Playfair Cipher C++ Program - The Coding Shala

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Playfair Cipher C++ Program

Here is the C++ program to implement the Playfair Cipher.

C++ Code: 

#include<bits/stdc++.h>

using namespace std;

void encrypt(string key){
 pair<char,pair<int,int> > pa[25]; //store i j value of char
 int visited[26] = {0};
 char a[5][5];
 char b[5][5];
 char ch1,ch2,c1,c2;
 int i,j,i1,j1,i2,j2;
 int count=0;
 for(i=0; i<5; i++){
  if(count==key.size()){
   break;
  }
  for(j=0; j<5; j++){
   if(count==key.size()){
    break;
   }else if(key[count]!='j'){ //skip j
    //cout << key[count] << endl;
    while(visited[key[count]-'a']==1){  //skip same char
     count++;
    }
    if(visited[key[count]-'a']==0){  //first time store it
     a[i][j]=key[count];
     visited[key[count]-'a']=1;
    }
   }
   count++;
  }
 }
 
 int p=i-1; 
 int q=j;
 i=0;  
 //making 5*5 matrix
 while(i<26){
  if(i!='j'-'a'){  //for j char
   if(visited[i]==0){
    if(q==5){
     q=0;
     p++;
    }
    a[p][q]='a'+i;
    q++;
   }
  }
  i++;
 }
 
 //copy a to b
 for(int i=0; i<5; i++){
  for(int j=0; j<5; j++){
   b[i][j] = a[i][j];
  }
 }
 
 
 //making pair
 for(int i=0; i<5; i++){
  for(int j=0; j<5; j++){
   pa[b[i][j]-'a'].first=a[i][j];
   pa[b[i][j]-'a'].second.first = i;
   pa[b[i][j]-'a'].second.second = j;
  }
 }
  
 FILE *fp1, *fp2;
 fp1 = fopen("encrypt.txt","r");
 fp2 = fopen("decrypt.txt","w");
 int flag=0;
 char c;
 do{
         if(flag==0){
          aa:
  ch1=fgetc(fp1);
  if(!isalpha(ch1) && ch1!=EOF) goto aa;
  }else{
  ch1=c;
  flag=0;
  }
  ab:
  ch2=fgetc(fp1);
  if(!isalpha(ch2) && ch2!=EOF) goto ch2;
  
  if(ch1==ch2){
  flag=1;
  c=ch2;
  ch2='x';
  }
  
  if(ch1!=EOF && ch2==EOF) {
  ch2='x';
  }
  if(ch1==EOF || ch2==EOF){
   break;
  }
  i1 = pa[ch1-'a'].second.first;  //take  first char row
  j1 = pa[ch1-'a'].second.second; // first char column
  i2 = pa[ch2-'a'].second.first;  // second char row
  j2 = pa[ch2-'a'].second.second;  // second char column

  if(pa[ch1-'a'].second.first==pa[ch2-'a'].second.first){   

                       //check same row
   c1 = a[i1][(j1+1)%5];      
   c2 = a[i2][(j2+1)%5];
  }else if(pa[ch1-'a'].second.second==pa[ch2-'a'].second.second){ 

                  // check same column
   c1 = a[(i1+1)%5][j1];
   c2 = a[(i2+1)%5][j2];
  }else{    // digonal interchange
   c1 = a[i1][j2];
   c2 = a[i2][j1];
  }
  //cout << ch1 << " " << ch2;
  fputc(c1,fp2);
  fputc(c2,fp2);
 }while(ch1 != EOF || ch2!=EOF);
 
 fclose(fp1);
 fclose(fp2);
 
 //fp2 = fopen("p2.txt","r");
 
 //do{
 // ch1=fgetc(fp2);
//cout << ch1;
 //}while(ch1!=EOF);


}

void decrypt(string key){
 pair<char,pair<int,int> > pa[25];
 int visited[26] = {0};
 char a[5][5];
 char b[5][5];
 char ch1,ch2,c1,c2;
 int i,j,i1,j1,i2,j2;
 int count=0;
 for(i=0; i<5; i++){
  if(count==key.size()){
   break;
  }
  for(j=0; j<5; j++){
   if(count==key.size()){
    break;
   }else if(key[count]!='j'){
    //cout << key[count] << endl;
    while(visited[key[count]-'a']==1){
     count++;
    }
    if(visited[key[count]-'a']==0){
     a[i][j]=key[count];
     visited[key[count]-'a']=1;
    }
   }
   count++;
  }
 }
 
 int p=i-1;
 int q=j;
 i=0;
 while(i<26){
  if(i!='j'-'a'){
   if(visited[i]==0){
    if(q==5){
     q=0;
     p++;
    }
    a[p][q]='a'+i;
    q++;
   }
  }
  i++;
 }
 
 for(int i=0; i<5; i++){
  for(int j=0; j<5; j++){
   b[i][j] = a[i][j];
  }
 }
 
 
 for(int i=0; i<5; i++){
  for(int j=0; j<5; j++){
   pa[b[i][j]-'a'].first=a[i][j];
   pa[b[i][j]-'a'].second.first = i;
   pa[b[i][j]-'a'].second.second = j;
  }
 }
  
 FILE *fp1, *fp2;
 fp1 = fopen("decrypt.txt","r");
 fp2 = fopen("encrypt.txt","w");
 
 do{
  ch1=fgetc(fp1);
  ch2=fgetc(fp1);
  if(ch1==EOF || ch2==EOF){
   break;
  }
  i1 = pa[ch1-'a'].second.first;
  j1 = pa[ch1-'a'].second.second;
  i2 = pa[ch2-'a'].second.first;
  j2 = pa[ch2-'a'].second.second;
  if(pa[ch1-'a'].second.first==pa[ch2-'a'].second.first){
   c1 = a[i1][(j1+4)%5];
   c2 = a[i2][(j2+4)%5];
  }else if(pa[ch1-'a'].second.second==pa[ch2-'a'].second.second){
   c1 = a[(i1+4)%5][j1];
   c2 = a[(i2+4)%5][j2];
  }else{
   c1 = a[i1][j2];
   c2 = a[i2][j1];
  }
  //cout << ch1 << " " << ch2;
  fputc(c1,fp2);
  fputc(c2,fp2);
 }while(ch1 != EOF || ch2!=EOF);
 
 fclose(fp1);
 fclose(fp2);
 
 //fp2 = fopen("p1.txt","r");
 
 //do{
 // ch1=fgetc(fp2);
 // cout << ch1;
//while(ch1!=EOF);


}

int main(){
 string key;
 int ch;
 start:
  cout<<"Playfair cipher\n";
 cout << "1.encrypt\n 2.decrypt\n 3.exit\n";
 cin >> ch;
 switch(ch){
  case 1:
   cout << "enter key  :\n";
   cin >> key;
   encrypt(key);
   goto start;
   break;
  case 2:
   cout << "enter key  :\n";
   cin >> key;
   decrypt(key);
   goto start;
   break;
  case 3:
   exit(0);
 }
 return 0;
}

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