[Practice Puzzle] Encryption/Decryption of Enigma Machine

Puzzles
#1

I have a problem reversing the Caesar cipher for this puzzle. My code passes all encoding tests but fails at decoding. I suspect there is an error at the last step where I try to decrypt the message. My logic is:

  • Find the index of the current letter of the encrypted message in the alphabet (eg. A = 0, B = 1, etc.)

  • Subtract the same offset and shift that would have been added to the original letter to produce the current one

  • If the result is negative that means the letter was produced via modulo 26, i.e. it exceeded the maximum index of ‘Z’, thus I add 26 to counteract the circular flow

  • Finally I add 65 to convert the index to the actual letter it represents.

      private static void encCeasar() {
          int offset = 0;
          char[] result = msg.toCharArray();
          for (int i = 0; i < msg.length(); i++) {
              result[i] = (char)((msg.charAt(i) + rng + offset - 65) % 26 + 65);
              offset++;
          }
          msg = String.valueOf(result);
      }
  
  private static void decCeasar() {
      int offset = 0;
      char[] result = msg.toCharArray();
      for (int i = 0; i < msg.length(); i++) {
          int index = (msg.charAt(i) - rng - offset - 65);
          while (index < 0) {
              index += 26;    
          }
          result[i] = (char)(index + 65);
          offset++;
      }
      msg = String.valueOf(result);
  }
#2

Please note that there is already a long thread about this puzzle.

#3

Hello,

There is a problem with the starter code (aka auto generated code) for C, it does not read in the input properly.
The line:
fgets(rotor, 27, stdin);
it should be
fgets(rotor, 27, stdin); fgetc(stdin);

Can this be corrected?

1 Like
Problem with C stub in contributions
#4

First of all, you probably should have posted in the other thread about this puzzle that I’ve already mentioned above (EDIT: actually you did, I assumed you didn’t because you had also posted here). And this thread should probably be closed (let’s invoke @TwoSteps).

That said, this issue is a known problem of about the stub generator that has not yet been solved by CG.
However, I fixed it manually by increasing the size of the expected string (now you’ll get fgets(rotor, 28, stdin); which is large enough to parse the 26 chars + "\n\0").

1 Like
#5

C stub issue thread

Enigma puzzle thread

closing this one, thanks @Niako

#6