How to change letters of string?

[Nathand]

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

[namezero111111]

What kind of string are you using?

char *, std::string or CString?

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

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

[Nathand]

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

[namezero111111]

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

[Nathand]

Ok, but what does pszString mean?

[laasunde]

How about;

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

Hello
Ifmmp

[Nathand]

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

[Mitsukai]

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...

[SuperKoko]

@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.

[Nathand]

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

[SuperKoko]

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

[TheCPUWizard]

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

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.

[Nathand]

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

Thanks,
Nathan

[SuperKoko]

TheCPUWizard: Wouldn't it be:

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

And if space characters have to be handled:

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.

[Nathand]

Ok, last question

When I use this 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