How to resolve the algorithm Playfair cipher step by step in the C++ programming language

Published on 7 June 2024 03:52 AM
#cpp

How to resolve the algorithm Playfair cipher step by step in the C++ programming language

Table of Contents

Problem Statement

Implement a Playfair cipher for encryption and decryption.

The user must be able to choose   J  =  I     or   no   Q   in the alphabet. The output of the encrypted and decrypted message must be in capitalized digraphs, separated by spaces.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Playfair cipher step by step in the C++ programming language

The provided code implements the Playfair cipher, a classic encryption technique.

Class Overview:

The code defines a playfair class that handles the encryption/decryption process.

Methods:

  • doIt(string k, string t, bool ij, bool e): The main function that takes the key, text, and flags for handling 'J' and 'Q' characters and the encryption/decryption mode (e for encryption, -e for decryption).

Private Methods:

  • doIt(int dir): Performs the actual encryption/decryption by iterating over the text, finding character positions in the grid, and applying the Playfair algorithm based on the direction (dir).
  • display(): Displays the encrypted/decrypted text.
  • getChar(int a, int b): Retrieves a character from the grid at the given position.
  • getCharPos(char l, int &a, int &b): Finds the position of a character in the grid and returns true if found.
  • getTextReady(string t, bool ij, bool e): Prepares the text for encryption by handling duplicate characters, replacing 'J' with 'I' (if ij is true), and removing 'Q' (if ij is false).
  • createGrid(string k, bool ij): Creates the 5x5 Playfair grid using the given key and handling 'J' and 'Q' characters.

Main Function:

  • The program starts by prompting the user to select encryption or decryption mode (e or d) and enters the key, 'J' handling mode, and the text to be encrypted/decrypted.
  • It then creates an instance of the playfair class and calls doIt() to perform the encryption/decryption, resulting in the encrypted/decrypted text.

Playfair Algorithm:

The Playfair algorithm works as follows:

  1. It creates a 5x5 grid using the key and the alphabet.
  2. It replaces 'J' with 'I' and removes 'Q' from the text.
  3. It divides the text into pairs of characters, adding 'X' if necessary.
  4. For each pair of characters, it finds their positions in the grid.
  5. If the characters are in the same row or column, it replaces them with the characters to their right or below, respectively.
  6. If the characters are not in the same row or column, it replaces them with the characters that form the opposite corners of the rectangle they form in the grid.
  7. The encrypted/decrypted text is formed by concatenating the replaced characters.

Source code in the cpp programming language

#include <iostream>
#include <string>

using namespace std;

class playfair
{
public:
    void doIt( string k, string t, bool ij, bool e )
    {
	createGrid( k, ij ); getTextReady( t, ij, e );
	if( e ) doIt( 1 ); else doIt( -1 );
	display();
    }

private:
    void doIt( int dir )
    {
	int a, b, c, d; string ntxt;
	for( string::const_iterator ti = _txt.begin(); ti != _txt.end(); ti++ )
	{
	    if( getCharPos( *ti++, a, b ) )
		if( getCharPos( *ti, c, d ) )
		{
		    if( a == c )     { ntxt += getChar( a, b + dir ); ntxt += getChar( c, d + dir ); }
		    else if( b == d ){ ntxt += getChar( a + dir, b ); ntxt += getChar( c + dir, d ); }
		    else             { ntxt += getChar( c, b ); ntxt += getChar( a, d ); }
		}
	}
	_txt = ntxt;
    }

    void display()
    {
	cout << "\n\n OUTPUT:\n=========" << endl;
	string::iterator si = _txt.begin(); int cnt = 0;
	while( si != _txt.end() )
	{
	    cout << *si; si++; cout << *si << " "; si++;
	    if( ++cnt >= 26 ) cout << endl, cnt = 0;
	}
	cout << endl << endl;
    }

    char getChar( int a, int b )
    {
	return _m[ (b + 5) % 5 ][ (a + 5) % 5 ];
    }

    bool getCharPos( char l, int &a, int &b )
    {
	for( int y = 0; y < 5; y++ )
	    for( int x = 0; x < 5; x++ )
		if( _m[y][x] == l )
		{ a = x; b = y; return true; }

	return false;
    }

    void getTextReady( string t, bool ij, bool e )
    {
	for( string::iterator si = t.begin(); si != t.end(); si++ )
	{
	    *si = toupper( *si ); if( *si < 65 || *si > 90 ) continue;
	    if( *si == 'J' && ij ) *si = 'I';
	    else if( *si == 'Q' && !ij ) continue;
	    _txt += *si;
	}
	if( e )
	{
	    string ntxt = ""; size_t len = _txt.length();
	    for( size_t x = 0; x < len; x += 2 )
	    {
		ntxt += _txt[x];
		if( x + 1 < len )
		{
		    if( _txt[x] == _txt[x + 1] ) ntxt += 'X';
		    ntxt += _txt[x + 1];
		}
	    }
	    _txt = ntxt;
	}
	if( _txt.length() & 1 ) _txt += 'X';
    }

    void createGrid( string k, bool ij )
    {
	if( k.length() < 1 ) k = "KEYWORD"; 
	k += "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; string nk = "";
	for( string::iterator si = k.begin(); si != k.end(); si++ )
	{
	    *si = toupper( *si ); if( *si < 65 || *si > 90 ) continue;
	    if( ( *si == 'J' && ij ) || ( *si == 'Q' && !ij ) )continue;
	    if( nk.find( *si ) == -1 ) nk += *si;
	}
	copy( nk.begin(), nk.end(), &_m[0][0] );
    }

    string _txt; char _m[5][5];
};

int main( int argc, char* argv[] )
{
    string key, i, txt; bool ij, e;
    cout << "(E)ncode or (D)ecode? "; getline( cin, i ); e = ( i[0] == 'e' || i[0] == 'E' );
    cout << "Enter a en/decryption key: "; getline( cin, key ); 
    cout << "I <-> J (Y/N): "; getline( cin, i ); ij = ( i[0] == 'y' || i[0] == 'Y' );
    cout << "Enter the text: "; getline( cin, txt ); 
    playfair pf; pf.doIt( key, txt, ij, e ); return system( "pause" );
}

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