someone explain this x-or encryption

**student2005** · 01-14-2004

Code:

#include <iostream>
#include <iomanip>
#include <string.h>
using namespace::std;

void encrypt(char string[], char key[]);
void decrypt(char string[], char key[]);

int main()
{
	int again;
    char string[81];
	char key[]="123456789ZBCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJK123456789";

	do
	{
	cout << "Enter a word or sentence up to 80 characters: " << endl;
	gets(string);

	encrypt(string, key);
	decrypt(string, key);
	
	cout << "Want to enter another? (1 for yes, 2 for no)" << endl;
	cin >> again;
	}
	while (again == 1);
	return 0;
}

void encrypt(char string[], char key[])
{
	for(int x = 0; x < strlen(string); x++)
	{
		string[x] = string[x]^key[x];
		cout << string[x];
	}
		cout<<endl;
}

  
void decrypt(char string[], char key[])
{
	for(int x=0; x < strlen(string); x++)
	{
		string[x] = string[x]^key[x];
		cout << string[x];
	}
		cout<<endl;
}

**alphaoide** · 01-14-2004

http://www.codeproject.com/cpp/bitba...asp?print=true
Go there to learn how the operator works, but basically..

Code:

encrypt: a ^ b = c   // a is the original and c is the result
decrypt: c ^ b = a   // perform the same operation with result you get the original

**gustavosserra** · 01-14-2004

Does this method really works? After the ^ you lost all the information about the original string.

**student2005** · 01-14-2004

when I run it

the first line is the string I entered
the second line is a bunch of goop
the third line is the string I entered again

**alphaoide** · 01-14-2004

Originally posted by alphaoide
http://www.codeproject.com/cpp/bitba...asp?print=true
Go there to learn how the operator works, but basically..

Code:

encrypt: a ^ b = c // a is the original and c is the result decrypt: c ^ b = a // perform the same operation with result you get the original

Code:

replace a and b with 0 or 1 and get the c based on this table

1   ^   1   ==   0
1   ^   0   ==   1
0   ^   1   ==   1
0   ^   0   ==   0

then do decrypting operation and you'll get the original a

**DougDbug** · 01-14-2004

Today, I'm making alot of assumptions, so I'll assume it's the bitwise-binary stuff.

First, you have to understand binary numbers. Everything inside the computer is stored in binary (ones and zeros). Getting a C++ program to display in binary is a little tricky... not really that hard, but not as trivial as displaying in decimal, octal, or hexadecimal.

The built-in Windows calculator will do decimal-binary conversion:
Start/Programs/Accessories/Calculator/View/Scientific/Bin

You can look-up the ASCII value for the character in question. The first character in your key is a space which is stored as 32 decimal, or 0010 0000 binary (leading zeros and space added for clarity).

Say the first character in your string is 'A'. This converts to 0100 0001.

Next, you have to understand bitwise operators. There is more info in the Programming FAQ

You bitwise-exclusive-or the first character in the key-string with the first character of your string to encrypt it:

0010 0000 ASCII space (key for first character)
0100 0001 ASCII 'A' (The first character in your string)
------------- XOR
0110 0001 ASCII '1' = 49 decimal (An ecncrypted A)

Now to decrypt:

0010 0000 ASCII space (key for first character)
0110 0001 ASCII '1' (An ecncrypted A)
------------- XOR
0100 0001 ASCII 'A' (The decrypted A) Ta-Da!!!

(I hope I didn't screw that up...)

A couple of comments:
I would expect a beginner's version of this program to use a single-character key... every character in your string XORed with the same key-character.

If you don't have a lot of experience with binary numbers, and you are not a "math whiz", don't expect to understand this stuff right-away. It takes some study, thinking, and practice to get comfortable with it. And, you can do alot of programming without binary numbers or bitwise operations. I'd say it's an intermediate topic. If you are a beginner, don't get "bogged-down" with this stuff... move-on to the next topic and come back to it later.

Nobody uses actual binary. (Well, almost nobody.) We use hexadecimal because,
1- You can't put binary numbers directly in your C++ source code.
2- Binary numbers get difficult to read. More than 8-bits, and you have to start counting the digits.
3- With a little practice, you can convert between binary and hexadecimal in your head. (binary-dicimal is not so easy.)

**joshdick** · 01-14-2004

Originally posted by student2005
when I run it

the first line is the string I entered
the second line is a bunch of goop
the third line is the string I entered again

That's a cipher, buddy.

**Cat** · 01-15-2004

Originally posted by gustavosserra
Does this method really works? After the ^ you lost all the information about the original string.

No, you lose none of the information about it, assuming you know what the key is.

For example, look at this snippet from alphaoide's post:

1 ^ 1 == 0
1 ^ 0 == 1
0 ^ 1 == 1
0 ^ 0 == 0

If you only know the key and the final value of a bit, then you have this:

? ^ 1 == 0
? ^ 0 == 1
? ^ 1 == 1
? ^ 0 == 0

And if you look at this, in each of the 4 possible cases, you can reconstruct the original value of the bit.

It's a reversible mathematical operation, and as long as you know the result and ONE of the operands, you know the other.

A mathematical example would be to assume I was going to "encrypt" a vector of numbers by adding 42 to each one. If I told you the "encrypted" vector was [183, 25, 47.2], surely you could tell me what the original was? You lose no information about the numbers.

**Salem** · 01-15-2004

It all stems from

K ^ K = 0

C ^ 0 = C

Since encryption followed by decryption is
( C ^ K ) ^ K

Exoring anything with the same value twice gets you back to where you started.

**digy** · 01-15-2004

Im a student of programming too.

Can someone explain this in plain english. I've never seen this before. I've typed in it...how does it encrypt to things like *&^%$#.
Thanks for anyones help..

**DougDbug** · 01-15-2004

digy,

Like I said above, first you have to understand ASCII, Character Array Strings, Binary Numbers, and Bitwise Operations. Explaining this stuff "in plain english" won't help unless you have the background. Once you have the background, the above posts will be easy to understand.

Programming is a difficult subject. You have to learn step-by-step... You can't learn it all at once!

So, you'll have to research and study-up on those topics, or wait till you run-across them in your normal programming-studies.

how does it encrypt to things like *&^%$#.

Everything in the computer is stored as a binary number. So, you can XOR encrypt anything. I doesn't even have to be a character... you can encrypt an mp3 file.

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someone explain this x-or encryption

I'm not sure what part(s) of the program you are having trouble with....

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