals to first and second line terminals respectively located at first and second local switching centers mutually linked by a switched telephone network, characterized in that, the network terminal and the line terminal of at least one of the subscriber lines being respectively equipped with network encryption/decryption means and with line encryption/decryption means, wherein the process comprises an encryption step in which information exchanged between the first and second subscriber sets is encrypted, at least on one of the two subscribe lines which is connected to the said respective network and line encryption/decryption means, on the basis of at least one encryption key generated by said network encryption/decryption means or by said line encryption/decryption means, and varying temporally in pseudo-random fashion by said network encryption/decryption means or by said line encryption/decryption means, the information exchanged between the two subscriber sets being transmitted in clear at least between each network terminal and the corresponding subscriber set.