Enigma

Cryptanalysis has advanced a long way.

Can you break Enigma without a known plaintext?

About this machine

The Enigma was a collection of cryptographic ciphering machines used by Germany and its allies prior to and during WWII.

This directory contains C++ code that emulates one particular Enigma machine: a 1939 M3 Enigma machine, with 3 rotor slots, rotor choices I - VIII (including the 3 Naval rotors), reflector B, and 10 plugboard pairs.

Message Format

All encrypted messages in this challenge are written in German.

Non-ASCII Letters

The Enigma only supports the 26 ASCII capital letters - it does not support ß, umlauts, punctuation, or spaces.

To get around this, German operators would encode messages using substitutes.

Letters

• Everything is capitalized.
• Umlaut vowels are replaced by their non-umlaut form, followed by an E. For example, the letter Ä is written AE.
• The letter ß is written SS.

Punctuation

• . is written X.
• , is written Y.
• -, —, /, and \ are all written YY.
• ? is written UD.
• : is written XX.
• Quotes (', ") are written J. So, "FOO" is written JFOOJ. This can also be used to stress text.
• Parentheses and other forms of brackets are written KK. So, (FOO) is written KKFOOKK.
• Spaces are completely skipped.

This encoding is obviously lossy, making deciphering messages rather annoying.

Message Key

In wartime use, Germany and its allies used both a fixed daily key and a per-message key. The first few characters of a message were encoded using the fixed daily key and specified the rotor positions that would be used for the rest of the message. Only the rotor positions were changed per-message; all other settings were too difficult to adjust and therefore only changed daily.

Due to a lack of available information on the encoding of the message key at the start of each message, and the fact that it is irrelevant to the cryptanalysis of a single message, separate message keys are not included in this challenge. All messages include only the message itself encrypted with the specified key.

Fernet files

We understand that trying to parse ICHS AGEH IERM ITJE INZI TATJ X into ICH SAGE HIERMIT "EIN ZITAT". is a task that depends greatly on whether your team has a member who speaks German. Without one, it would be difficult even after your team has broken the Enigma. Our goal with this challenge is not to test your command of German - Bletchley Park would have simply employed German speakers for that purpose. Therefore, we have provided the original messages encrypted with the modern Fernet cipher as well, which supports all the detailed punctuation we could need. The key is the rotor/ring/plugboard settings in the exact same format as used by machine.cc.

Example:

./encrypt_modern.py decrypt $(cat messages/may_09_2022/settings) < messages/may_09_2022/comet.fernet.txt > may_09_original

The file may_09_original will be byte-for-byte identical with messages/may_09_2022/comet.original.txt.

Directory Layout

enigma

This directory contains the code for the machine itself. Makefile and Bazel BUILD rules are provided. enigma.h provides a library implementation; machine.cc provides a command-line interface.

encode.py

This is a simple utility script for cleaning up text and formatting with only the 26 ASCII capital letters.

messages

For testing, we provide a number of previously-decrypted sample messages (in messages/may_[0-9]{2}_2022. Files are as follows:

• .original.txt: The original message to be encrypted.
• .plaintext.txt: The same message, formatted for Enigma (via encode.py).
• .ciphertext.txt: The ciphertext after being encrypted via the Enigma machine.
• settings: The settings used for that ciphertext.
• .fernet.txt: The original message, encoded with the Fernet cipher (via encrypt_modern.py).

In wartime, there would often be multiple messages that discussed the same topic, and breaking any message would leak sensitive information. That pattern happens here: there are two messages with the flag. (messages/may_12_2022 and messages/may_14_2022). They are not the same message, but they both describe the flag. You only need to break one of them.

encrypt_modern.py

Encrypts or decrypts a message using Fernet. The key is passed as a series of space-separated command-line arguments. Usage is:

./encrypt_modern.py [encrypt|decrypt] {key pieces} < input > output

Constraints

While an Enigma machine could accept multiple copies of the same rotor, each machine was only shipped with one copy of each. Therefore, no key includes a duplicated rotor.

Germany generated its codebooks by rolling dice. However, it enforced reules meant to ensure there was "enough variation" between days. If those rules were violated, dice would be rerolled. Those rules are: - Every day must include at least one "naval rotor" (rotors VI-VIII). - The same naval rotor cannot be used in the same position on two consecutive days.

These constraints apply in this challenge. While certain other randomness constraints were applied in some divisions during certain time periods, only the restrictions mentioned here or enforced by machine.cc apply to this challenge. Keys are otherwise random.