Multi-Byte XOR Encoding : Part 2 of a series

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>

main(int argc, char *argv[])
{
    FILE *input, *output = stdout;
    char *prog = argv[0];
    char ch;
    unsigned masklen, j;
    unsigned long filelen, i;
    int mask[]= {'t','e','s','t', '1',0,0,0, 0,'2',0,0, 0,0,'3',0,};
        
    if(argc == 1 || argc == 2)
    {
        fprintf(stderr, "ERROR: %s requires a Source and Destination file.\n", prog);
        exit(1);
    }
    else
    {
        if((input = fopen(*++argv, "rb")) == NULL)
        {
            fprintf(stderr, "ERROR: %s can not open file %s.\n", prog, *argv);
            exit(1);
        }
        if((output = fopen(*++argv, "ab")) == NULL)
        {
            fprintf(stderr, "ERROR: %s can not create file %s.\n", prog, *argv);
            exit(2);
        }
    }

    fprintf(stdout, "Multi-Byte XOR Encoder/Decoder by typedeaF # www.osix.net\n\n");
    
    masklen = sizeof(mask)/sizeof(mask[0]);

    fseek(input, 0L, SEEK_END);
    filelen = ftell(input);
    fseek(input, 0L, SEEK_SET);

    fprintf(stdout, "Using mask: \"");

    for(i = 0L; i < masklen; i++)
    {
        if(mask[i] == 0)
            fprintf(stdout, "-");
        else
            fprintf(stdout, "%c", mask[i]);
    }
    fprintf(stdout, "\"\n");
 
    for(i = 0L, j = 0; i < filelen; i++, j++)
    {
        ch = fgetc(input);
        if(j == masklen)
            j = 0;
        if(mask[j] == 0)
            fputc(ch, output);
        else
            fputc((mask[j]^ch)&0xFF, output);
    }
    
    fclose(input);
    fclose(output);

    return 0;
}

This is the real data in the plain text file. (plain text)
test1----2-----3-test1----2----3-test1----2-- (mask)
XXXXXis tXe reaX XXXXXin tXe plXiXXXXXt fiXe. (results (X is just symbolic here))