Program to perform a letter frequency attack on a monoalphabetic substitution cipher

#include <bits/stdc++.h>

using namespace std;

  

void printString(string S, int N)

{

  

    

    

    string plaintext[5];

  

    

    

    int freq[26] = { 0 };

  

    

    

    int freqSorted[26];

  

    

    int Used[26] = { 0 };

  

    

    for (int i = 0; i < N; i++) {

        if (S[i] != ' ') {

            freq[S[i] - 'A']++;

        }

    }

  

    

    for (int i = 0; i < 26; i++) {

        freqSorted[i] = freq[i];

    }

  

    

    

    

    string T = "ETAOINSHRDLCUMWFGYPBVKJXQZ";

  

    

    sort(freqSorted, freqSorted + 26, greater<int>());

  

    

    for (int i = 0; i < 5; i++) {

  

        int ch = -1;

  

        

        for (int j = 0; j < 26; j++) {

  

            if (freqSorted[i] == freq[j] && Used[j] == 0) {

                Used[j] = 1;

                ch = j;

                break;

            }

        }

        if (ch == -1)

            break;

  

        

        

        int x = T[i] - 'A';

  

        

        

        x = x - ch;

  

        

        

        string curr = "";

  

        

        

        for (int k = 0; k < N; k++) {

  

            

            if (S[k] == ' ') {

                curr += ' ';

                continue;

            }

  

            

            

            int y = S[k] - 'A';

            y += x;

  

            if (y < 0)

                y += 26;

            if (y > 25)

                y -= 26;

  

            

            

            curr += 'A' + y;

        }

  

        plaintext[i] = curr;

    }

  

    

    for (int i = 0; i < 5; i++) {

        cout << plaintext[i] << endl;

    }

}

  

int main()

{

    

    string S = "B TJNQMF NFTTBHF";

    int N = S.length();

  

    

    printString(S, N);

  

    return 0;

}

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