Notes

**********Caeser Cipher**********
#include <stdio.h>
int main()
{
char message[100], ch;
int i, key;
printf("Enter a message to encrypt: ");
gets(message);
printf("Enter key: ");
scanf("%d", &key);
for (i = 0; message[i] != ''; ++i)
{
ch = message[i];
if (ch >= 'a' && ch <= 'z')
{
ch = ch + key;
if (ch > 'z')
{
ch = ch - 'z' + 'a' - 1;
}
message[i] = ch;
}
else if (ch >= 'A' && ch <= 'Z')
{
ch = ch + key;
if (ch > 'Z')
{
ch = ch - 'Z' + 'A' - 1;
}
message[i] = ch;
}
}
printf("Encrypted message: %s", message);
return 0;
}
********MONOALPHABETIC**********
#include<bits/stdc++.h>
using namespace std;
int main() {
char pt[26] = {'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'};
char ct[26] = {'Z', 'Y', 'X', 'W', 'V', 'U', 'T', 'S', 'R', 'Q', 'P',
'O', 'N', 'M', 'L', 'K', 'J', 'I', 'H', 'G', 'F', 'E', 'D', 'C', 'B',
'A'};
string plain, cipher = "", decipher = "";
int i, j;
cout << "Enter Plain text : ";
getline(cin, plain);
int flg = 0;
if (islower(plain[0])) {
flg = 1;
}
transform(plain.begin(), plain.end(), plain.begin(), ::toupper);
int k = 0;
for (i = 0; i < plain.size(); i++) {
if (plain[i] == ' ') {
cipher += ' ';
continue;
}
for (j = 0; j < 26; j++) {
if (pt[j] == plain[i]) {
cipher += ct[j];
}
}
}
if (flg) {
string t = cipher;
transform(t.begin(), t.end(), t.begin(), ::tolower);
cout << "After Encryption: " << t << endl;
}
else
cout << "After Encryption: " << cipher << endl;
k = 0;
for (i = 0; i < cipher.size(); i++)
{
if (cipher[i] == ' ') {
decipher += ' ';
continue;
}
for (j = 0; j < 26; j++)
{
if (ct[j] == cipher[i])
{
decipher += pt[j];
}
}
}
if (flg) {
string t = decipher;
transform(t.begin(), t.end(), t.begin(), ::tolower);
cout << "After Decryption: " << t << endl;
}
else
cout << "After Decryption: " << decipher << endl;
}


*********Hill Cipher**********
#include <stdio.h>
#include <string.h>
int main()
{
unsigned int a[3][3] = {{6, 24, 1}, {13, 16, 10}, {20, 17, 15}};
unsigned int b[3][3] = {{8, 5, 10}, {21, 8, 21}, {21, 12, 8}};
int i, j;
unsigned int c[20], d[20];
char msg[20];
int determinant = 0, t = 0;
printf("Enter plain text: ");
scanf("%s", msg);
for (i = 0; i < 3; i++)
{
c[i] = msg[i] - 65;
printf("%d ", c[i]);
}
for (i = 0; i < 3; i++)
{ t = 0;
for (j = 0; j < 3; j++)
{
t = t + (a[i][j] * c[j]);
}
d[i] = t % 26;
}
printf("nEncrypted Cipher Text :");
for (i = 0; i < 3; i++)
printf(" %c", d[i] + 65);
for (i = 0; i < 3; i++)
{
t = 0;
for (j = 0; j < 3; j++)
{
t = t + (b[i][j] * d[j]);
}
c[i] = t % 26;
}
printf("nDecrypted Cipher Text :");
for (i = 0; i < 3; i++)
printf(" %c", c[i] + 65);
return 0;
}


***********Polyaphabetic*************

#include<bits/stdc++.h>
using namespace std;
string generateKey(string str, string key)
{
int x = str.size();
for (int i = 0; ; i++)
{
if (x == i)
i = 0;
if (key.size() == str.size())
break;
key.push_back(key[i]);
}
return key;
}
string cipherText(string str, string key)
{
string cipher_text;
for (int i = 0; i < str.size(); i++)
{
// converting in range 0-25
char x = (str[i] + key[i]) %26;
// convert into alphabets(ASCII)
x += 'A';
cipher_text.push_back(x);
}
return cipher_text;
}
string originalText(string cipher_text, string key)
{
string orig_text;
for (int i = 0 ; i < cipher_text.size(); i++)
{
// converting in range 0-25

char x = (cipher_text[i] - key[i] + 26) %26;
// convert into alphabets(ASCII)
x += 'A';
orig_text.push_back(x);
}
return orig_text;
}
int main()
{
string str;
string keyword;
cout<<"Enter Plain text: ";
cin>>str;
cout<<"Enter key: ";
cin>>keyword;
string key = generateKey(str, keyword);
string cipher_text = cipherText(str, key);
cout << "Ciphertext : "
<< cipher_text << "n";
cout << "Original/Decrypted Text : "
<< originalText(cipher_text, key);
return 0;
}

