Caesar Cipher is the most common encryption decryption algorithm in cryptography. It is a type of substitution cipher technique.

Caesar Cipher.cpp

/*
Md Tahmid Hasan
CSE, BSMRSTU
tahmidhasan3003
*/

#include<bits/stdc++.h>
using namespace std;

string encDec(string inputText, int key, bool encryption)
{
    string outputText;
    char temp;
    for(int i=0; i<inputText.length(); i++)
    {
        temp = toupper(inputText[i]);
        if(temp > 64 && temp < 91)
            outputText = encryption ? outputText=outputText + (char)(65+(temp-65+key)%26) : outputText=outputText + (char)(65+(26+temp-65-key)%26);
        else
            outputText += temp;
    }
    return outputText;
}

int main()
{
    string inputText;
    int key=0, op;
    while(1)
    {
        cout<<"Choose any...\n1. Key Setup\n2. Encryption\n3. Decryption\n4. Exit"<<endl;
        cin>>op;
        getchar();
        switch (op)
        {
        case 1:
            cout<<"Input Key:"<<endl;
            cin>>key;
            getchar();
            cout<<endl<<"Your new key is\t: "<<key<<endl;
            key %= 26;
            break;
        case 2:
            cout<<endl<<"Input original message:"<<endl;
            getline(cin, inputText);
            cout<<endl<<"Plain Text\t: "<<inputText<<endl;
            cout<<"Cipher Text\t: "<<encDec(inputText, key, true)<<endl;
            cout<<"Used key \t: "<<key<<endl;
            break;
        case 3:
            cout<<endl<<"Input encrypted message:"<<endl;
            getline(cin, inputText);
            cout<<endl<<"Cipher Text\t: "<<inputText<<endl;
            cout<<"Plain Text\t: "<<encDec(inputText, key, false)<<endl;
            cout<<"Used key \t: "<<key<<endl;
            break;
        case 4:
            exit(0);
        default:
            cout<<"wrong input. Please input again..."<<endl;
        }
        cout<<endl;
    }
    return 0;
}