C code to Encrypt Message using PlayFair (Monarchy) Cipher

C program to implement PlayFair Cipher to encrypt a given message.

#include<stdio.h>
#include<conio.h>

void main()
{
char v,w,ch,string[100],arr[5][5],key[10],a,b,enc[100];
int temp,i,j,k,l,r1,r2,c1,c2,t,var;
FILE * fp;
fp=fopen("sk.txt","r");   //keep message in sk.txt (e.g. jamia)
clrscr();
printf("Enter the key\n");
fflush(stdin);
scanf("%s",&key);
l=0;
while(1)
{
ch=fgetc(fp);
    if(ch!=EOF)
        {
        string[l++]=ch;
        }
    if(ch==EOF)
    break;
}
string[l]='\0';
puts(string);
for(i=0;key[i]!='\0';i++)
{
    for(j=i+1;key[j]!='\0';j++)
    {
    if(key[i]==key[j])
        {
            temp=1;
            break;
        }
    }
}
if(temp==1)
printf("invalid key");
else
{
k=0;
a='a';
//printf("%c",b);
for(i=0;i<5;i++)
{
    for(j=0;j<5;j++)
    {
    if(k<strlen(key))
     arr[i][j]=key[k];
    else if(k==strlen(key))
    {
    b:
    for(l=0;l<strlen(key);l++)
    {
        if(key[l]==a)
        {
  a++;
  goto b;
        }
    }
    arr[i][j]=a;
    if(a=='i')
     a=a+2;
    else
     a++;
    }
     if(k<strlen(key))
     k++;
    }
}
printf("\n");
printf("The matrix is\n");
for(i=0;i<5;i++)
{
    for(j=0;j<5;j++)
    {
     printf("%c",arr[i][j]);
    }
    printf("\n");
}
t=0;
if(strlen(string)%2!=0)
var=strlen(string)-1;
for(i=0;i<var;)
{
    v=string[i++];
    w=string[i++];
    if(v==w)
    {
     enc[t++]=v;
     enc[t++]='$';
    }
    else
    {
    for(l=0;l<5;l++)
        {
            for(k=0;k<5;k++)
            {
      if(arr[l][k]==v||v=='j'&&arr[l][k]=='i')
      {
       r1=l;
       c1=k;
      }
      if(arr[l][k]==w||w=='j'&&arr[l][k]=='i')
      {
       r2=l;
       c2=k;
      }
            }
        }
        if(c1==c2)
        {
  r1++;
  r2++;
  if(r1==5||r2==5)
  {
   r1=0;
   r2=0;
  }
        }
        else if(r1==r2)
        {
  c1++;
  c2++;
  if(c1==5||c2==5)
  {
   c1=0;
   c2=0;
  }
        }
        else
        {
  temp=r1;
  r1=r2;
  r2=temp;
        }
        enc[t++]=arr[r1][c1];
        enc[t++]=arr[r2][c2];
       
    }
}
if(strlen(string)%2!=0)
 enc[t++]=string[var];
enc[t]='\0';
}
printf("The encrypted text is\n");
puts(enc);
getch();
}

Output of the above program:-

Result of PlayFair (Monarchy) Cipher 

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