*********Railfence************
#include <bits/stdc++.h>
using namespace std;
string encryptRailFence(string text, int key)
{
char rail[key][(text.length())];
for (int i = 0; i < key; i++)
for (int j = 0; j < text.length(); j++)
rail[i][j] = 'n';
bool dir_down = false;
int row = 0, col = 0;
for (int i = 0; i < text.length(); i++)
{
if (row == 0 || row == key - 1)
dir_down = !dir_down;
rail[row][col++] = text[i];
dir_down ? row++ : row--;
}
string result;
for (int i = 0; i < key; i++)
for (int j = 0; j < text.length(); j++)
if (rail[i][j] != 'n')
result.push_back(rail[i][j]);
return result;
}
string decryptRailFence(string cipher, int key)
{
char rail[key][cipher.length()];
for (int i = 0; i < key; i++)
for (int j = 0; j < cipher.length(); j++)
rail[i][j] = 'n';
bool dir_down;
IU2041050128 | PARAM SUTHAR
Page | 15
int row = 0, col = 0;
for (int i = 0; i < cipher.length(); i++)
{
// check the direction of flow
if (row == 0)
dir_down = true;
if (row == key - 1)
dir_down = false;
rail[row][col++] = '*';
dir_down ? row++ : row--;
}
int index = 0;
for (int i = 0; i < key; i++)
for (int j = 0; j < cipher.length(); j++)
if (rail[i][j] == '*' && index < cipher.length())
rail[i][j] = cipher[index++];
string result;
row = 0, col = 0;
for (int i = 0; i < cipher.length(); i++)
{
if (row == 0)
dir_down = true;
if (row == key - 1)
dir_down = false;
if (rail[row][col] != '*')
result.push_back(rail[row][col++]);
dir_down ? row++ : row--;
}
return result;
}
int main()
{
cout<<"Plain Text: Cryptography n";
cout <<"After Encryption: "<< encryptRailFence("Cryptography ", 3) <<
endl;
cout <<"After Decryption: "<< decryptRailFence("Cta rporpyygh", 3) <<
endl;
return 0;
}

************RSA******************
#include<iostream>
#include<math.h>
using namespace std;

int gcd(int a, int b) {
int t;
while(1) {
t= a%b;
if(t==0)
return b;
a = b;
b= t;
}
}

int main() {
double p = 13;
double q = 11;
double n=p*q;
double track;
double phi= (p-1)*(q-1);

double e=7;
while(e<phi) {
track = gcd(e,phi);
if(track==1)
break;
else
e++;
}

double d1=1/e;
double d=fmod(d1,phi);
double message = 9;
double c = pow(message,e);
double m = pow(c,d);
c=fmod(c,n);
m=fmod(m,n);
cout<<"Original Message = "<<message;
cout<<"n"<<"First prime number, p = "<<p;
cout<<"n"<<"Second prime number, q = "<<q;
cout<<"n"<<"n = p*q = "<<n;
cout<<"n"<<"phi = (p-1)*(q-1) = "<<phi;
cout<<"n"<<"encrypt = "<<e;
cout<<"n"<<"decrypt = "<<d;
cout<<"n"<<"Encrypted message = "<<c;
cout<<"n"<<"Decrypted message = "<<m;
return 0;
}

**********Diffie-hellmen************
#include <iostream>
#include <cmath>
using namespace std;

long long int power(long long int a, long long int b, long long int P)
{
if (b == 1)
return a;
else
return (((long long int)pow(a, b)) % P);
}

int main()
{
long long int P, G, x, a, y, b, ka, kb;
P = 23;
cout << "A prime number P : " << P << endl;
G = 9;
cout << "A primitive root for P, G : " << G << endl;

a = 4;
cout << "The private key a for Person1 : " << a << endl;
x = power(G, a, P);

b = 3;
cout << "The private key b for Person2 : " << b << endl;
y = power(G, b, P);

ka = power(y, a, P);
kb = power(x, b, P);

cout << "Secret key for the Person1 is : " << ka << endl;
cout << "Secret key for the Person2 is : " << kb << endl;
return 0;
}