how encrypt and decrypt file extension using hill cipher method

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Program to implement  HILL CIPHER  method.
#include<stdio.h>
#include<conio.h>
int n,i,len,j,m,l,ptxt[3][3],k[3][3],aa[3];
char pt[20],ct[20],rf[20];
void getd();
void display1();
void encrypt();
void decrypt();
int select(int);
int euclid(int,int);
int display();
void main(){
int c;
do{
clrscr();
f1:
display();
printf("Enter Your Choice:");
scanf("%d",&c);
 
if(c>4 || c<1){
clrscr();
printf("\nEnter proper value\n");
goto f1;
}
select(c);
}while(c!=4);
getch();
}
int display(){
printf("Hill Cipher program\n");
printf("———————–\n");
printf(" option Functions\n");
printf("———————–\n");
printf(" [1] Enter The Text\n");
printf(" [2] Encrypted Text\n");
printf(" [3] Decrypted Text\n");
printf(" [4] Exit\n");
printf("———————–\n");
return 0;
}
int select(int c){
int i;
switch(c){
case 1:{clrscr();
display();
fflush(stdin);
getd();
display1();
for(i=0;ct[i]!=";i++)
ct[i]=";
break;
}
case 2:{clrscr();
display();
encrypt();
break;
}
case 3:{clrscr();
display();
decrypt();
break;
}
case 4: break;
}
return c;
}
void getd(){
printf("Enter the plain text:");
gets(pt);
printf("Ente the key in number associate alphabet:\n");
for(i=0;i<3;i++){
for(j=0;j<3;j++){
f: printf("k[%d][%d]= ",i,j);
scanf("%d",&k[i][j]);
if(k[i][j]>25 || k[i][j]<0){
printf("Enter Wrong Value reenter");
goto f; } }  }  }
void display1(){
printf("plain text:%s\n",pt);
printf("key:\n");
for(i=0;i<3;i++){
for(j=0;j<3;j++){
printf("%3d",k[i][j]);}
printf("\n");
}
}
void encrypt(){
 
j=0;
l=n=0;
len=strlen(pt);
for(i=0;i<len;){
for(m=0;m<3;m++,i++){
if(i>len) ptxt[m][0]=0;
else ptxt[m][0]=abs((65-pt[i]));
}
for(j=0;j<3;j++,n++){
aa[j]=0;
for(l=0;l<3;l++)
aa[j]=aa[j]+(k[j][l]*ptxt[l][0]);
ct[n]=(65+(aa[j]%26));
}
} ct[n]=";
printf("\n%s",ct);
getch();
}
int euclid(int a,int b)
{
int r1,r2,t1,t2,q,r,t,inverse,temp;
if(a<b){
temp=b;
b=a;
a=temp;
}
r1=a;
r2=b;
t1=0;
t2=1;
while(r2!=0)
{
q=r1/r2;
r=r1%r2;
t=t1-(q*t2);
t1=t2;
t2=t;
r1=r2;
r2=r;
}
inverse=t1%t2;
if(inverse<0)
{
inverse=inverse+t2;
}
return inverse;
}
void decrypt()
{ int b[3],in[3][3],det,abc,d1;
det=k[0][0]*(k[1][1]*k[2][2]-k[2][1]*k[1][2])-k[0][1]*(k[1][0]*k[2][2]-k[1][2]*k[2][0])+k[0][2]*(k[1][0]*k[2][1]-k[1][1]*k[2][0]);
abc=det;
if(abc<0) det=26-((abs(abc))%26);
else det=(abc%26);
det=euclid(det,26);
printf("\n Determinant :%d \n",det);
in[0][0]=((k[1][1]*k[2][2])-(k[2][1]*k[1][2]));
in[1][0]=-((k[1][0]*k[2][2])-(k[1][2]*k[2][0]));
in[2][0]=((k[1][0]*k[2][1])-(k[2][0]*k[1][1]));
in[0][1]=-((k[0][1]*k[2][2])-(k[0][2]*k[2][1]));
in[1][1]=((k[0][0]*k[2][2])-(k[0][2]*k[2][0]));
in[2][1]=-((k[0][0]*k[2][1])-(k[2][0]*k[0][1]));
in[0][2]=((k[0][1]*k[1][2])-(k[0][2]*k[1][1]));
in[1][2]=-((k[0][0]*k[1][2])-(k[1][0]*k[0][2]));
in[2][2]=((k[0][0]*k[1][1])-(k[1][0]*k[0][1]));
printf("\n Inverse is : \n");
for(i=0;i<3;i++){
for(j=0;j<3;j++) {
if(in[i][j]<0) in[i][j]=26-((abs(in[i][j]))%26);
else in[i][j]=(in[i][j]%26);
in[i][j]=((in[i][j]*det)%26);
printf("%4.2d ",in[i][j]);}
printf("\n");  }
len=strlen(ct);
j=0;
l=n=0;
for(i=0;i<len;){
for(m=0;m<3;m++,i++){
if(i>len) ptxt[m][0]=0;
else
 ptxt[m][0]=abs((65-ct[i]));
printf("%2d ",ptxt[m][0]); }
printf("\n");
for(j=0;j<3;j++,n++){
b[j]=0;
for(l=0;l<3;l++)
b[j]=b[j]+((in[j][l]*ptxt[l][0]));
aa[j]=b[j];
rf[n]=(65+((26+aa[j])%26));  }  }
 rf[n]=";
for(i=0;i<strlen(ct);i++)
printf("%c",rf[i]);
getch();
}