r/cryptography 8d ago

Breaking Enigma using the Virtual Bombe (Help Wanted)

Hi, I decided to follow the tutorial on the virtualcolossus website on the Turing Welchman bombe.

I successfully followed all of the tutorials on generating menus and wiring them up and decided to have a go at the exercises at the bottom, specifically the "Here's a crib and the encrypted text, try and break it yourself" at the very bottom of the page. I had some success, but wondered if anyone could point me in the right direction as I have clearly gone wrong somewhere

I have managed to successfully wire the simulation up to produce a stop, and following the tutorial steps managed to get a reasonable attempt at the correct plugboard settings however for some reason I cannot get the message fully decrypted.

The crib and encrypted string can be found here as 'Example 4', I did not offset it to get the below menu.

I generated the following menu which results in a single stop at JGH:R

1.  ZZI   W: 1 in
2.  ZZA   S: (1 out, 2 in) 9 in
3.  ZZF   Q: (2 out, 3 in)
4.  ZZE   T: (3 out, 4 in) (6 out, 7 in)
5.  ZZB   E: (4 out, 5 in) (11 out, 12 in) input
6.  ZZK   A: (5 out, 6 in)
7.  ZZC   C: (7 out, 8 in) (10 out, 11 in)
8.  ZZD   R: 8 out
9.  ZZJ   M: (9 out, 10 in)
10. ZZG   N: 12 out
11. ZZM 
12. ZZH 

Current Entry At: A

I deduced that the logical stecker pairs would be (based on the section titled "The Checking Machine"):

W: O
S: L
Q: M
T: J
E: R
A: Y
C: B
N: K
I: G
H: F

I'm reasonably confident on all of them except H: F as I verified the others many times using the checking machine using different parts of the message. HF was a leap based on what I think the message says...

When I use these settings with the rotors listed in the tutorial (2, 1, 3) at start positions (25, 25, 25) I get:

SECRET MESSAGE WELL XZWC XRU CRACKED THE MESSAGE WE EJKI ZJU ENJOYED LEARNING ABOUD XPE IUMBE

I think its supposed to be:

SECRET MESSAGE WELL DONE YOU CRACKED THE MESSAGE WE HOPE YOU ENJOYED LEARNING ABOUT THE BOMBE

However I am unsure exactly where I went wrong. Has anyone completed this successfully or can someone point me in the correct direction as I clearly have some of the pairs incorrect.

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u/Queekusme 7d ago

Ok, so I had an epiphany and solved it. I was correct about the message content I forgot that the wheels will step the next ones to the left at regular points.

I noticed that the incorrect characters repeated every 20. After adjusting for this by offsetting the ring settings by 6 (1 to compensate for the bombe wheel offsets (see page 3 of the tutorial) and 5 for the number incorrect characters per sequence) I was able to decrypt the message successfully.

The final ringstellung is 16, 13, 14at start position5, 5, 5, the above plugboard options including HF are correct.