How to change letters of string?

Hi, I'm working my way through a book called "Information Security: Principles and Practice". At the end of each chapter there are problems that the author deals out to make sure you comprehend what you just read. Well, a lot of the problems are encrypted strings using the ceasar cipher, and I'm trying to shift the letters in a string to decrypt the string. How can I do this in C++?

Thanks for the help,
Nathan

What kind of string are you using?

char *, std::string or CString?

In case it is a char *, you can do the following:

Code:

---


char *pCursor = pszString;

while('\0' != *pCursor)
{
  *pCursor = *pCursor + whatever; // Here you can modify every single char

  pCursor++; // advance to next char
}

---

In case of std::string or CString, there are functions for replacing strings.
Or, you could get a ptr to the bytes and then use what I wrote above.


Hope that helps,
Andy

I haven't written the program yet, cause I wasn't sure how :) In your code you posted does it actually change a letter? I don't understand it very well, sorry. I'm kind of new to C++ so thanks for the help :)

No, but in the loop you can add/subtract the number you want (I understand that is what Caesarian cipher is, adding 3 to 'A', so it becomes 'D'.

E.g.

'A' + 3 = 'D' and
'D' - 3 = 'A'

If you don't understand why, look at an ASCII table.


-Andy

Ok, but what does pszString mean?

How about;

Code:

---

int main()
{
    std::string s = "Hello";
    std::cout << s << std::endl;

    for(int index=0; index < s.length(); index++)
    {
      s[ index ] = s[ index ] + 1;
    }

    std::cout << s << std::endl;

    system("PAUSE");
    return 0;
}

---

Output

Code:

---

Hello
Ifmmp

---

So adding a number to a letter will shift the letter by the numbers amount?

Quote:

---

Originally Posted by Nathand

So adding a number to a letter will shift the letter by the numbers amount?

---

as suggested earlier look at the asciitable.
search asciitable on google...

@namezero111111: Warning, your encoded string may contain nul character.
Or, assuming that the source character set contains only A-Za-z and space, the destination character set doesn't match the source character set.

When I try to compile that C++ program I get: http://rafb.net/paste/results/fHSKTq95.html.

@Nathand: Check that you include the <string> and <iostream> headers (both of them must be included).

A proper cipher (assuming only capital letters in the source)

Code:

---

char *in = source;
char *out = *dest; // assuming dest is pre-allocated
while (*in)
{
  *out = ((*in -'A') + offset) %26;
  in++;  // not optimal, but possibly easier to read for a beginner...
  out++;
}
*out = 0; // terminate the output.

---

You were right :) I forgot to add "#include <string>.

Thanks,
Nathan

TheCPUWizard: Wouldn't it be:

Code:

---

*out = 'A' + ((*in -'A') + offset) %26;

---

:p
And if space characters have to be handled:

Code:

---

if (*in == ' ') *out=*in;
else *out = 'A' + ((*in -'A') + offset) %26;

---

Yes, I know, space characters are not encrypted, it's very weak, but AFAIK, the original caesar encryption algorithm does that.

Ok, last question :)

When I use this code:

Code:

---

#include <iostream>
#include <string>

int key = 1;
int main()
{
    std::string s = "CSYEVIXIVQMREXIH";
    std::cout << s << std::endl;
        for(int i = 0; i < 26; i++)
        {
                for(int index=0; index < s.length(); index++)
                {
                  s[ index ] = s[ index ] + key;
                }
               
                std::cout << s << std::endl;
                key += 1;
        }

    system("PAUSE");
    return 0;
}

---

The caesar's cipher is "youareterminated", but the program returns "_ouareterminated". Why would this be? This also happens with some other ciphers I tried.

And if someone feels so inclined to compile and run this on their system, let me know if you get a very strange "beeping" noise coming from your computer?

Thanks,
Nathan