Decrypt plaintext by letter frequencies

freqdecrypt.cpp

#include <iostream>
#include <fstream>
#include <string> 
#include <ctype.h>
 
using namespace std;
 
char freq[26] =        {'E', 'T', 'A', 'O', 'I', 'N', 'S', 'H', 
                        'R', 'D', 'L', 'C', 'U', 'M', 'W', 'F',
                        'G', 'Y', 'P', 'B', 'V', 'K', 'J', 'X',
                        'Q', 'Z'};
 
string getKey();
string getEncryptedMsg();
string getDecryptedMsg(int *key, string enMsg);
string getDecryptedMsg(string key, string enMsg);
int getScore(string deMsg);
int* initKey(int *msgFreq);
void initGuess(int* key, string enMsgStr);
void computeMsgFreq(string enMsgStr, int* msgFreq);
void printMsgFreq(string enMsgStr, int* msgFreq);
string getAlphaKey(int* key);
 
int getScore(string deMsg)
{
    string word;
    int score = 0;
    ifstream file;
    file.open("dictionary.txt");
 
    while(!file.eof())
    {
      getline(file, word);
      if (deMsg.find(word) != string::npos)
      {
        score++;
      } 
    }
    file.close();
    return score;
}
 
string getDecryptedMsg(int *key, string enMsg)
{
  string deMsg = "";
  char msgArray[enMsg.size()];
  strcpy(msgArray, enMsg.c_str());
 
  for(int i = 0; i < enMsg.length(); i++)
  {
    deMsg.push_back(freq[key[toupper(msgArray[i]) - 'A']]);
  }
  return deMsg;
}
 
string getDecryptedMsg(string key, string enMsg)
{
  string deMsg = "";
  char msgArray[enMsg.size()];
  strcpy(msgArray, enMsg.c_str());
 
  for(int i = 0; i < enMsg.length(); i++)
  {
    deMsg.push_back(key[toupper(msgArray[i]) - 'A']);
  }
  return deMsg;
}
 
void initGuess(int* key, string enMsgStr)
{
  int tempScore;
  char tempChar;
  int inc = 1;
  int score = getScore(getDecryptedMsg(key, enMsgStr));
  for(int i = 0; i < 26; i++)
  {
    if(i+inc <= 25)
    {
      tempChar = freq[i];
      freq[i] = freq[i+inc];
      freq[i+inc] = tempChar;
      tempScore = getScore(getDecryptedMsg(key, enMsgStr));
      if(score >= tempScore)
      {
        tempChar = freq[i];
        freq[i] = freq[i+inc];
        freq[i+inc] = tempChar;
      }
      else
      {
        score = tempScore;
      }
    }
  }
}
 
int* initKey(int *msgFreq)
{
  int *key = new int[26];
  int j;
  for(j = 0; j < 26; j ++)
  {
    int max = -1, indexOfMax = -1;
    int i;
    for(i = 0; i < 26; i++)
    {
      if(msgFreq[i] > max)
      {
        max = msgFreq[i];
        indexOfMax = i;
      }
    }
    msgFreq[indexOfMax] = -1;
    key[indexOfMax] = j;
  }
  return key;
}
 
string getKey()
{
  string key = "";
  string upperKey = "";
  cout << "\nEnter key (or 'quit' to quit):\n " << endl; 
  getline(cin, key);
  for(int i = 0; i < key.size(); i++)
  {
    upperKey += toupper(key[i]);
  }
  return upperKey;
}
 
string getEncryptedMsg()
{
    string msg, str;
 
    ifstream file;
    file.open("encrypted_message.txt");
 
    while(!file.eof())
    {
      getline(file, str);
      str = str.substr(0, str.length()-1);
      msg.append(str);
    }
    file.close();
    return msg;
}
 
void computeMsgFreq(string enMsgStr, int* msgFreq)
{
  char enMsgArray[enMsgStr.size()];
  strcpy(enMsgArray, enMsgStr.c_str());
 
  for(int i = 0; i < enMsgStr.length(); i++)
  {
    msgFreq[toupper(enMsgArray[i]) - 'A']++;
  }
}
 
void printMsgFreq(string enMsgStr, int* msgFreq)
{
  cout << "\nOriginal Encrypted Message Frequencies:" << endl;
  for(int i = 0; i < 26; i++)
  {
    cout << char(toupper(i) + 'A') << ": " 
         << ((msgFreq[i] * 1.0  / enMsgStr.length() * 1.0 ) * 100.0) 
         << '%' << endl;
  }
}
 
string getAlphaKey(int* key)
{
  string str = "";
  for(int i = 0; i < 26; i++)
  {
    str.push_back(freq[key[i]]);
  }
  return str;
}
 
int main()
{
 
  string enMsgStr = getEncryptedMsg();
  int msgFreq[26] = {0};
  computeMsgFreq(enMsgStr, msgFreq);
  printMsgFreq(enMsgStr, msgFreq);
  int *key = initKey(msgFreq);
 
  cout << "\nDecrypting message...\n" << endl;
  initGuess(key, enMsgStr);
 
  cout << "Decrypted Message (using Key - " << getAlphaKey(key) 
       << "):\n" << getDecryptedMsg(key, enMsgStr) << endl;
 
  bool running = true;
  while(running)
  {
    string keyStr = getKey();
    if(keyStr != "QUIT")
    {
      cout << "Decrypted Message (using Key - " << keyStr << "):\n" 
           << getDecryptedMsg(keyStr, enMsgStr) << endl;
    }
    else
    {
      running = false;
    }
  } 
  return 0;
}