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

void encipher(FILE *out, FILE *in, char *key)
{
  size_t klen = strlen(key);
  unsigned char ch;
  size_t i = 0;

  while(fread(&ch, 1, 1, in))
  {
    ch ^= key[i++];
    fwrite(&ch, 1, 1, out);
    i %= klen;
  }
}

int main(int argc, char **argv)
{
  if(argc > 3)
  {
    FILE *in = fopen(argv[1], "rb");
    if(in != NULL)
    {
      FILE *out = fopen(argv[2], "wb");
      if(out != NULL)
      {
        encipher(out, in, argv[3]);

        if(ferror(out))
        {
          fputs("Output error.\n", stderr);
        }
        fclose(out);
      }
      else
      {
        fputs("Can't open output file.\n", stderr);
      }
      if(ferror(in))
      {
        fputs("Input error.\n", stderr);
      }
      fclose(in);
    }
    else
    {
      fputs("Can't open input file.\n", stderr);
    }
  }
  else
  {
    fputs("Arguments required are: inputfile outputfile key.\n", stderr);
  }
  return 0;
}