src/Zend/Filter/Encrypt/Mcrypt.php
public function encrypt($value)
    {
        // compress prior to encryption
        if (!empty($this->_compression)) {

            $compress = new Zend_Filter_Compress($this->_compression);
            $value    = $compress->filter($value);
        }

        $cipher  = $this->_openCipher();
        $this->_initCipher($cipher);
        $encrypted = mcrypt_generic($cipher, $value);
        mcrypt_generic_deinit($cipher);
        $this->_closeCipher($cipher);

        return $encrypted;
    }
public function encrypt($value)
    {
        // compress prior to encryption
        if (!empty($this->_compression)) {

            $compress = new Zend_Filter_Compress($this->_compression);
            $value    = $compress->filter($value);
        }

        $cipher  = $this->_openCipher();
        $this->_initCipher($cipher);
        $encrypted = mcrypt_generic($cipher, $value);
        mcrypt_generic_deinit($cipher);
        $this->_closeCipher($cipher);

        return $encrypted;
    }
Defined by Zend_Filter_Interface

Encrypts $value with the defined settings

@param  string $value The content to encrypt
@return string The encrypted content