MonoAlphabetic Substitution Cipher : Part 5 of a Series

#include <stdio.h>
#include <time.h>
#include <limits.h>
#include <stdlib.h>
#include <ctype.h>
 
int CheckArgs(int argc, char *argv[])
{
      unsigned char *prog = argv[0];
 
      if(argc != 3)
      {
            fprintf(stdout, "ERROR: %s requires plain and cipher file arguements.n", prog);
            return 0;
      }
      else
            return 1;
}
 
unsigned int GetFileLen(FILE *fp)
{
      unsigned int len = 0;
 
      fseek(fp, 0L, SEEK_END);
      len = ftell(fp);
      fseek(fp, 0L, SEEK_SET);
 
      return len;
}
 
int main(int argc, char *argv[])
{
      FILE *infile;
      FILE *outfile;
      unsigned char key[26];
      unsigned char *data;
      unsigned int datalen;
      unsigned int i = 0;
      unsigned int j = 0;
      unsigned int temp = 0;
 
      printf("nMonoAplhabetic Substitution Cipher by typedeaF @ www.osix.netn");
      if(!CheckArgs(argc, argv))
      {
            return EXIT_FAILURE;
      }
 
      if((infile = fopen(argv[1], "rb")) == NULL)
      {
            fclose(infile);
            printf("ERROR: %s could not open %s for reading.n", argv[0], argv[1]);
            return EXIT_FAILURE;
      }
 
      if((outfile = fopen(argv[2], "wb")) == NULL)
      {
            fclose(infile), fclose(outfile);
            printf("ERROR: %s could not open %s for writing.n", argv[0], argv[2]);
            return EXIT_FAILURE;
      }
 
      if((keyfile = fopen("key.txt", "wb")) == NULL)
      {
            fclose(infile), fclose(outfile);
            printf("ERROR: %s could not open keyfile for writing.n", argv[0], argv[2]);
            return EXIT_FAILURE;
      }
            
 
      datalen = GetFileLen(infile);
      if((data = malloc(datalen)) == NULL)
      {
            fclose(infile), fclose(outfile), free(data);
            printf("ERROR: Insuffecient Memoryn");
            return EXIT_FAILURE;
      }
      
      if(!fread(data, datalen, 1, infile))
      {
            fclose(infile), fclose(outfile), free(data);
            printf("ERROR: read error.n");
            return EXIT_FAILURE;
      }
 
      /* initilize key */
      for(i = 0; i < 26; i++)
      {
            key[i] = i + 'a';
      }
 
      srand((unsigned)time(NULL));
      
      /* shuffle the key */
      for(i = 0; i < 26; i++)
      {
            j = (unsigned int)((26) * (rand() / ((double)RAND_MAX + 1.0)));
            temp = key[i];
            key[i] = key[j];
            key[j] = temp;
      }
      
      /* save the key */
      for(i = 0; i < 26; i++)
            putc(key[i], keyfile);
 
      printf("nProcessing");
 
      for(i = 0;(unsigned int)i < datalen; i++)
      {
            printf(".");
            fflush(stdout);
            if(isalpha(data[i]))
                  fputc(key[tolower(data[i])-'a'], outfile);
            else
                  fputc(data[i], outfile);
      }
      printf("nDONE");
      fclose(infile), fclose(outfile), free(data);
 
      return EXIT_SUCCESS;
}