The Shark Cipher

About the Cipher

The operation of this cipher is outlined in Letter #111 from the First Eric Sanderson (presented on pages 73--76 of The Raw Shark Texts by Steven Hall). To use the cipher in the exact manner outlined in the book, the intended direction needs to be set with the --encrypt or --decipher flags.

An alternate version is run with the default --reversible flag. In this variant, only links where the both output and input contain the other in their surrounding letters are considered. While both versions can, in theory, unambiguously reverse back to cleartext if --fixed is used instead of the default --random, the differing numbers of valid substitutions per keycap means that it would be a notably more complicated process than reversing mod 8 arithmetic.

The ambiguity of both methods is conceptual shark-repellent, not a bug. Run ./cipher.py --help to read about all the various options.

System Requirements

Python 3.8+ with Click installed (see requirements.txt)

Tutorial

We will encode example phrases on a Dvorak keyboard using both the symmetric and original ciphers. Next, we run the ciphertext through the program to show that the letter of the input are in the possibilities of the output. Your exact results will vary due to the random selection of which possibility is chosen.

1. Encode

Note how both commands have a - immediately before the file redirection. That hyphen means that input is read from stdin and allows us to pipe our input.

><)> echo "What you are refering to as Linux…" | ./cipher.py -k Dvorak --only-one - > symmetric.swp
><)> cat symmetric.swp
hmqnxqklzocogkcbcdhqslvdvjf
><)> echo "…is, in fact, GNU+Linux." | ./cipher.py -k Dvorak --only-one --encrypt --include - > original.swp
><)> cat original.swp
…fa, uv ylwh, tvp+rxzik

As you may guess, using --include makes decoding vastly easier by telling us where the spaces and punctuation reside.

2. Decode

Since .swp files were created in step 1, we will now use the files for the TEXT argument in place of the hyphen. As you may note, the ability to accept input from stdin means you can also do such galaxy-brained things such as piping in a cat filename.txt or using < file redirection.

In the following examples, the cleartext has been inserted next to the ciphertext of the output for demonstration purposes.

><)> ./cipher.py -k Dvorak symmetric.swp
hmqnxqklzocogkcbcdhqslvdvjf
whatyouarereferingtoaslinux
---------------------------
wbezpaevrqgqminfgbwzzvchlkd
ttotfzxrlenjdjmghhdenpnxcpy
mdavijupnjhahxwxrxbolarirub
cfzwdepsvawetyhmnicjrntmwem
ghjlbojasqrqcetivggaaslbsoh
dweskayzqevjfugdwyfzvrsfzqi
fgocyzinajmawpvytftezzzgnkg
bcarujevratebirhmmmonvwytpx
---------------------------
whatyouarereferingtoaslinux
hmqnxqklzocogkcbcdhqslvdvjf

If you are bored, check that each letter of the original message exists in the column above it.

><)> ./cipher.py -k Dvorak --decipher --include original.swp
…fa, uv ylwh, tvp+rxzik
…is, in fact, GNU+Linux
-----------------------
…dl, lr bvrw, vzy+tdpxy
…gz, yn fqvc, grx+wkvpl
…ho, jz drmm, csk+cfqbi
…xs, et isnd, nni+nulfe
…iq, xc uahb, wcj+lyndp
…bl, ks kpcf, hte+sbakx
…yz, pw xztg, mll+zisuj
…mo, il pngt, rwu+vpryu
-----------------------
…is, in fact, GNU+Linux
…fa, uv ylwh, tvp+rxzik

Isn't it amazing how much nicer it is to have spaces and punctuation?

Getting things wrong

The exclamation points in the fencing indicate a non-exhaustive list of positions where the cleartext letter is not among the options.

Decoding with the wrong layout

Decoded using the default QWERTY keyboard after being encoded Dvorak.

><)> ./cipher.py symmetric.swp
hmqnxqklzocogkcbcdhqslvdvjf
whatyouarereferingtoaslinux
!---!!--!!-!-!-!!!-!-!!!!-!
jkxbspmisispbirrswjwqifsrig
mhataajpxlxihleyvrmxeaexdht
buzuzxlampektmdvrcbzzorefmv
yisjdwiopkrlnoffxeysapbfcuc
nzwgqzokaivpyjxgfvupxkcwtkd
gypmcsuiwldifuvhestadigcenr
ujxhwpmpqpfkristdfgwcatrbye
tnayeajaskwlvlwnwxnxwodvgib
!---!!--!!-!-!-!!!-!-!!!!-!
whatyouarereferingtoaslinux
hmqnxqklzocogkcbcdhqslvdvjf

• Every instance of o, r, e, i, and n is a mismatch—indicates that the letters are not linked in the same way
• Due to the large difference between Dvorak and QWERTY, there are more mismatches here than a Colemak/QWERTY conflict

Using a mismatching direction option

In this particular case, --encrypt was used a second time rather than --decipher. Using --decipher after --reversible will give similar results. Likewise, a default --reversible will not decode a ciphertext generated with --encrypt.

><)> ./cipher.py -k Dvorak --encrypt --include original.swp
…fa, uv ylwh, tvp+rxzik
…is, in fact, GNU+Linux
----------------!------
…bz, js puhf, wrj+suvpp
…ys, ec bzmg, rss+tklfj
…gl, yt xkrd, hni+nfnui
…mq, pl intt, gcz+ldokx
…ho, iz upvm, nte+vprye
…dz, kr dsgb, vwl+ciaxu
…is, xw kacw, clk+wysdy
…xl, jn fvnc, mzy+zbqbp
----------------!------
…is, in fact, GNU+Linux
…fa, uv ylwh, tvp+rxzik

Note how the only missing letter here is a single instance of u.

Getting --encrypt and --decipher backwards

><)> echo "Hello (good 2B back)!" | ./cipher.py --decipher --only-one - | ./cipher.py --encrypt -
jdooibaixvyxsl
hellogoodbback
----------!---
irikknxmwcndwo
mepimypkaguzeq
uflplhsjsrtwci
hcklofoudfbeqa
nsikutloqbjazz
kwpijvzlcegqxp
yxlpkrwmztmsdk
ivklmgqkedhcao
----------!---
hellogoodbback
jdooibaixvyxsl

Again, only a single b is missing.

Ideas for Extension

These are ideas that you, the user, are free to run with. I currently lack the interest to see these projects through to completion, but they would be valuable additions towards the use of this tool for cracking these ciphers.

• Automate using a dictionary to find words in a ciphered text
• An interactive tool to use a dictionary to find words from ciphertext
• Implement unambiguous reversal of --fixed ciphertexts

Running Tests

./test.py --help will tell you how to test with custom phrases