From 960edafd0fda5c47249cf20a0da9f9ca90455fcc Mon Sep 17 00:00:00 2001
From: Kshitij <kshitijk@tuta.io>
Date: Sun, 11 Aug 2024 21:56:51 +0530
Subject: [PATCH] Merged CRC, i.e. Code-A4 from testing branch.

---
 Codes/Code-A4.cpp | 102 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)
 create mode 100644 Codes/Code-A4.cpp

diff --git a/Codes/Code-A4.cpp b/Codes/Code-A4.cpp
new file mode 100644
index 0000000..c8bb556
--- /dev/null
+++ b/Codes/Code-A4.cpp
@@ -0,0 +1,102 @@
+#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;
+}