public PageReference directlogin(){
    Cookie getPasswd = ApexPages.currentPage().getCookies().get('password');
    String decryptedPassword= null;
    if(getPasswd !=null){
        Blob cryptoKey = Crypto.generateAesKey(256);
        Blob datac = EncodingUtil.base64Decode(getPasswd.getValue());
         try{
           Blob decryptedData = Crypto.decryptWithManagedIV('AES256', cryptoKey,datac ); // here error throwing
           decryptedPassword = decryptedData.toString();
         }catch(exception e){
            ex=e;
         }
    }
    return null;
}