core-bundle/src/Resources/contao/forms/FormCaptcha.php
public function validate()
	{
		if (!isset($_POST[$this->strCaptchaKey]) || !\in_array(Input::post($this->strCaptchaKey.'_hash'), $this->generateHashes((int) Input::post($this->strCaptchaKey)), true) || (isset($_POST[$this->strCaptchaKey.'_name']) && Input::post($this->strCaptchaKey.'_name')))
		{
			$this->class = 'error';
			$this->addError($GLOBALS['TL_LANG']['ERR']['captcha']);
		}
	}
public function validate()
	{
		if (!isset($_POST[$this->strCaptchaKey]) || !\in_array(Input::post($this->strCaptchaKey.'_hash'), $this->generateHashes((int) Input::post($this->strCaptchaKey)), true) || (isset($_POST[$this->strCaptchaKey.'_name']) && Input::post($this->strCaptchaKey.'_name')))
		{
			$this->class = 'error';
			$this->addError($GLOBALS['TL_LANG']['ERR']['captcha']);
		}
	}
Validate the input and set the value