fn pub hchacha20(key: ref ByteArray, nonce: ref ByteArray) -> ByteArray {
  if key.size != KEY_SIZE {
    panic('HChaCha20 key sizes must be exactly ${KEY_SIZE} bytes')
  }

  if nonce.size != HCHACHA_NONCE_SIZE {
    panic('HChaCha20 nonce sizes must be exactly ${HCHACHA_NONCE_SIZE} bytes')
  }

  let out = ByteArray.filled(with: 0, times: 32)
  let matrix = Matrix(
    [
      0x61707865,
      0x3320646E,
      0x79622D32,
      0x6B206574,
      little.read_u32(from: key, at: 0),
      little.read_u32(from: key, at: 4),
      little.read_u32(from: key, at: 8),
      little.read_u32(from: key, at: 12),
      little.read_u32(from: key, at: 16),
      little.read_u32(from: key, at: 20),
      little.read_u32(from: key, at: 24),
      little.read_u32(from: key, at: 28),
      little.read_u32(from: nonce, at: 0),
      little.read_u32(from: nonce, at: 4),
      little.read_u32(from: nonce, at: 8),
      little.read_u32(from: nonce, at: 12),
    ],
  )

  matrix.perform_rounds
  little.write_u32(matrix.words.get(0), into: out, at: 0)
  little.write_u32(matrix.words.get(1), into: out, at: 4)
  little.write_u32(matrix.words.get(2), into: out, at: 8)
  little.write_u32(matrix.words.get(3), into: out, at: 12)
  little.write_u32(matrix.words.get(12), into: out, at: 16)
  little.write_u32(matrix.words.get(13), into: out, at: 20)
  little.write_u32(matrix.words.get(14), into: out, at: 24)
  little.write_u32(matrix.words.get(15), into: out, at: 28)
  out
}