ParagonIE_Sodium_Core_Salsa20::salsa20_xor_ic

    public static function salsa20_xor_ic($m, $n, $ic, $k)
    {
        $mlen = self::strlen($m);
        if ($mlen < 1) {
            return '';
        }
        $kcopy = self::substr($k, 0, 32);
        $in = self::substr($n, 0, 8);
        // Initialize the counter
        $in .= ParagonIE_Sodium_Core_Util::store64_le($ic);

        $c = '';
        while ($mlen >= 64) {
            $block = self::core_salsa20($in, $kcopy, null);
            $c .= self::xorStrings(
                self::substr($m, 0, 64),
                self::substr($block, 0, 64)
            );
            $u = 1;
            for ($i = 8; $i < 16; ++$i) {
                $u += self::chrToInt($in[$i]);
                $in[$i] = self::intToChr($u & 0xff);
                $u >>= 8;
            }

            $mlen -= 64;
            $m = self::substr($m, 64);
        }

        if ($mlen) {
            $block = self::core_salsa20($in, $kcopy, null);
            $c .= self::xorStrings(
                self::substr($m, 0, $mlen),
                self::substr($block, 0, $mlen)
            );
        }
        try {
            ParagonIE_Sodium_Compat::memzero($block);
            ParagonIE_Sodium_Compat::memzero($kcopy);
        } catch (SodiumException $ex) {
            $block = null;
            $kcopy = null;
        }

        return $c;
    }