Program 237: Decryption of Playfair Cipher using C

Program 237: See Encryption To understand Decryption
 

#include<stdio.h>
#include<string.h>
#include<ctype.h>
int removerepeated(int size,int a[]);
int insertelementat(int position,int a[],int size);
main()
{
 int i,j,k,numstr[100],numcipher[100],numkey[100],lenkey,templen,tempkey[100],flag=-1,size,cipherkey[5][5],lennumstr,row1,row2,col1,col2;
 char str[100],key[100];
 printf("Enter a string\n");
 gets(str);
 //converting entered string to Capital letters
 for(i=0,j=0;i<strlen(str);i++)
 {
  if(str[i]!=' ')
  {
   str[j]=toupper(str[i]);   
   j++;
  }

 }
 str[j]='\0';
 printf("Entered String is %s\n",str);
 //Storing string in terms of ascii and to restore spaces I used -20
 size=strlen(str);
 for(i=0;i<size;i++)
 {
  if(str[i]!=' ')
  numstr[i]=str[i]-'A';
 }
 lennumstr=i;
 //Key processing
 printf("Enter the key (Non repeated elements if possible)\n");
 gets(key);
 //converting entered key to Capital letters
 for(i=0,j=0;i<strlen(key);i++)
 {
  if(key[i]!=' ')
  {
   key[j]=toupper(key[i]);   
   j++;
  }
 }
 key[j]='\0';
 printf("%s\n",key);
 //Storing key in terms of ascii
 k=0;
 for(i=0;i<strlen(key)+26;i++)
 {
  if(i<strlen(key))
  {
   if(key[i]=='J')
   {
    flag=8;
    printf("%d",flag);
   }
       numkey[i]=key[i]-'A';   
  }
  else
  {
    if(k!=9 && k!=flag)//Considering I=J and taking I in place of J except when J is there in key ignoring I
    {
            numkey[i]=k; 
      }
      k++;

  }
 }
 templen=i;
 lenkey=removerepeated(templen,numkey);
 printf("Entered key converted according to Play Fair Cipher rule\n");
 for(i=0;i<lenkey;i++)
 {
     printf("%c",numkey[i]+'A');  
 }
 printf("\n");
 //Arranging the key in 5x5 grid
 k=0;
 for(i=0;i<5;i++)
 {
  for(j=0;j<5;j++)
  {
   cipherkey[i][j]=numkey[k];   
   k++;
  }
 }
 
 printf("Arranged key\n");
 for(i=0;i<5;i++)
 {
  for(j=0;j<5;j++)
  {

   printf("%c ",cipherkey[i][j]+'A');

  }
  printf("\n");
 }
   
   //Message Processing
 
   for(i=0;i<lennumstr;i+=2)
   {
      if(numstr[i]==numstr[i+1])
      {
       insertelementat(i+1,numstr,lennumstr);
       lennumstr++;
      }
   }
   if(lennumstr%2!=0)
   {
    insertelementat(lennumstr,numstr,lennumstr);
    lennumstr++;
   }
   printf("Entered String/Message After Processing according to Play fair cipher rule\n");
   for(i=0;i<lennumstr;i++)
   {
    printf("%c",numstr[i]+'A');
   }
   for(k=0;k<lennumstr;k+=2)
   {
    for(i=0;i<5;i++)
    {
     for(j=0;j<5;j++)
     {
      if(numstr[k]==cipherkey[i][j])
      {
         row1=i;
         col1=j;       
      }
      if(numstr[k+1]==cipherkey[i][j])
      {
         row2=i;
         col2=j;       
      }
      
     }
    }
    //Only change between Ecryption to decryption is changing + to -
    //If negative add 5 to that row or column
    if(row1==row2)
    {
     col1=(col1-1)%5;
     col2=(col2-1)%5;
     if(col1<0)
     {
      col1=5+col1;
     }
     if(col2<0)
     {
      col2=5+col2;
     }
     numcipher[k]=cipherkey[row1][col1];
     numcipher[k+1]=cipherkey[row2][col2];
    }
    if(col1==col2)
    {
     row1=(row1-1)%5;
     row2=(row2-1)%5;
      if(row1<0)
     {
      row1=5+row1;
     }
     if(row2<0)
     {
      row2=5+row2;
     }
     numcipher[k]=cipherkey[row1][col1];
     numcipher[k+1]=cipherkey[row2][col2];
    }
    if(row1!=row2&&col1!=col2)
    {
     numcipher[k]=cipherkey[row1][col2];
     numcipher[k+1]=cipherkey[row2][col1];
    }
   } 
   printf("\nCipher Text is\n");

   for(i=0;i<lennumstr;i++)
   {
    if((numcipher[i]+'A')!='X')//Should remove extra 'X' which were created during Encryption
      printf("%c",numcipher[i]+'A'); 
   }
   printf("\n");

 
}

int removerepeated(int size,int a[])
{
 int i,j,k;
 for(i=0;i<size;i++)
  {
 for(j=i+1;j<size;)
 {
    if(a[i]==a[j])
    {
     
     for(k=j;k<size;k++)
     {
      a[k]=a[k+1];
     }
         size--;
        }
    else
    {
      j++;
     } 
 }
 }
return(size);
}

int insertelementat(int position,int a[],int size)
{
       int i,insitem=23,temp[size+1];
    for(i=0;i<=size;i++)
        {
        if(i<position)
        {
            temp[i]=a[i];
        }
        if(i>position)
        {
         temp[i]=a[i-1];    
        }
        if(i==position)
        {
            temp[i]=insitem;
        }
        
        }
        
        for(i=0;i<=size;i++)
        {
         a[i]=temp[i];
        }
}

Explanation:

//Coming Soon... 

Output: