True Random Number Generator core implemented in Verilog.


This repo contains the design of a True Random Number Generator (TRNG) for the Cryptech OpenHSM project.

Design inspiration, ideas and principles

The TRNG MUST be a really good one. Furthermore it must be trustable by its users. That means it should not do wild and crazy stuff. And users should be able to verify that the TRNG works as expected.

  • Follow best practice
  • Be conservative - No big untested ideas.
  • Support transparency - The parts should be testable.

Some of our inspiration comes from:

  • The Fortuna RNG by Ferguson and Schneier as described in Cryptography Engineering.

  • /dev/random in OpenBSD

System description

The TRNG consists of a chain with three main subsystems

  • Entropy generation
  • Entropy mixing
  • Random generation

Entropy generation

The entropy generation subsystems consists of at least two separate entropy generators. Each generator collects entropy from an independent physical process. The entropy sources MUST be of different types. For example avalance noise from a reversed bias P/N junction as one source and RSSI LSB from a receiver.

The reason for having multiple entropy sources is both to provide redundancy as well as making it harder for an attacker to affect the entropy collection by forcing the attacker to try and affect different physical processes simultaneously.

A given entropy generator is responsible for collecting the entropy (possibly including A/D conversion.). The entropy generator MUST implement some on-line testing of the physical entropy source based on the entropy collected. The tests shall be described in detail here but will at least include tests for:

  • No long run lengths in generated values.
  • Variance that exceeds a given threshhold.
  • Mean value that don't deviate from expected mean.
  • Frequency for all possible values are within expected variance.

If the tests fails over a period of generated values the entropy source MUST raise an error flag. And MAY also block access to the entropy it otherwise provides.

There shall also be possible to read out the raw entropy collected from a given entropy generator. This MUST ONLY be possible in a specific debug mode when no random generation is allowed. Also the entropy provided in debug mode MUST NOT be used for later random number generation.

The entropy generator SHALL perform whitening on the collected entropy before providing it as 32-bit values to the entropy accumulator.

Entropy mixing

The entropy mixer subsystems reads 32-bit words from the entropy generators to build a block of bits to be mixed.

When 1024 bits of mixed entropy has been collected the entropy is used as a message block which is fed into a hash function.

The hash function used is SHA-512 (NIST FIPS 180-4).

The digest is then extracted and provided to the random generation as as a seed.

Random generation

The random generation consists of a cryptographically secure pseudo random number generator (CSPRNG). The CSPRNG used in the trng is the stream cipher ChaCha.

ChaCha is seeded with:

  • 512 bits block
  • 256 bits key
  • 64 bits IV
  • 64 bits counter

In total the seed used is: 896 bits. This requires getting two seed blocks of 512 bits from the mixer.

The number of rounds used in ChaCha is conservatively selected. We propose that the number of rounds shall be at least 24 rounds. Possibly 32 rounds. Given the performance in HW for ChaCha and the size of the keystream block, the TRNG should be able to generate plentiful of random values even with 32 rounds.

The random generator shall support the ability to test its functionality by seeding it with a user supplied value and then generate a number of values in a specific debug mode. The normal access to generated random values MUST NOT be allowed during the debug mode. The random generator MUST also set an error flag during debug mode. Finally, when exiting the debug mode, reseeding MUST be done.

Finally the random generator provides random numbers as 32-bit values. the 512 bit keystream blocks from ChaCha are divided into 16 32-bit words and provided in sequence.

Implementation details

The core supports multpiple entropy sources as well as a CSPRNG. For each entropy source there are some estimators that checks that the sources are not broken.

There are also an ability to extract raw entropy as well as inject test data into the CSPRNG to verify the functionality.

The core will include one FPGA based entropy source but expects the other entropy source(s) to be connected on external ports. It is up to the user/system implementer to provide physical entropy souces. We will suggest and provide info on how to design at least one such source.

For simulation there are simplistic fake entropy sources that can be found in the tb/fake_modules directory. This modules SHOULD NOT be used as real sources.

For synthesis there are wrappers for the real entropy source cores to adapt their interfaces to what we need in the trng. These wrappers should not be included during simulation.


Normal operation:

  • Extract 32-bit random words.

Config parameters:

  • Number of blocks in warm-up.
  • Number of keystream blocks before reseeding.

Debug access

  • Enable/disable entropy generator X
  • Check health of entropy generator X
  • Read raw entropy from entropy generator X as 32-bit word.
  • Write 256 bit seed value as 8 32-bit words
  • Read out one or more 512 bit keystream blocks as 32-bit words.


*** (2014-09-11) ***

The first version of the CSPRNG is debugged and completed. This version supports automatic reseeding and an output fifo.