Cipher_Algorithms.cpp

/*
 * This file contains the implementation of the algorithms used to create ciphers.
 * 
 * That includes:
 * -------------------------------
 * caesar cipher
 * RSA encryp and decrypt
 * base64 encrypt and ecrypt
 * -------------------------------
 * 
*/

#include <bits/stdc++.h>

using namespace std;

const string menu = "Enter 1 to print menu\n"
                    "Enter 2 to rotate every character in the string\n"
                    "Enter 3 to encode it to base64\n"
                    "Enter 4 to decode it from base64\n"
                    "Enter 5 to RSA encrypt a number\n"
                    "Enter 6 to RSA decrypt a number\n"
                    "Enter 7 to end program\n"
                    "-->";

const string charSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";


/*
 * The function binpow() is used for RSA encryption and ecryption.
 * It uses the concept of binary exponentiation under modulus.
 * It takes three parameters , the number to be encrypted/decrypted (a), the encryption key(b), the modulo number(m)
 */

long long binpow(long long a, long long b, long long m)
{
    a %= m;
    long long res = 1;
    while (b > 0)
    {
        if (b & 1)
            res = res * a % m;
        a = a * a % m;
        b >>= 1;
    }
    return res;
}


/*
 * The function rotateCharacters is used to rotate every character by some factor.
 */

void rotateCharacters(string s, int val)
{

    val %= 26;
    for (auto &i : s)
    {
        if (i >= 'A' and i <= 'Z')
            i = (i - 'A' + val) % 26 + 'A';
    }
    cout << s << endl;
    return;
}

/*
 * The function encodeToBase64 is used to encode the givent text into base64. 
 */

void encodeToBase64(string str)
{
    string result = "";
    int i;
    int j;
    int temp;
    int index;
    int len = str.length();
    int numberOfBits = 0;
    int padding = 0;
    int value = 0;
    int count = 0;

    for (i = 0; i < len; i += 3)
    {
        value = 0;
        count = 0;
        numberOfBits = 0;

        for (j = i; j < len && j <= i + 2; j++)
        {
            value = value << 8;
            value = value | str[j];
            count += 1;
        }

        numberOfBits = 8 * count;
        padding = numberOfBits % 3;

        while (numberOfBits != 0)
        {

            if (numberOfBits >= 6)
            {

                temp = numberOfBits - 6;
                index = (value >> temp) & 63;
                numberOfBits -= 6;
            }
            else
            {
                temp = 6 - numberOfBits;
                index = (value << temp) & 63;
                numberOfBits = 0;
            }

            result.push_back(charSet[index]);
        }
    }

    for (i = 1; i <= padding; i++)
    {
        result.push_back('=');
    }

    cout << result << endl;
    return;
}

int findIndexInCharSet(char c)
{
    for (int i = 0; i < 64; i++)
    {
        if (c == charSet[i])
        {
            return i;
        }
    }
    return -1;
}

/*
 * The function decodeToBase64 is used to decode the given base64 into raw data text. 
 */

void decodeFromBase64(string str)
{
    string result = "";
    int i;
    int j;
    int value = 0;
    int numberOfBits = 0;
    int len = str.length();
    for (i = 0; i < len; i += 4)
    {
        value = 0;
        numberOfBits = 0;
        for (j = i; j < len && j <= i + 3; j++)
        {
            if (str[j] != '=')
            {
                value = value << 6;
                numberOfBits += 6;
            }

            int idx = findIndexInCharSet(str[j]);
            if (idx != -1)
            {
                value = value | idx;
            }
            else
            {
                value = value >> 2;
                numberOfBits -= 2;
            }
        }
        while (numberOfBits != 0)
        {
            numberOfBits -= 8;
            result.push_back((value >> numberOfBits) & 255);
        }
    }

    cout << result << endl;
    return;
}

int main()
{
    while (1)
    {
        string s;
        int val;
        cout << menu;
        int option;
        cin >> option;
        switch (option)
        {
        case 1:
            cout<<menu;
            break;

        case 2:
            cout << "Enter the input string: ";
            cin >> s;
            cout << "Enter the number of character places to rotate:";
            cin >> val;
            cout << "\nOutput string: " << endl;
            rotateCharacters(s, val);
            cout << endl;
            break;

        case 3:
            cout << "Enter the input string: ";
            cin >> s;
            cout << "\nOutput string: " << endl;
            encodeToBase64(s);
            cout << endl;
            break;

        case 4:
            cout << "Enter the input string: ";
            cin >> s;
            cout << "\nOutput string: " << endl;
            decodeFromBase64(s);
            cout << endl;
            break;

        case 5:
        {
            cout << "Enter the value to be encrypted: ";
            long long num;
            cin >> num;
            cout << "Enter value of e: ";
            long long e;
            cin >> e;
            cout << "Enter value of N: ";
            long long N;
            cin >> N;
            long long ans = binpow(num, e, N);
            cout << "Cipher is : " << ans << endl<<endl;
            break;
        }
        case 6:
        {
            cout << "Enter the value to be decrypted: ";
            long long num;
            cin >> num;
            cout << "Enter value of d: ";
            long long d;
            cin >> d;
            cout << "Enter value of N: ";
            long long N;
            cin >> N;
            long long ans = binpow(num, d, N);
            cout << "Cipher is : " << ans << endl<<endl;
            break;
        }

        case 7:
            return 0;

        default:
            cout << "Enter a valid option" << endl;
            break;
        }
    }
    return 0;
}