ComputerNetworksAndSecurity/Codes/CRC.cpp

#include <iostream>
#include <cstring>
using namespace std;

string XOR(string data, string key) {
    // Dividend is data
    // Divisor is the primary key, i.e. the key
    string result = "";
    int dataLen = data.length();
    int keyLen = key.length();

    // Perform XOR operation
    for (int i=0; i<keyLen; i++) {
        if (i < dataLen) {
            // Only perform XOR if within the length of data
            if (data[i] == key[i]) {
                result += '0';
            }
            else {
                result += '1';
            }
        } else {
            // If data length exceeded, append the key
            result += key[i];
        }
    }
    return result;
}

string encoder(string data, string key) {
    int keyLen = key.length();

    // Append n-1 zeroes to the data
    string dataWithZeroes = data + string(keyLen-1, '0');

    // Perform XOR operation with the key
    string remainder = XOR(dataWithZeroes, key);

    // Get the remainder (last n-1 bits)
    string crc = remainder.substr(remainder.length() - (keyLen-1));

    // Append the CRC to the original data
    return data + crc;
}

string performDivision(string data, string key) {
    int keyLen = key.length();

    // Initialize with the data to be divided
    string temp = data.substr(0, keyLen);

    // Perform XOR operations on each segment
    for (int i = keyLen; i < data.length(); i++) {
        if (temp[0] == '1') {  // Only perform XOR if the leading bit is 1
            temp = XOR(temp, key);
        }
        temp = temp.substr(1) + data[i];  // Shift left and add the next bit
    }

    // Perform the final XOR operation
    if (temp[0] == '1') {
        temp = XOR(temp, key);
    }

    // Extract the remainder (last keyLen-1 bits)
    return temp.substr(temp.length() - (keyLen - 1));
}

// Function to check the correctness of received data
bool checkData(string data, string key) {
    string remainder = performDivision(data, key);
    return (remainder.find('1') == string::npos); // No '1' means remainder is all zeros
}

int main() {
    string data, key;

    cout << endl << "Enter data:\t";
    getline(cin, data);
    cout << "Enter primary key:\t";
    getline(cin, key);

    cout<<endl<<"Original data:\t"<<data;
    cout<<endl<<"Key:\t"<<key;
    string messageToSend = encoder(data, key);
    cout<<endl<<"----------------------------------------"<<endl;
    cout<<"Message to be sent:\t"<<messageToSend;
    cout<<endl<<"----------------------------------------"<<endl;

    string receivedData;
    cout<<endl<<"HINT: Received data should be the same as message to be sent.";
    cout<<endl<<"Enter received data:\t";
    getline(cin, receivedData);

    if (receivedData == messageToSend) {
        cout<<"The received data is correct."<<endl;
    } else {
        cout<<"The received data appears to be tampered."<<endl;
    }

    return 0;
}