This is basic c++ program to implement the autokey Encryption.

1. Sanjay Kumar Chakravarti
//IMPLEMENTATION OF AUTO KEY IN C++
#include<iostream.h>
#include<conio.h>
#include<string.h>
void main(){
clrscr();
char *plain,*key,alp[]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
int plen,klen,i,count=0,ct[50];
cout<<"nntt*******This Program Done By:Sanjay Kumar Chakravarti**********";
cout<<"nEnter Text:";
cin>>plain;
cout<<"nEnter Key:";
cin>>key;
plen=strlen(plain);
klen=strlen(key);
if(klen<plen){
for(i=klen;i<plen;i++){
key[i]=plain[count];
count++;
}
}
cout<<"nNow Key will be:";
for(i=0;i<plen;i++){
cout<<key[i];
}

2. int j,k;
for(i=0;i<plen;i++){
for(j=0;j<26;j++){
if(plain[i]==alp[j]){
ct[i]=j;
for(k=0;k<26;k++){
if(key[i]==alp[k]){
ct[i]+=k;
}
}
}
}
}
char *cih;
cout<<"nCipher Text will be:";
for(i=0;i<plen;i++){
ct[i]=ct[i]% 26;
cih[i]=alp[ct[i]];
//cout<<ct[i]<<" ";
cout<<alp[ct[i]];
}
cout<<"nttDecryption:";
int ct1[50];
for(i=0;i<plen;i++){
for(j=0;j<26;j++){
if(cih[i]==alp[j]){
ct1[i]=j;
for(k=0;k<26;k++){

3. if(key[i]==alp[k]){
ct1[i]-=k;
}
}
}
}
}
cout<<"nOriginal Plain Text will be:";
for(i=0;i<plen;i++){
ct1[i]=ct1[i]% 26;
if(ct1[i]<0){
ct1[i]=ct1[i]+26;
}
//cih[i]=alp[ct1[i]];
//cout<<ct[i]<<" ";
cout<<alp[ct1[i]];
}
getch();
}