ParagonIE_Sodium_Crypto::aead_chacha20poly1305_decrypt

    public static function aead_chacha20poly1305_decrypt(
        $message = '',
        $ad = '',
        $nonce = '',
        $key = ''
    ) {
        /** @var int $len - Length of message (ciphertext + MAC) */
        $len = ParagonIE_Sodium_Core_Util::strlen($message);

        /** @var int  $clen - Length of ciphertext */
        $clen = $len - self::aead_chacha20poly1305_ABYTES;

        /** @var int $adlen - Length of associated data */
        $adlen = ParagonIE_Sodium_Core_Util::strlen($ad);

        /** @var string $mac - Message authentication code */
        $mac = ParagonIE_Sodium_Core_Util::substr(
            $message,
            $clen,
            self::aead_chacha20poly1305_ABYTES
        );

        /** @var string $ciphertext - The encrypted message (sans MAC) */
        $ciphertext = ParagonIE_Sodium_Core_Util::substr($message, 0, $clen);

        /** @var string The first block of the chacha20 keystream, used as a poly1305 key */
        $block0 = ParagonIE_Sodium_Core_ChaCha20::stream(
            32,
            $nonce,
            $key
        );

        /* Recalculate the Poly1305 authentication tag (MAC): */
        $state = new ParagonIE_Sodium_Core_Poly1305_State($block0);
        try {
            ParagonIE_Sodium_Compat::memzero($block0);
        } catch (SodiumException $ex) {
            $block0 = null;
        }
        $state->update($ad);
        $state->update(ParagonIE_Sodium_Core_Util::store64_le($adlen));
        $state->update($ciphertext);
        $state->update(ParagonIE_Sodium_Core_Util::store64_le($clen));
        $computed_mac = $state->finish();

        /* Compare the given MAC with the recalculated MAC: */
        if (!ParagonIE_Sodium_Core_Util::verify_16($computed_mac, $mac)) {
            throw new SodiumException('Invalid MAC');
        }

        // Here, we know that the MAC is valid, so we decrypt and return the plaintext
        return ParagonIE_Sodium_Core_ChaCha20::streamXorIc(
            $ciphertext,
            $nonce,
            $key,
            ParagonIE_Sodium_Core_Util::store64_le(1)
        );
    }