Caesar.cpp

//  Created by Евгений on 11.10.16.

#include <iostream>
#include <string>

std::string encCaesar(const std::string& message, int shift) {
    
    if (shift > 26)
        shift = shift % 26;
    
    auto sLen = message.length();
    std::string encrypted = "";
    encrypted.reserve(sLen);
    
    for (int i = 0; i < sLen; i++) {
        
        auto cCode = int(message[i]);
        
        if (cCode >= 97 && cCode <= 122) {
            char encChar = (cCode+shift <=122) ? (char)(cCode+shift) : (char)(cCode - 26 + (shift % 26));
            
            encrypted += encChar;
        }
    }
    
    return  encrypted;
}

std::string decCaesar(const std::string& message, int shift) {
    
    auto sLen = message.length();
    std::string decrypted;
    decrypted.reserve(sLen);
    
    if (shift != 0) {
        for (int i = 0; i < sLen; i++) {
            
            auto cCode = int(message[i]);
            
            if (cCode >= 97 && cCode <= 122) {
                
                char decChar;
                auto newValue = cCode-shift;
                
                if (newValue <= 122 && newValue >=97)
                    decChar = (char)newValue;
                
                else if (newValue < 97) {
                    decChar = (char)(newValue + 26);
                }
                
                else {
                    decChar = (char)(cCode - 26 - (shift % 26));
                }
                
                decrypted += decChar;
            }
        }
    }
    
    else {
        for (int i = 1; i < 27 ;i++) {
            std::cout << "decrypted message with shift " << i << "\n";
            
            shift = i;
            decrypted = "";
            
            for (int i = 0; i < sLen; i++) {
                
                auto cCode = int(message[i]);
                
                if (cCode >= 97 && cCode <= 122) {
                    
                    char decChar;
                    auto newValue = cCode-shift;
                    
                    if (newValue <= 122 && newValue >=97)
                        decChar = (char)newValue;
                    
                    else if (newValue < 97) {
                        decChar = (char)(newValue + 26);
                    }
                    
                    else {
                        decChar = (char)(cCode - 26 - (shift % 26));
                    }
                    
                    decrypted += decChar;
                }
            }
            std::cout << decrypted <<"\n\n";
        }
    }
    
    return decrypted;
}

int main(int argc, const char * argv[]) {
    std::string plain = "qwertyuzxc";
    auto encrypted = encCaesar(plain, 3);
    
    std::cout
    << "plain: " << plain
    << "\n\n" << "encrypted: (key == 3)\n" << encrypted
    << "\n\n" << "decrypted (key == 3):\n" << decCaesar(encrypted, 3)
    << "\n\n" << "brootforced:\n" << decCaesar(encrypted, 0);
}