FiReBuG - Fast Random Bit Generator

APA Consulting has developed the ORB Open Random Bit Generator that continues to be a reliable stand-alone chip for cryptographic entropy generation. This new design, the FiReBuG (Fast Random Bit Generator), is intended for design onto silicon as part of a larger security-enhanced processor chip. The raw bits produced by the FiReBuG will include some statistical imperfections (autocorrelation) due to the nature of the circuitry, but these can be easily removed with digital processing, and in certain applications may be tolerable. The FiReBuG is capable of at least 10⁸ bits/sec, compared with 10³ bits/sec for the ORB.

The FiReBuG's principle of operation is illustrated in the block diagram and flowchart that follow. The circuit includes both digital logic and analog elements. The logic controls the timing sequence and a decision process for the analog elements. Two capacitors hold charge. They are connected interchangeably through solid-state switches. The capacitors switch roles as the source of input potential, and as the receptacle for output potential of the active analog elements. The circuit approximates an endless binary expansion of the initial charge on C₁. Except for the precision with which the initial charge is known, the output bits are random.

Analog Block Diagram

In the first half of an operating cycle, the charge on C₁ is connected to the input of the analog section which determines if it is greater than 1/2 (of the signal range). If so C₁'s potential less 1/2 is doubled, else just its potential is doubled. The resulting potential is applied to C₂for storage. In the second half of a cycle, C₂ is connected as input and the result is stored in C₁. The process then repeats.

Logic Flowchart

A simulation program has been written to verify the practicality of the FiReBuG system, and to explore parameters needed to plan a VLSI realization. The simulation is discussed in the report FiReBuG ODE Simulation.

The FiReBuG has numerous advantages over other random bit generators. Among them are its simplicity, robustness, and the rate at which it can produce a random bitstream. Also, it produces a fixed, guaranteed bitrate that can easily be synchronized with other digital circuitry. It is conceivable that 10⁴ replications could be placed on a single silicon die, and operate in parallel to produce the 10¹² bits/sec needed for the new "everlasting security" systems developed by Michael Rabin.