ParagonIE_Sodium_File::secretbox_encrypt

    protected static function secretbox_encrypt($ifp, $ofp, $mlen, $nonce, $key)
    {
        if (PHP_INT_SIZE === 4) {
            return self::secretbox_encrypt_core32($ifp, $ofp, $mlen, $nonce, $key);
        }

        $plaintext = fread($ifp, 32);
        if (!is_string($plaintext)) {
            throw new SodiumException('Could not read input file');
        }
        $first32 = self::ftell($ifp);

        /** @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 = str_repeat("\x00", 32);

        /** @var int $mlen - Length of the plaintext message */
        $mlen0 = $mlen;
        if ($mlen0 > 64 - ParagonIE_Sodium_Crypto::secretbox_xsalsa20poly1305_ZEROBYTES) {
            $mlen0 = 64 - ParagonIE_Sodium_Crypto::secretbox_xsalsa20poly1305_ZEROBYTES;
        }
        $block0 .= ParagonIE_Sodium_Core_Util::substr($plaintext, 0, $mlen0);

        /** @var string $block0 */
        $block0 = ParagonIE_Sodium_Core_Salsa20::salsa20_xor(
            $block0,
            $realNonce,
            $subkey
        );

        $state = new ParagonIE_Sodium_Core_Poly1305_State(
            ParagonIE_Sodium_Core_Util::substr(
                $block0,
                0,
                ParagonIE_Sodium_Crypto::onetimeauth_poly1305_KEYBYTES
            )
        );

        // Pre-write 16 blank bytes for the Poly1305 tag
        $start = self::ftell($ofp);
        fwrite($ofp, str_repeat("\x00", 16));

        /** @var string $c */
        $cBlock = ParagonIE_Sodium_Core_Util::substr(
            $block0,
            ParagonIE_Sodium_Crypto::secretbox_xsalsa20poly1305_ZEROBYTES
        );
        $state->update($cBlock);
        fwrite($ofp, $cBlock);
        $mlen -= 32;

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

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

        /*
         * 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).
         */
        fseek($ifp, $first32, SEEK_SET);

        while ($mlen > 0) {
            $blockSize = $mlen > self::BUFFER_SIZE
                ? self::BUFFER_SIZE
                : $mlen;
            $plaintext = fread($ifp, $blockSize);
            if (!is_string($plaintext)) {
                throw new SodiumException('Could not read input file');
            }
            $cBlock = ParagonIE_Sodium_Core_Salsa20::salsa20_xor_ic(
                $plaintext,
                $realNonce,
                $iter,
                $subkey
            );
            fwrite($ofp, $cBlock, $blockSize);
            $state->update($cBlock);

            $mlen -= $blockSize;
            $iter += $incr;
        }
        try {
            ParagonIE_Sodium_Compat::memzero($block0);
            ParagonIE_Sodium_Compat::memzero($subkey);
        } catch (SodiumException $ex) {
            $block0 = null;
            $subkey = null;
        }
        $end = self::ftell($ofp);

        /*
         * Write the Poly1305 authentication tag that provides integrity
         * over the ciphertext (encrypt-then-MAC)
         */
        fseek($ofp, $start, SEEK_SET);
        fwrite($ofp, $state->finish(), ParagonIE_Sodium_Compat::CRYPTO_SECRETBOX_MACBYTES);
        fseek($ofp, $end, SEEK_SET);
        unset($state);

        return true;
    }