How do you decrement using modulo in this specific example?

Question

I have this bit of code:

[deleted due to unwanted plagiarism]

This is a simplified version of the Enigma machine used during World War II. The decrypting part of this code does not work well and returns HFFDGRHC instead of HELLOBOY. Would anyone know how to fix this problem?

Assignment: Imitate a Enigma cipher: The twist of the ENIGMA machine was that there were actually 26 different rotating cyphers, and a new cypher was chosen for each letter based on the output of the previous letter. Hint: Add a sum of the previous letter and previous cipher key for encryption. Using this information, figure how to decrypt rather than encrypt.

Define "NOT WORKING WELL" - what are the inputs, the expected output, and the actual output? — Oliver Charlesworth, Nov 25 '17 at 17:21
The inputs are key = 1, text = "IZXVWKLW". The expected output is HELLOBOY, but instead it outputs HFFDGRHC. — ShadowWolf, Nov 25 '17 at 17:25

score 1 · Accepted Answer · answered Nov 25 '17 at 18:28

1

The clue is here:

Add a sum of the previous letter and previous cipher key for encryption.

You were not adding the previous key to the new key.

Change the key rolling line to:

key = (key + newLetter) % 26;

Output is now "HELLOBOY"

answered Nov 25 '17 at 18:28

Bohemian

412,405
93
575
722

Because the 'sum' portion was given as a hint for encryption rather than decryption – ShadowWolf Nov 25 '17 at 18:32
@ShadowWolf Obviously (to me anyway) if the key rolls during encryption it must roll for decryption too. I admit guessing `% 26` part. – Bohemian Nov 25 '17 at 18:34
Ah because it doesn't seem to work with encryption and yields IAUBSKUL instead of IZXVWKLW. But key = newLetter; works perfectly. – ShadowWolf Nov 25 '17 at 18:40

How do you decrement using modulo in this specific example?

1 Answers1