# Write a C Program to Implement the Play Fair Substitution Technique

#### Aim:

To write a C Program to Implement the Simple Substitution Technique Named Play Fair.

#### Algorithm:

1. In playfair cipher unlike traditional cipher we encrypt a pair of alphabets(digraphs) instead of a single alphabet.
2. It was used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the Australians during World War II.
3. The plaintext is split into pairs of two letters (digraphs). If there is an odd number of letters, a Z is added to the last letter.
4. If both the letters are in the same column: Take the letter below each one.

#### Program:

```#include<stdio.h>
#include<stdlib.h>
#include<stdbool.h>
char matrix[5][5];

struct max{
int r;
int c;
};
struct min{
char qr;
char qc;
};
char* repeat(char* k)
{
char* res=(char*)malloc(100*sizeof(char));
int i=1;
int j=0;
int ch;
int kl=1;
res[0]=k[0];
while(k[i]!='\0')
{
ch=0;
for(j=0;j<i;j++)
{
if(k[i]==res[j])
{
ch=0;
j=i+1;
}
else
ch=1;
}
if(ch==1)
{
if(k[i]=='j')
k[i]='i';
res[kl]=k[i];
kl++;
}
i++;
}
return res;
}
bool check(int count,char *ptr )
{
int i=0;
while(ptr[i]!='\0')
{
if((count)==ptr[i])
return 0;
i++;
}
return 1;
}
void constmatrix(char *key)
{
int i=0;
int j=0;
int ch=0;
int count=97;
char *ptr=repeat(key);
while(ptr[ch]!='\0')
{
matrix[i][j]=ptr[ch];
ch++;

if(j==4)
{
j=-1;
i++;
}
j++;
}

for(;i<5;i++)
{
int n=0;
for(;j<5;j++)
{
if(check(count,ptr)){
if(count==106)
{
count++;
j--;
}
else{
matrix[i][j]=count;
count++;
}
}
else
{
j--;
count++;
}
n=1;

}
if(n==1)
j=0;
}
for(i=0;i<5;i++){
for(j=0;j<5;j++)
printf("%c ",matrix[i][j]);
printf("\n");}
free (ptr);
}
struct max find(char a)
{
int i,j;
struct max z;
for(i=0;i<5;i++)
for(j=0;j<5;j++)
{
if(matrix[i][j]==a)
{
z.r=i;
z.c=j;
return z;
}
}
}
struct min  plafair(char a,char b)
{
struct max kat[2];
struct min kate;
kat[0]=find(a);
kat[1]=find(b);
if(kat[0].r==kat[1].r)
{
int col=kat[0].c+1;
if(col>=5)
col=0;
kate.qc=matrix[kat[0].r][col];
col=kat[1].c+1;
if(col>=5)
col=0;
kate.qr=matrix[kat[1].r][col];
return kate;
}
else if(kat[0].c==kat[1].c)
{
int row=kat[0].r+1;
if(row>=5)
row=0;
kate.qc=matrix[row][kat[0].c];
row=kat[1].r+1;
if(row>=5)
row=0;
kate.qr=matrix[row][kat[1].c];
return kate;
}
else
{
kate.qc=matrix[kat[0].r][kat[1].c];
kate.qr=matrix[kat[1].r][kat[0].c];
return kate;
}
}
struct min  deplafair(char a,char b)
{
struct max kat[2];
struct min kate;
kat[0]=find(a);
kat[1]=find(b);
if(kat[0].r==kat[1].r)
{
int col=kat[0].c-1;
if(col<0)
col=4;
kate.qc=matrix[kat[0].r][col];
col=kat[1].c-1;
if(col<0)
col=4;
kate.qr=matrix[kat[1].r][col];
return kate;

}
else if(kat[0].c==kat[1].c)
{
int row=kat[0].r-1;
if(row<0)
row=4;
kate.qc=matrix[row][kat[0].c];
row=kat[1].r-1;
if(row<0)
row=4;
kate.qr=matrix[row][kat[1].c];
return kate;
}
else
{

kate.qc=matrix[kat[0].r][kat[1].c];
kate.qr=matrix[kat[1].r][kat[0].c];
return kate;
}

}
int main()
{
char a[25],mes[100];
struct min res[100];
printf("ENTER THE KEY :\n");
scanf("%s",a);
printf("ENTER THE MESSAGE :\n");
scanf("%s",mes);
constmatrix(a);
int i=0;
int count=0;
while(mes[i]!='\0')
{
count++;
i++;
}

i=0;
int ko=0;
int later[100],o=0;
while(i<count)
{
if(i==count-1)
{
res[ko++]=plafair(mes[i],'x');
o=10;
i++;
}
else if(mes[i]!=mes[i+1])
{
res[ko++]=plafair(mes[i],mes[i+1]);

i=i+2;
}
else{
res[ko++]=plafair(mes[i],'x');
later[o++]=i+1;
i++;
}
}
int j;
printf("ENCRPTED MESSAGE = ");
for(j=0;j<ko;j++)
printf("%c%c",res[j].qc,res[j].qr);
printf("\n");
struct min dem[100];
i=0;
j=0;
char se[100];
int h_h=0,k_k=0;
while(i<ko)
{
dem[i]=deplafair(res[i].qc,res[i].qr);
se[h_h++]=dem[i].qc;
se[h_h++]=dem[i].qr;

i++;
}
int lj=0;
if(o==10)
{
se[h_h-1]='\0';
}
printf("DECRPTED MESSAGE = ");
while(se[k_k]!='\0')
{
if(k_k!=later[lj])
{
printf("%c",se[k_k]);
}
else{
lj++;
}
k_k++;
}
}```

#### Execution:

```Input:
dawn evening

Output:

ENTER THE KEY :
ENTER THE MESSAGE :
d a w n b
c e f g h
i k l m o
p q r s t
u v x y z
ENCRPTED MESSAGE = kagamdfy
DECRPTED MESSAGE = evening```

#### Result:

Thus implementation of the Play Fair program was executed Successfully.