Abstract: In order to improve the security of message transmission from a terminal apparatus in an electronic banking or other data communications system a check-sum or MAC is computed from the data within the message in dependence upon a cryptographic key. This MAC is issued as a "challenge" to the user who is also equipped with a separate portable token for computing a "response" in dependence upon a second cryptographic key which is unique to his token. This "response" is then entered into the terminal and appended to the message as its authentication code before transmission. A recipient of the message and authentication code equipped with the same cryptographic keys can therefore check both the contents of the message and the correct identity of the sender by computing an expected authentication code from the received message and comparing it with the code received.

Abstract: A pseudo-random binary sequence generator comprises at least one shift register (S, T) arranged in a recirculating loop and having a plurality of logic gates (G) for logically combining the outputs of selected stages of the register to provide a pseudo-random sequence, and a multiplexer (M), having a p data inputs and q address inputs all connected to selected shift register stages, and which selects at any instant one of the p data input bits in accordance with the q-bit address word to provide the generator output. The number s of logic gates is especially high and is related to the total number r of shift register stages (r>p+q) by the expression: 2.sup.s .gtoreq.r.sup.2. Some of the shift register stages of the or each shift register are connected to selected data inputs of the multiplexer and others of the stages of the same shift register are connected to selected address inputs of the multiplexer.

Abstract: A data transmission system comprises a transmitter transmitting data messages to n receivers by means of a link. Each message is preceded by a preamble. Each preamble is such that it can either be recognized only by one of the receivers (to whom, alone, the message is directed) or it can be recognized by all receivers when the message is intended for all of them. Each preamble is recognized if at least x of its y bits are recognized. At each receiver, the received preamble is sampled at a multiple of the bit rate and the respective pulse trains produced by the same-numbered samples in each bit period are individually compared with the two predetermined preambles to be recognized by that receiver.