Shift Cipher without knowing the key in c-CodePudding

I am writing a program to solve a random code.(the code will not have capital letters,any form of signs like full stop) The code is using shift cipher without knowing the key,it can also encrypt code but i will work on that one later.For now i am trying to output all 25 result first.

However if i enter things like "back to back",it will output a question mark at the end.

Here's my code

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

int main() {
  char code[400];
  char after[400];  // variable initialization
  int i, x, choice, z, temp, y;
  printf("                     Welcome\n");
  printf("Enter your code here:\n");
  scanf("%[^'\n']s", code);
  printf("Press 1 to Decrypt\nPress 2 to encrypt\nPress 3 to Exit\n");
  scanf("%d", &choice);

  // case 1 is decryption
  switch (choice) {
    case 1:
      for (x = 1; x <= 25; x  ) {
        for (i = 0; (i <= 400 && code[i] != 0); i  )  // for loop
        {
          if (code[i] == 32) {  // 32 means space in ascii code
            after[i] = 32;
            continue;  // if there is a space it will continue
          }
          if (code[i]   x > 122) {  // 122 is z in ascii code
            after[i] = code[i]   x;  // if it is higher than 'z'
            after[i] = (after[i] - 122)   96;  // it start counting form 'a'
            continue;
          } else {
            after[i] = code[i]   x;
            // if nothing is wrong,it starting adding the key
          }
        }
        printf("After decrypting:%s\n", after);
        printf("%s\n", code);
      }
      break;

    // case 2 is ecnrypt so its not important
    case 2:
      x = 5;
      for (i = 0; (i <= 400 && code[i] != 0); i  )  // for loop
      {
        if (code[i] == ' ') {
          continue;
        }
        if (code[i]   x > 122) {
          code[i] = code[i]   x;
          code[i] = (code[i] - 122)   96;
          continue;
        } else {
          code[i] = code[i]   x;
        }
      }
      printf("After decrypting:%s", code);
      break;

    // not important as well
    case 3:
      printf("GOODBYE");
      break;

    // not important as well
    default:
      printf("Errors");
      return 0;
  }
}

CodePudding user response：

To solve a shift cipher all you need is a dictionary and the first ten words or so of your ciphertext. Try all possibilities and test each “decoded” plaintext word against your dictionary. Choose the cipher with the highest match rate.

CodePudding user response：

You forgot to null terminate after.

Perhaps you should do:

    for (i = 0; (i <= 400 && code[i] != 0); i  ) {
      if (code[i] == 32) {
        after[i] = ...
      } else if (code[i]   x > 122) {
        after[i] = ...
      } else {
        after[i] = ...
      }
    }
    after[i] = '\0'; // <-- This line is missing.
    printf("After decrypting:%s\n", after);
    printf("%s\n", code);