ParagonIE_Sodium_Compat::unpad

    public static function unpad($padded, $blockSize, $dontFallback = false)
    {
        /* Type checks: */
        ParagonIE_Sodium_Core_Util::declareScalarType($padded, 'string', 1);
        ParagonIE_Sodium_Core_Util::declareScalarType($blockSize, 'int', 2);

        $padded = (string) $padded;
        $blockSize = (int) $blockSize;

        if (self::useNewSodiumAPI() && !$dontFallback) {
            return (string) sodium_unpad($padded, $blockSize);
        }
        if ($blockSize <= 0) {
            throw new SodiumException('block size cannot be less than 1');
        }
        $padded_len = ParagonIE_Sodium_Core_Util::strlen($padded);
        if ($padded_len < $blockSize) {
            throw new SodiumException('invalid padding');
        }

        # tail = &padded[padded_len - 1U];
        $tail = $padded_len - 1;

        $acc = 0;
        $valid = 0;
        $pad_len = 0;

        $found = 0;
        for ($i = 0; $i < $blockSize; ++$i) {
            # c = tail[-i];
            $c = ParagonIE_Sodium_Core_Util::chrToInt($padded[$tail - $i]);

            # is_barrier =
            #     (( (acc - 1U) & (pad_len - 1U) & ((c ^ 0x80) - 1U) ) >> 8) & 1U;
            $is_barrier = (
                (
                    ($acc - 1) & ($pad_len - 1) & (($c ^ 80) - 1)
                ) >> 7
            ) & 1;
            $is_barrier &= ~$found;
            $found |= $is_barrier;

            # acc |= c;
            $acc |= $c;

            # pad_len |= i & (1U + ~is_barrier);
            $pad_len |= $i & (1 + ~$is_barrier);

            # valid |= (unsigned char) is_barrier;
            $valid |= ($is_barrier & 0xff);
        }
        # unpadded_len = padded_len - 1U - pad_len;
        $unpadded_len = $padded_len - 1 - $pad_len;
        if ($valid !== 1) {
            throw new SodiumException('invalid padding');
        }
        return ParagonIE_Sodium_Core_Util::substr($padded, 0, $unpadded_len);
    }