Synchronous stream cipher

A synchronous data-stream generator for generating a stream of output data items of at least one bit in synchronisation with a clock trigger;

• the data-stream generator comprising:
• a plurality of subgenerators Mi, i≧1, each subgenerator Mi comprising a respective clock input and a respective output;
• each subgenerator Mi being operative to generate a data item of at least one bit on the respective output in response to a trigger received via the respective clock input;
• means for combining a generated data item of each of the subgenerators Mi, forming the output data item of the data-stream generator;
• a control subgenerator C comprising an input for receiving the clock trigger and an output;
• the control subgenerator C being operative to generate a control data item of at least one bit on the output in response to the clock trigger;
• and
• control means operative to provide a trigger to at least one of the clock inputs of the subgenerators Mi in dependence on the control data item of the control subgenerator C;
• characterised in that the control means comprises for at least one subgenerator Mi an associated number selector Si for, in dependence on the control data item of the control subgenerator C, selecting a number ni,j from a group Hi of different integer numbers;
• at least two numbers of the group Hi being larger than zero;
• the group Hi being associated with the number selector Si;
• and in that the control means is operative to cause the associated subgenerator Mi to provide at the output the selected ni,j-th data item successive to a last generated data item.