ParagonIE_Sodium_Core_ChaCha20_Ctx::__construct

    public function __construct($key = '', $iv = '', $counter = '')
    {
        if (self::strlen($key) !== 32) {
            throw new InvalidArgumentException('ChaCha20 expects a 256-bit key.');
        }
        if (self::strlen($iv) !== 8) {
            throw new InvalidArgumentException('ChaCha20 expects a 64-bit nonce.');
        }
        $this->container = new SplFixedArray(16);

        /* "expand 32-byte k" as per ChaCha20 spec */
        $this->container[0]  = 0x61707865;
        $this->container[1]  = 0x3320646e;
        $this->container[2]  = 0x79622d32;
        $this->container[3]  = 0x6b206574;
        $this->container[4]  = self::load_4(self::substr($key, 0, 4));
        $this->container[5]  = self::load_4(self::substr($key, 4, 4));
        $this->container[6]  = self::load_4(self::substr($key, 8, 4));
        $this->container[7]  = self::load_4(self::substr($key, 12, 4));
        $this->container[8]  = self::load_4(self::substr($key, 16, 4));
        $this->container[9]  = self::load_4(self::substr($key, 20, 4));
        $this->container[10] = self::load_4(self::substr($key, 24, 4));
        $this->container[11] = self::load_4(self::substr($key, 28, 4));

        if (empty($counter)) {
            $this->container[12] = 0;
            $this->container[13] = 0;
        } else {
            $this->container[12] = self::load_4(self::substr($counter, 0, 4));
            $this->container[13] = self::load_4(self::substr($counter, 4, 4));
        }
        $this->container[14] = self::load_4(self::substr($iv, 0, 4));
        $this->container[15] = self::load_4(self::substr($iv, 4, 4));
    }