C :: Fixing Command Prompt File Input

Im writing program for a rail fence cipher that should run from the command prompt and take in two .txt files, one containing the key for how many rails there should be (between 1 and 25) and the other containing the message to be encrypted. Both of those txt files should be entered from the command prompt, taken in with the program's arguments, not asking the user for input I think I have the logistics of how to get the program to encrypt it worked out, but every time I try to run the program it keeps on crashing.

The program should run when I enter this:./railcipher samplekey.txt samplemessage.txt

I'm pretty sure my issue lies within my first few lines of code, with the main(), or with the FILE*, but I dont know what I would need to change it to for it to work properly.

Code:
#include<stdio.h>
#include<string.h>
#include <stdlib.h>
int main(int argc, char **argv[])

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