xor encryption and streams

I tried to write a simple XOR encryption program. But it's not working properly. For some files is the output file longer, for some other shorter then it should be. Here is my source code:

Code:

/*

   XOR encryption - simple encryption algorithm

   use: xorenc [key] filename

   It encrypts file 'filename'; output is in file with the same name, but
   extension is changed to .enc

*/

/*
   problems:
	 *** should I use delete or delete[] operator?
		 see ~Key()
	 *** how to know whether it's EOF or not using cin and <iostream.h>?
		 see main() : while loop
	 *** is it good to use variable definitions in loop?
		 see main() : while loop
*/

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

const int MAX_KEY_LENGTH = 16;

class Key {
private:
  char *key;
  int next;
  int length;

public:
  Key(void) {
	key = new char[MAX_KEY_LENGTH];
  }

  Key(const char *s) { init(s); }

  ~Key(void) { delete key; }

  void init(const char *s);
  char get_next_char(void);
};

void
Key::init(const char *s)
{
  length = strlen(s);

  if (length == 0) {
	cerr << "Key is too short" << endl;
  }
  else if (length > MAX_KEY_LENGTH) {
	length = MAX_KEY_LENGTH;
  }

  key = new char[length+1];
  strncpy(key, s, length);
  key[length] = 0;

  next = 0;
}

char
Key::get_next_char(void)
{
  if (next < length)
	return key[next++];

  // we should repeat the key from beginning
  next = 1;
  return key[0];
}

void
usage(const char *program)
{
  cout << endl << "Use " << program << " [key] filename" << endl << endl;
}

char *
get_only_file_name(char *s)
{
  int i = 0;

  while (s[i] != '\0' && s[i] != '.')
	i++;

  if (s[i] != '\0')
	s[i] = '\0';

  return s;
}

char *
prompt_for_key(void)
{
  char s[100];

  cout << "Enter key for encrypting: ";
  return fgets(s, 100, stdin); // using fgets doesn't seem good..
}


int
main(int argc, char *argv[])
{
  ifstream in;
  ofstream out;
  Key key;

  if (argc <= 1) {
	cout << "Too few arguments" << endl;
	usage(argv[0]);
	return 0;
  }
  else if (argc == 2) {
	in.open(argv[1]);
	out.open(strcat(get_only_file_name(argv[1]), ".enc"));

	key.init(prompt_for_key());
  }
  else if (argc >= 3) {
	in.open(argv[2]);
	out.open(strcat(get_only_file_name(argv[2]), ".enc"));

	// creates an instance of Key class an initialize it
	key.init(argv[1]);
  }

  if (!in)
	cerr << endl << "Input file does not exist!" << endl << endl;

  if (!out)
	cerr << endl << "Cannot open output file!" << endl << endl;

  int c;

  while (1) {
	in >> c;
	if (in.eof())
	  break;
	out.put(c ^ key.get_next_char());
  }

  in.close();
  out.close();
  return 0;
}

And another question is about streams. How should I test whether the file is on its end? And how about formatting the output? (in C we have %6.2f for example) Do you know about some tutorial or what, so I'd be able to read it?

If you're smashing the data with XOR, you may need to open your files in binary mode.

>>out.open(strcat(get_only_file_name(argv[1]), ".enc"));
Here, you are appending to argv[1] which isn't a good thing to do. You don't know that there's enough room for you to do so, and you might overrun a buffer.

>>return fgets(s, 100, stdin); // using fgets doesn't seem good..
cin.getline() maybe?

>>in >> c;
might do better using
> while ((c = in.get()) != EOF)

Originally posted by mazo
And another question is about streams. How should I test whether the file is on its end? And how about formatting the output? (in C we have %6.2f for example) Do you know about some tutorial or what, so I'd be able to read it?

C++ format modifiers

Thanks a lot, Hammer! I have fixed my bugs(like follows down here) and it's working now excellent!

Code:

 else if (argc == 2) {
	in.open(argv[1], ios::binary);

	char *filename;
	filename = new char[strlen(argv[1]) + 5];
	strcpy(filename, argv[1]);
	strcat(get_only_file_name(filename), ".enc");
	out.open(filename, ios::binary);
	delete filename;

	key.init(prompt_for_key());
  }

I have one more question: should I use

Code:

delete filename;

or

Code:

delete[] filename

?

You allocated an array with this:
>filename = new char[strlen(argv[1]) + 5];

so delete the array with this:
>delete [] filename;