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bomm

bomm, initiated as part of the Bachelor thesis Breaking Enigma ciphertext in 2023 by Fränz Friederes, is a C-based command-line program designed to break Enigma ciphertext in a ciphertext-only scenario.

The thesis script will be made available, soon.

Getting started

First, download the latest stable release or build the program from source (see instructions below).

Command line usage:

Usage: bomm data/queries/kr-blitz.json
Options:
  -h, --help        display this help message
  -n, --num-hold    number of hold elements to collect
  -t, --num-threads number of concurrent threads to use
  -q, --quiet       quiet mode
  -v, --verbose     verbose mode

To evaluate a ciphertext messages with bomm, a query needs to be composed and passed as the only argument. It contains the ciphertext itself, the key space to be searched (referencing known or custom wheels and wirings), and a set of passes that describe the strategies (e.g. hill climbing) to be applied. A schema for such query files can be found at data/schemas/query.json. Example queries are stored in data/queries.

Exemplary, the following command and query can be used to run an attack against the KR Blitz message, targeting the practical key space of Enigma I with UKW-B using the E-Stecker technique.

bomm data/queries/kr-blitz.json

This will result in the following view that is kept up-to-date while processing the query, displaying the top scoring keys accompanied by a preview of the putative plaintext:

┌──────┬───────────────────────────────────────────────────────────────────────┐
│ Bomm │ Progress      0.228 % │ Elapsed      00:03:44 │ Remaining    26:06:54 │
├──────┴───────────────────────────────────────────────────────────┬───────────┤
│ tgenpjnrjdqubenahrbefohsamtjnenexsiesgalieldninsnptcqssenidfqxan   -8.702661 │
│ UKW-B,III,IV,V aaaa aber bf ch dt in kv or ps qz wy                          │
├──────────────────────────────────────────────────────────────────┬───────────┤
│ rlverarmxplinsxjerpisjdsixfeheiarvifuetentxschxefojaxungthxeutha   -8.737875 │
│ UKW-B,II,III,V aaaa abjt bz cy dp fv hi jm lt ou qr                          │
├──────────────────────────────────────────────────────────────────┬───────────┤
│ rderdsjxsseliseilunidpbenidxzhlqhtbeqnafkorprexrgvniniweantbenix   -8.739881 │
│ UKW-B,V,II,IV aaaa aadj ad bj cq es fo iy kt lr mz nx pw uv                  │
├──────────────────────────────────────────────────────────────────┬───────────┤
│ eciemxkztpyxejhmlerkxdiesierregsieigfaratewfgarkyhydjzbnhoilonss   -8.745330 │
│ UKW-B,II,III,V aaaa aapy ad bt ek fq gu ir jy lw ns oz pv                    │
├──────────────────────────────────────────────────────────────────┬───────────┤
│ rlgsigezeodnfstaurmitgensexonfcsienenssgewapxximnrkxsonsaangreit   -8.756366 │
│ UKW-B,V,III,IV aaaa aaxo am do ip lz nw qt rx vy                             │
├──────────────────────────────────────────────────────────────────┬───────────┤
│ tgenpjnrjdqubenahrbefobsartjnenexsiesialieldninsnptqqssenuqfqxan   -8.762827 │
│ UKW-B,III,IV,V aaaa aber bf ch dt in or ps qz vy                             │
╞══════════════════════════════════════════════════════════════════╤═══════════╡
│ hitlutsunqrtliabftqrnuwlqvnitrsctnqipklmaheefdcabcuworubswyrbggf   -10.09713 │
│ Unchanged ciphertext (97 letters)                                            │
└──────────────────────────────────────────────────────────────────────────────┘

By default, half the number of detected CPU cores is used as the number of parallel threads spawned. This may be overridden by the -t flag.

When the query completes or the program is terminated by the SIGINT (pressing Ctrl+C) or SIGTERM signal, the full hold is printed out before exiting.

Wheels

In a query key space, wheels can be referenced by their name. The following options are available:

  • Entry wheels (all models)
    ETW-ABC, ETW-QWE
  • Enigma I, M1, M2, M3, and Enigma M4
    I, II, III, IV, V, VI, VII, VIII, beta, gamma, UKW-A, UKW-B, UKW-C, UKW-B-thin UKW-C-thin
  • Enigma B 207
    B207-I, B207-II, B207-III, B207-UKW
  • Enigma S “Sondermaschine”
    S-I, S-II, S-III, S-UKW
  • “Spanish Enigma”
    ES-I, ES-II, ES-III, ES-IV, ES-V, ES-UKW
  • Enigma “Zagreb Delta”
    ZD-I, ZD-II, ZD-III, ZD-IV, ZD-V, ZD-UKW

If a wheel is missing in the built-in library, it can be specified in the wheels array in the query object:

{
  "name": "B207-I",
  "wiring": "iqynwgavkozscpfbumlextjdhr",
  "turnovers": "q"
}

Measures

In the query, built-in measures (or scoring functions) are referenced by string tokens like ic or sinkov_trigram. The following options are available:

  • Sinkov statistic or log-weight statistic for n-grams; Requires a frequency map
    Available options: sinkov_monogram, sinkov_bigram, sinkov_trigram, sinkov_quadgram, sinkov_pentagram, sinkov_hexagram
  • Index of coincidence measure
    Available options: ic, ic_bigram, ic_trigram, ic_quadgram, ic_pentagram, ic_hexagram
  • Shannon entropy measure
    Available options: entropy, entropy_bigram, entropy_trigram, entropy_quadgram, entropy_pentagram, entropy_hexagram

Build from source

First, make sure the libraries jansson and criterion are available to the compiler.

Build and test the program using make:

make clean && make test
# Creates build/bomm

A non-Latin alphabet may be used with an especially compiled version of the program:

make clean && make test ALPHABET=0123456789
# Creates build/bomm-0123456789

Note, that as of now, only path-safe ASCII alphabets are allowed here.

Data sources

  • The frequency tables for English text en-bigram.txt and en-trigram.txt were generated by Practical Cryptography from around 4.5 billion characters of English text, sourced from Wortschatz.
  • The frequency tables for raw 1941 Enigma message decrypts enigma1941-bigram.txt and enigma1941-trigram.txt have been published on Frode Weierud's CryptoCellar.

License

The source code in this repository is published under the MIT license. See LICENSE.txt.