I have installed Sharepoint 2013 in my app server and then in web sever I gave same paraphrase for both. Now when I was installing CU in app server my windows server crashed, i didn't take snapshot So the server has been rebuilt. Now Do I need to re-install SharePoint in webserver too or If I give same paraphrase that i gave before in both servers will that be ok.
I dont think, there is any link between farm crash with passphrase. It is just another security check.
Now the question is what CU you were installing and what you mean by crashed the server.
- if it is totally lost then you need to install the SharePoint and join to the farm using the same passphrase.
installing CU on WFE will crash the server as well? no but we have to see what issue you got which did this.