ParagonIE_Sodium_File::secretbox_decrypt

    protected static function secretbox_decrypt($ifp, $ofp, $mlen, $nonce, $key)
    {
        if (PHP_INT_SIZE === 4) {
            return self::secretbox_decrypt_core32($ifp, $ofp, $mlen, $nonce, $key);
        }
        $tag = fread($ifp, 16);
        if (!is_string($tag)) {
            throw new SodiumException('Could not read input file');
        }

        /** @var string $subkey */
        $subkey = ParagonIE_Sodium_Core_HSalsa20::hsalsa20($nonce, $key);

        /** @var string $realNonce */
        $realNonce = ParagonIE_Sodium_Core_Util::substr($nonce, 16, 8);

        /** @var string $block0 */
        $block0 = ParagonIE_Sodium_Core_Salsa20::salsa20(
            64,
            ParagonIE_Sodium_Core_Util::substr($nonce, 16, 8),
            $subkey
        );

        /* Verify the Poly1305 MAC -before- attempting to decrypt! */
        $state = new ParagonIE_Sodium_Core_Poly1305_State(self::substr($block0, 0, 32));
        if (!self::onetimeauth_verify($state, $ifp, $tag, $mlen)) {
            throw new SodiumException('Invalid MAC');
        }

        /*
         * Set the cursor to the end of the first half-block. All future bytes will
         * generated from salsa20_xor_ic, starting from 1 (second block).
         */
        $first32 = fread($ifp, 32);
        if (!is_string($first32)) {
            throw new SodiumException('Could not read input file');
        }
        $first32len = self::strlen($first32);
        fwrite(
            $ofp,
            self::xorStrings(
                self::substr($block0, 32, $first32len),
                self::substr($first32, 0, $first32len)
            )
        );
        $mlen -= 32;

        /** @var int $iter */
        $iter = 1;

        /** @var int $incr */
        $incr = self::BUFFER_SIZE >> 6;

        /* Decrypts ciphertext, writes to output file. */
        while ($mlen > 0) {
            $blockSize = $mlen > self::BUFFER_SIZE
                ? self::BUFFER_SIZE
                : $mlen;
            $ciphertext = fread($ifp, $blockSize);
            if (!is_string($ciphertext)) {
                throw new SodiumException('Could not read input file');
            }
            $pBlock = ParagonIE_Sodium_Core_Salsa20::salsa20_xor_ic(
                $ciphertext,
                $realNonce,
                $iter,
                $subkey
            );
            fwrite($ofp, $pBlock, $blockSize);
            $mlen -= $blockSize;
            $iter += $incr;
        }
        return true;
    }