ParagonIE_Sodium_Crypto32::secretbox

    public static function secretbox($plaintext, $nonce, $key)
    {
        /** @var string $subkey */
        $subkey = ParagonIE_Sodium_Core32_HSalsa20::hsalsa20($nonce, $key);

        /** @var string $block0 */
        $block0 = str_repeat("\x00", 32);

        /** @var int $mlen - Length of the plaintext message */
        $mlen = ParagonIE_Sodium_Core32_Util::strlen($plaintext);
        $mlen0 = $mlen;
        if ($mlen0 > 64 - self::secretbox_xsalsa20poly1305_ZEROBYTES) {
            $mlen0 = 64 - self::secretbox_xsalsa20poly1305_ZEROBYTES;
        }
        $block0 .= ParagonIE_Sodium_Core32_Util::substr($plaintext, 0, $mlen0);

        /** @var string $block0 */
        $block0 = ParagonIE_Sodium_Core32_Salsa20::salsa20_xor(
            $block0,
            ParagonIE_Sodium_Core32_Util::substr($nonce, 16, 8),
            $subkey
        );

        /** @var string $c */
        $c = ParagonIE_Sodium_Core32_Util::substr(
            $block0,
            self::secretbox_xsalsa20poly1305_ZEROBYTES
        );
        if ($mlen > $mlen0) {
            $c .= ParagonIE_Sodium_Core32_Salsa20::salsa20_xor_ic(
                ParagonIE_Sodium_Core32_Util::substr(
                    $plaintext,
                    self::secretbox_xsalsa20poly1305_ZEROBYTES
                ),
                ParagonIE_Sodium_Core32_Util::substr($nonce, 16, 8),
                1,
                $subkey
            );
        }
        $state = new ParagonIE_Sodium_Core32_Poly1305_State(
            ParagonIE_Sodium_Core32_Util::substr(
                $block0,
                0,
                self::onetimeauth_poly1305_KEYBYTES
            )
        );
        try {
            ParagonIE_Sodium_Compat::memzero($block0);
            ParagonIE_Sodium_Compat::memzero($subkey);
        } catch (SodiumException $ex) {
            $block0 = null;
            $subkey = null;
        }

        $state->update($c);

        /** @var string $c - MAC || ciphertext */
        $c = $state->finish() . $c;
        unset($state);

        return $c;
    }