csprng.h

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#pragma once
 
#include <Arduino.h>
#include <ChaCha.h>
 
static const uint8_t CSPRNG_BLOCK_BYTES = 64u;
 
static ChaCha  s_chacha;
 
static uint8_t s_keystreamBuf[CSPRNG_BLOCK_BYTES];
 
static uint8_t s_keystreamUsed = CSPRNG_BLOCK_BYTES;
 
static void s_fillKeystreamBlock() {
  // Zero the cache so the in-place XOR yields the raw keystream.
  memset(s_keystreamBuf, 0, CSPRNG_BLOCK_BYTES);
 
  // XOR the all-zero input with ChaCha20's keystream -> raw keystream output.
  // The library advances its internal block counter after every 64-byte call.
  s_chacha.encrypt(s_keystreamBuf, s_keystreamBuf, CSPRNG_BLOCK_BYTES);
 
  s_keystreamUsed = 0u;
}
 
void initCSPRNG(const uint8_t* hardwareSeed) {
  // Install the 256-bit hardware-entropy key into the ChaCha20 engine.
  s_chacha.setKey(hardwareSeed, 32u);
 
  // Set a zero IV (nonce). Safe because the key is unique per session.
  // The IV is 8 bytes for djb-ChaCha (not the 12-byte IETF variant used for packets).
  static const uint8_t zeroIV[8] = {};
  s_chacha.setIV(zeroIV, sizeof(zeroIV));
 
  // Pre-compute the first keystream block so the first caller incurs no stall.
  s_fillKeystreamBlock();
}
 
uint32_t getSecureRandom32() {
  // Transparently refill the cache when fewer than 4 bytes remain.
  if (s_keystreamUsed + 4u > CSPRNG_BLOCK_BYTES) {
    s_fillKeystreamBlock();
  }
 
  // Assemble four consecutive keystream bytes as a little-endian uint32_t.
  const uint32_t value =
       static_cast<uint32_t>(s_keystreamBuf[s_keystreamUsed])             |
      (static_cast<uint32_t>(s_keystreamBuf[s_keystreamUsed + 1u]) <<  8u) |
      (static_cast<uint32_t>(s_keystreamBuf[s_keystreamUsed + 2u]) << 16u) |
      (static_cast<uint32_t>(s_keystreamBuf[s_keystreamUsed + 3u]) << 24u);
 
  s_keystreamUsed += 4u;
  return value;
}