ParagonIE_Sodium_Crypto::secretbox_open

    public static function secretbox_open($ciphertext, $nonce, $key)
    {
        /** @var string $mac */
        $mac = ParagonIE_Sodium_Core_Util::substr(
            $ciphertext,
            0,
            self::secretbox_xsalsa20poly1305_MACBYTES
        );

        /** @var string $c */
        $c = ParagonIE_Sodium_Core_Util::substr(
            $ciphertext,
            self::secretbox_xsalsa20poly1305_MACBYTES
        );

        /** @var int $clen */
        $clen = ParagonIE_Sodium_Core_Util::strlen($c);

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

        /** @var string $block0 */
        $block0 = ParagonIE_Sodium_Core_Salsa20::salsa20(
            64,
            ParagonIE_Sodium_Core_Util::substr($nonce, 16, 8),
            $subkey
        );
        $verified = ParagonIE_Sodium_Core_Poly1305::onetimeauth_verify(
            $mac,
            $c,
            ParagonIE_Sodium_Core_Util::substr($block0, 0, 32)
        );
        if (!$verified) {
            try {
                ParagonIE_Sodium_Compat::memzero($subkey);
            } catch (SodiumException $ex) {
                $subkey = null;
            }
            throw new SodiumException('Invalid MAC');
        }

        /** @var string $m - Decrypted message */
        $m = ParagonIE_Sodium_Core_Util::xorStrings(
            ParagonIE_Sodium_Core_Util::substr($block0, self::secretbox_xsalsa20poly1305_ZEROBYTES),
            ParagonIE_Sodium_Core_Util::substr($c, 0, self::secretbox_xsalsa20poly1305_ZEROBYTES)
        );
        if ($clen > self::secretbox_xsalsa20poly1305_ZEROBYTES) {
            // We had more than 1 block, so let's continue to decrypt the rest.
            $m .= ParagonIE_Sodium_Core_Salsa20::salsa20_xor_ic(
                ParagonIE_Sodium_Core_Util::substr(
                    $c,
                    self::secretbox_xsalsa20poly1305_ZEROBYTES
                ),
                ParagonIE_Sodium_Core_Util::substr($nonce, 16, 8),
                1,
                (string) $subkey
            );
        }
        return $m;
    }