Question

Jack is a successful lawyer who runs a law firm that deals with sensitive cases. In 2017, after the widely known ransomware attacks, their data was compromised. As a protective measure, he requested that all employees must encrypt their messages to each other. Employees generally exchange instructions or case updates with messages that are no longer than thirty characters each. For the purpose of hiding the meaning of the messages, they were told to encrypt them using Caesar cipher substitution then using another substitution where the key is 567. Once the message is processed through the aforementioned methods, they added an extra layer of security by encrypting the message with One Time Pad that increments by one each time it’s used but remains less or equals to 15 for encryption and decryption. For one particular message between two employees, that key was: 7,15,12,6,8,9,4,2,1,13,12,5,3,1,8,15,6,4,8,12,8,10,9,14,6,11,13,2,4,6 When the receiver received the message, he/she received the following ciphertext: LC DOMX IZY XVHP XMJQSH AANW FIHABRT What is the plaintext?

Answer 1

//Decryption for caeser cipher

#include <iostream>

#include <cstring>

using namespace std;

int main()

{

    int key;

    char msg[100],cur;

    cout<<"Enter Key";

    cin>>key;

    cout<<"Enter msg";

    cin>>msg;

    for(int i=0;i<strlen(msg);i++)

    {

        cur=msg[i];

        if(cur>='A'&&cur<='Z')

        {

            cur=cur-key;

            if(cur<'A')

            {

                cur=cur+'Z'-'A'+1;

            }

            msg[i]=cur;

        }

        if(cur>='a'&&cur<='z')

        {

            cur=cur-key;

            if(cur<'a')

            {

                cur=cur+'z'-'a'+1;

            }

            msg[i]=cur;

        }

        if(cur>='0'&&cur<='9')

        {

            cur=cur-key;

            if(cur<'0')

            {

                cur=cur+'9'-'0'+1;

            }

            msg[i]=cur;

        }

    }

    cout<<"Decrypted message"<<msg;

    return 0;

}

//Substitution Cipher

#include<stdlib.h>
#include<string.h>
#include<ctype.h>
#include<stdbool.h>
#include<stdio.h>

bool Cipher(char* str, char* plainAlphabet, char* key, char* cipherText)
{
   int strLen = strlen(str);

   if (strlen(plainAlphabet) != strlen(key))
       return false;

   for (int i = 0; i < strLen; ++i)
   {
       const char* ptr = strchr(plainAlphabet, tolower(str[i]));
       int oldCharIndex = ptr - plainAlphabet;
  
       if (ptr && oldCharIndex >= 0)
           cipherText[i] = isupper(str[i]) ? toupper(key[oldCharIndex]) : key[oldCharIndex];
       else
           cipherText[i] = str[i];
   }

   cipherText[strLen] = '\0';
   return true;
}

bool Encipher(char* str, char* key, char* cipherText)
{
   char* plainAlphabet = "abcdefghijklmnopqrstuvwxyz";
   return Cipher(str, plainAlphabet, key, cipherText);
}

bool Decipher(char* str, char* key, char* cipherText)
{
   char* plainAlphabet = "abcdefghijklmnopqrstuvwxyz";
   return Cipher(str, key, plainAlphabet, cipherText);
}

int main()
{
char* str[100];
char* key[26];
printf("Enter text:\t");
scanf("%s",str);
printf("Key:\t");
scanf("%s",key);
char* cipherText = (char*)malloc(strlen(str));
char* originalText = (char*)malloc(strlen(cipherText));

bool encipherResult = Encipher(str, key, cipherText);
bool decipherResult = Decipher(cipherText, key, originalText);
printf("\n%s",originalText);
printf("\n%s",cipherText);

return 0;
}