文字頻度攻撃のための単一文字置換暗号プログラム

String T=ETAOINSHRDLCUMWFGYPBVKJXQZ”
Given string Str = "SGHR HR SGD BNCD";

Output:
THIS IS THE CODE
FTUE UE FTQ OAPQ
LZAK AK LZW UGVW
PDEO EO PDA YKZA
IWXH XH IWT  RDST

#include <stdio.h>
#include <string.h>
// Define a function to decrypt given monoalphabetic substitution cipher by implementing the method of frequency analysis or an attack
void printTheString(char Str[], int K){

   // this stores the final 5 feasible plaintext //which are deciphered
   char ptext[5][K+1];
   
   // the frequency of every letter in the
   // cipher text is stored
   int fre[26] = { 0 }; 
   
   // The letter frequency of the cipher text is stored in the order of descendence
   int freSorted[26]; 
   
   // this stores the used alphabet 
   int Used[26] = { 0 }; 
   
   // Traversing the given string named Str
   for (int i = 0; i < K; i++) {
      if (Str[i] != ' ') {
         fre[Str[i] - 'A']++;
      }
   } 
   
   // Copying the array of frequency
   for (int i = 0; i < 26; i++) {
      freSorted[i] = fre[i];
   } 
   
   //by concatenating the english letters in //decreasing frequency in the english alphabet , the string T is //obtained
   char T[] = "ETAOINSHRDLCUMWFGYPBVKJXQZ"; 
   
   // Sorting the array in the order of descendence
   for (int i = 0; i < 26; i++) {
      for (int j = i + 1; j < 26; j++) {
         if (freSorted[j] > freSorted[i]) {
            int temp = freSorted[i];
            freSorted[i] = freSorted[j];
            freSorted[j] = temp;
         }
      }
   } 
   
   // Iterating in the range between [0, 5]
   for (int i = 0; i < 5; i++) {
      int ch = -1; 
      
      // Iterating in the range between [0, 26]
      for (int m = 0; m < 26; m++) { 
         if (freSorted[i] == fre[m] && Used[m] == 0) {
            Used[m] = 1;
            ch = m;
            break;
         }
      }
      if (ch == -1)
         break; 
         
      //  here numerical equivalent of letter is stored ith index of array letter_frequency
      int x = T[i] - 'A'; 
      
      //  now probable shift is calculated in the monoalphabetic cipher
      x = x - ch; 
      
      // defining a temporary string cur to create one plaintext at a time or at the current time
      char cur[K+1]; 
      
      //  ith plaintext is generated by making use of the shift calculated 
      for (int T = 0; T < K; T++) { 
      
         // whitespaces is inserted without any //change
         if (Str[T] == ' ') {
            cur[T] = ' ';
            continue;
         } 
         
         // Shifting the Tth cipher letter by x we get
         int y = Str[T] - 'A';
         y =y+x; 
         if (y < 0)
            y =y+ 26;
         if (y > 25)
            y -=26; 
            
         // Adding the kth calculated letter to the temporary string cur 
         cur[T] = 'A' + y;        
      }
      cur[K] = '\0';
      
      // The ith feasible plaintext is printed
      printf("%s\n", cur);
   }
}
int main(){
   char Str[] = "SGHR HR SGD BNCD";
   int K = strlen(Str);
   printTheString(Str, K);
   return 0;
}

THIS IS THE CODE
FTUE UE FTQ OAPQ
LZAK AK LZW UGVW
PDEO EO PDA YKZA
IWXH XH IWT RDST