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Method and apparatus for generating a key stream

  • US 7,769,169 B2
  • Filed: 01/23/2003
  • Issued: 08/03/2010
  • Est. Priority Date: 01/23/2002
  • Status: Active Grant
First Claim
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1. A method of generating a key stream from a precomputed state information table including state information, said method being performed by a correspondent in a communication system and comprising:

  • a) initializing a counter and an accumulator with non-zero values by;

    i. initializing said counter;

    ii. combining with a value of the accumulator state information identified by a value of the counter and key information identified by the value of the counter to obtain another value of the accumulator;

    iii. combining state information identified by the value of the counter with a first of said non-zero values;

    iv. combining state information identified by said another value of the accumulator with a second of said non-zero values;

    v. swapping the state information identified by the value of the counter with the state information identified by the another value of the accumulator;

    vi. incrementing the counter to generate another value for the counter; and

    vii. repeating steps ii to vi to produce said non-zero values;

    b) obtaining a constant odd value, c greater than 1, derived from a publicly available session identifier;

    c) combining with a current value for the accumulator state information identified by a current value for the counter to obtain a next value for the accumulator;

    d) swapping said state information identified by the current value for the counter with state information identified by said next value for the accumulator;

    e) combining with said state information identified by said next value for the accumulator said state information identified by the current value for the counter to generate a combined value;

    f) outputting the state information identified by the combined value as a byte of the key stream;

    g) combining said odd value c with the current value for the counter to generate a next value for said counter; and

    h) repeating steps c) to g) to produce each byte of the key stream.

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