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# trng
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2023-05-10 11:30:58 +00:00
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2022-10-19 07:37:58 +00:00
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Implementation of the Tillitis True Random Number Generator (TRNG).
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## Introduction
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Applications running on the Tillitis TKey device may have a need of
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random numbers. For unpredictable initial vectors, challnges, random
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tokens etc.
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The Tillitis TRNG supports these applications by providing a hardware
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based source of entropy (digital noise) with a uniform distribution.
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Note that the data provided by the TRNG is entropy, not processed
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random numbers. The data should NOT be used directly, but used as
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seed for a (cryptographically safe) random number generator (CSPRNG)
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algorithm.
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## Status
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First version of TKey completed. The TRNG has been tested and provides
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good entropy suitable as seed for a CSPRNG.
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## API
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The TRNG API provides to readable addresses:
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```
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ADDR_STATUS: 0x09
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STATUS_READY_BIT: 0
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ADDR_ENTROPY: 0x20
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```
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The STATUS_READY_BIT in the status register indicates that there is a
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new word of entropy available to read out. When the word is read from
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ADDR_ENTROPY register, the ready bit is cleared.
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Applications requiring multiple words of entropy MUST wait for the
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ready bit to be set again before reading ut subseqent words. Not
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waiting for the ready bit to be set will lead to reading out (at least
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parts of) the same entropy data more than once.
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Applications that need cryptographically safe random number should use
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the output from the TRNG as seed to a Digital Random Bit Generator
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(DRBG), for example a Hash_DRBG.
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## Implementation details
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The implementation is based on free running digital oscillators. The
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implementation creates two sets of oscillators by instantiating a
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number if LCs configured as one bit inverter gates, where the output
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of the inverter is connected to its own input. The oscillators will
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have a toggle rate based on the given internal gate delay and the wire
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delay through given by the feedback circuit.
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After a given number of clock cycles (4096) the outputs from the
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oscillators in each group are XOR combined and sampled into two
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separate registers. This process is repeated a second time, producing
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two more bits, one for each group. These two sets of two bits are then
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XOR combined to produce a single entropy bit. This means that an
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entropy bit is the XOR combined result from two oscillator groups over
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two sampling events.
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Entropy bits are collected into an entropy word. When at least 32 bits
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have been collected, the ready bit is set, indicating to SW that a new
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entropy word is available.
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Note that the entropy word is not held for the SW to read
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out. Sampling and collection is running continuosly, and the word read
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by SW will contain the latest 32 bits collected. Entropy bits not read
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by SW will be discarded at the same rate as new bits are collected.
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Currently the following build time parameters are used to configure
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the implementation:
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- 4096 cycles between sampling
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- 16 oscillators in each group
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- 64 bits collected before setting the ready flag
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With the TKey device running at 18 MHz this means that we sample bits
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at 4.3 kbps. Since we sample twice to produce a single bit, the
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effective raw bitrate is 2.1 kbps.
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The 64 bits collected means that there is a separation of at least 32
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collected entropy bits between bits in the words read out.
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---
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