[C++]Help in starting the code

[MasterTrainer]

I want to make a program that does a basic substitution cipher based on an input integer. For example, I type first '3', then the next message I type is "ABC", then each letter is moved an n number of letters on the alphabet line. So "ABC" becomes "DEF".

What I can't figure out is, how do I initialize and set all of this up? Do i create variables for each of the letter of the alphabet and for each lower and uppercase? But also, how do I make the code skip the spaces?

I would appreciate any type of help.

 

[Cokemonkey11]

This is what you want - http://en.wikipedia.org/wiki/ASCII#ASCII_printable_code_chart

Simply add a value to each char in the input char* and subtract a value if the result is out of range

 

[MasterTrainer]

Oooh nice I also found this video on YoutTube that does the Caesar Cipher:
https://www.youtube.com/watch?v=_EQ6TJs_TEk

I guess that's it for me right now.
Thanks. Solved.

 

[Dr Super Good]

In theory you can do it with an ascii chart as mentioned above. However in reality your program description is a bit rubbish as not all localities use the English alphabet.

What one should do is to specific an encoding which defines the boundaries of the cipher. One must also specify what happens to characters outside this boundary (are they deleted? or ignored? etc).

 

[Cokemonkey11]

Of course, unicode and test cases are something a software engineer would consider, but I don't think that's necessary for someone writing their first toy C++ program

 

[MasterTrainer]

#include <iostream>
#include <string>

	
using namespace std;

int main()
{
	string input;
	int count = 0, length;

	cout <<"Enter your phrase \n";
	getline(cin, input);

	length = (int)input.length();

	for(count = 0; count < length; count++)
	{
		if(isalpha(input[count]))
		{
			input[count] = tolower(input[count]);
			for (int i = 0; i < 13; i++)
			{
               if(input[count] == 'z')
               	  input[count] = 'a';
               	else
               		input[count]++
		}
			}
	}

	cout << "Results: \n"  << input << endl;
}

Hmm...
So I made this code based on a video I found. It sets the letters 13 spaces from where it is originally from. But this is only for encryption. How do I make the code that decrypts?

I tried transforming all the ++ symbols to --, but that just messed things up. The video also said that I could change the index from 13 to any other number, but I'd have to have two of these:

for(count = 0; count < length; count++)
	{
		if(isalpha(input[count]))
		{
			input[count] = tolower(input[count]);
			for (int i = 0; i < 13; i++)
			{
               if(input[count] == 'z')
               	  input[count] = 'a';
               	else
               		input[count]++
		}
			}
	}

And that I should change the other one's ++s to --s. But that also does not work. What is going wrong?

 

[GhostWolf]

You don't need a loop.

You know that your letter space so to speak is from 'a' to 'z' (since you used the tolower function).
That means that if you are between 'a' and 'z', you don't need to do anything. If you are above 'z', and you want to cycle back, you need to reduce 'z'-'a', and if you are below 'a' and want to cycle back, you need to add 'z'-'a'.

For example, imagine your number space is 0 to 10 (remember that the char/id notation 'x' is just a number).
If you have 5, that's fine.
If you have 15, reduce 10-0, and you get 5.
If you have -5, add 10-0, and you get 5.
If you want to handle large numbers that are twice as big, or larger, than the size of your space (not that relevant to this specific case, but still), you can use the modulo operator.

After you implement this sort of cycle, it's a simple matter of adding X to every input to encode, and subtracting X from every output to decode (or vice versa).

Also use the highlight option instead of code in the future: [code=cpp][/code]

 

[Dr Super Good]

For encryption I would recommend something like AES (a highly reputable standard algorithm). If you want the output to be human readable then convert it into something like base 64. That is what I did with StarCraft II for encryption.

 

[GhostWolf]

Can you please, just please stop suggesting advanced and unrelated suggestions to every thread ever...

 

[edo494]

what the fuck DSG? Does he look like he is going to write professional encryption? He just wanted to make Caesar cypher, and you come here suggesting AES.

 

[GhostWolf]

But, how xould a letter be beyond 'a' - 'z'?

Again, that notation is just a nice way to write numbers using ASCII letters.

'a' == 97
'b' == 98
...
'z' == 122

If you want to move a letter up by some N letters, you just add a number.

'a' + 3 == 'd' == 100

Basically all you need to do is the following, if, for example, your encoding process adds 5 to each character:

char encoded = C + 5; // Where C is your input character

if (encoded > 'z') {
    encoded -= 'z' - 'a'; // Cycle from z to a
} else if (encoded < 'a') {
    encoded += 'z' - 'a'; // Cycle from a to z
}

And of course, 'z' - 'a' is just the constant 25.