What is yii csrf

/**
 * Returns the token used to perform CSRF validation.
 * 返回用于执行CSRF验证的token
 * This token is a masked version of [[rawCsrfToken]] to prevent [BREACH attacks](http://breachattack.com/).
 * This token may be passed along via a hidden field of an HTML form or an HTTP header value
 * to support CSRF validation.
 * @param boolean $regenerate whether to regenerate CSRF token. When this parameter is true, each time
 * this method is called, a new CSRF token will be generated and persisted (in session or cookie).
 * @return string the token used to perform CSRF validation.
 */
public function getCsrfToken($regenerate = false)
{
    if ($this->_csrfToken === null || $regenerate) {
        if ($regenerate || ($token = $this->loadCsrfToken()) === null) {    //loadCsrfToken()就是在cookie或者session中获取token值
            $token = $this->generateCsrfToken();        //如果token为空则调用generateCsrfToken()去生成
        }
        // the mask doesn't need to be very random
        $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-.';
        $mask = substr(str_shuffle(str_repeat($chars, 5)), 0, static::CSRF_MASK_LENGTH);
        // The + sign may be decoded as blank space later, which will fail the validation
        $this->_csrfToken = str_replace('+', '.', base64_encode($mask . $this->xorTokens($token, $mask)));
    }

    return $this->_csrfToken;
}

/**
 * Loads the CSRF token from cookie or session.
 * @return string the CSRF token loaded from cookie or session. Null is returned if the cookie or session
 * does not have CSRF token.
 */
protected function loadCsrfToken()
{
    if ($this->enableCsrfCookie) {
        return $this->getCookies()->getValue($this->csrfParam);         //cookie中获取csrf的token 
    } else {
        return Yii::$app->getSession()->get($this->csrfParam);          //session中获取csrf的token
    }
}

/**
 * Creates a cookie with a randomly generated CSRF token.
 * Initial values specified in [[csrfCookie]] will be applied to the generated cookie.
 * @param string $token the CSRF token
 * @return Cookie the generated cookie
 * @see enableCsrfValidation
 */
protected function createCsrfCookie($token)
{
    $options = $this->csrfCookie;
    $options['name'] = $this->csrfParam;
    $options['value'] = $token;
    return new Cookie($options);
}

/**
 * Generates  an unmasked random token used to perform CSRF validation.
 * @return string the random token for CSRF validation.
 */
protected function generateCsrfToken()
{
    $token = Yii::$app->getSecurity()->generateRandomString();      //生成随机的安全字符串
    if ($this->enableCsrfCookie) {
        $cookie = $this->createCsrfCookie($token);                  //createCsrfCookie()用于生成csrf的key=>value形式的token
        Yii::$app->getResponse()->getCookies()->add($cookie);       //将生成key=>value保存到cookies 
    } else {
        Yii::$app->getSession()->set($this->csrfParam, $token);     //将csrf的token存在session中
    }
    return $token;
}

/**
 * 每次调用控制器中的方法的时候都会调用下面的Yii::$app->getRequest()->validateCsrfToken()验证
 * @inheritdoc
 */
public function beforeAction($action)
{
    if (parent::beforeAction($action)) {
        if ($this->enableCsrfValidation && Yii::$app->getErrorHandler()->exception === null && !Yii::$app->getRequest()->validateCsrfToken()) {         
            throw new BadRequestHttpException(Yii::t('yii', 'Unable to verify your data submission.'));
        }
        return true;
    } else {
        return false;
    }
}


/**
 * 校验方法
 * Performs the CSRF validation.
 *
 * This method will validate the user-provided CSRF token by comparing it with the one stored in cookie or session.
 * This method is mainly called in [[Controller::beforeAction()]].
 *
 * Note that the method will NOT perform CSRF validation if [[enableCsrfValidation]] is false or the HTTP method
 * is among GET, HEAD or OPTIONS.
 *
 * @param string $token the user-provided CSRF token to be validated. If null, the token will be retrieved from
 * the [[csrfParam]] POST field or HTTP header.
 * This parameter is available since version 2.0.4.
 * @return boolean whether CSRF token is valid. If [[enableCsrfValidation]] is false, this method will return true.
 */
public function validateCsrfToken($token = null)
{
    $method = $this->getMethod();
    // only validate CSRF token on non-"safe" methods http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1
    if (!$this->enableCsrfValidation || in_array($method, ['GET', 'HEAD', 'OPTIONS'], true)) {
        return true;
    }

    $trueToken = $this->loadCsrfToken();

    if ($token !== null) {
        return $this->validateCsrfTokenInternal($token, $trueToken);
    } else {
        return $this->validateCsrfTokenInternal($this->getBodyParam($this->csrfParam), $trueToken)
            || $this->validateCsrfTokenInternal($this->getCsrfTokenFromHeader(), $trueToken); 
            //getCsrfTokenFromHeader()这个我也不太理解，还请指点一下
    }
}

/**
 * @return string the CSRF token sent via [[CSRF_HEADER]] by browser. Null is returned if no such header is sent.
 */
public function getCsrfTokenFromHeader()
{
    $key = 'HTTP_' . str_replace('-', '_', strtoupper(static::CSRF_HEADER));
    return isset($_SERVER[$key]) ? $_SERVER[$key] : null;
}

/**
 * Validates CSRF token
 *
 * @param string $token
 * @param string $trueToken
 * @return boolean
 */
private function validateCsrfTokenInternal($token, $trueToken)
{
    $token = base64_decode(str_replace('.', '+', $token));      //解码从客户端获取的csrf的token
    $n = StringHelper::byteLength($token);
    if ($n <= static::CSRF_MASK_LENGTH) {
        return false;
    }
    $mask = StringHelper::byteSubstr($token, 0, static::CSRF_MASK_LENGTH);
    $token = StringHelper::byteSubstr($token, static::CSRF_MASK_LENGTH, $n - static::CSRF_MASK_LENGTH);
    $token = $this->xorTokens($mask, $token);

    return $token === $trueToken;       //验证从客户端获取的csrf的token和真实的token是否相等
}