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@@ -1,4 +1,38 @@ rosc_entropy ============ -Entropy source based on jitter between multiple, digital ring oscillators. +Digital entropy source based on on jitter between multiple, digital ring +oscillators. The entropy source is used in the TRNG as one of several +entropy sources feeding the mixer. + + +## Functionality ## + +The digital oscillators are created using adders. The carry out from the +adder are inverted and fed back into the adder as carry in. In +combination with operand values we basically get an inverted signal (the +carry out) that toggles reapeatedly. By having the operands externally +defined, synthesis tools in general will not optimize them away. + +The carry out signal is sampled with a clock that toggles at a much +slower rate than the intrinsic toggle rate of the carry out signal. In a +modern FPGA, the toggle rate may be 400+ MHz while the sample rate might +be 10 kHz. This sample time allows the differences in intrinsic toggle +frequency between separate oscillators to drift apart after sampling. + +The entropy source contains 32 separate oscillators. The outputs from +the oscillators are XOR-combined to create a single entropy bit. Entropy +bits are collected into 32-bit words which are provided to entropy +consumers. + + +## Implementation Results ## + +The entropy source has been implemented, tested and shown to produce +good quality entropy (using the ent estimation tool etc) in Altera +Cyclone-IV and Cyclone-V devices as well as in Xilinx Spartan-6. + +The Xilinx synthesis tool will try to optimize the combinational loop +away. (More specifically, it claims that the oscillator sample registers +will have a fixed value.). There is therefore an attribute in the source +code to force the tool to preserve the register. |