# Breaking Gnomon Cipher [closed]

[First time posting so apologies if this is an inappropriate question for this site.]

I recently finished reading the book “Gnomon” by Nick Harkaway. At the start of the book there’s a set of numbers that represent a code of some sort.

The code itself is - " DCAC:/ 3455 6671 1643 2776 6655 5443 2147 7654 5667 7122 7543 1177 7666 5543 2511 7656 7711 2331 6542 2111 7776 6543 6221 7671 1223 4427 6533 2221 7671 1223 4427 6533 2221 7671 1223 4427 6533 2221 1177 6547 3321 7122 3345 5317 6443 3322 2117 6514 4322 3445 5677 5321 6655 […] "

The author posted some hints on Twitter 4 years ago they are listed below:

1. who am I? I have zero skills in this area [computer based cryptography]; I intend this to be breakable; I did not intend that you should need a supercomputer…
2. What’s the book about? What kind of book is it?
3. Who did Bletchley Park recruit apart from mathematicians?
4. To solve this you need the book; a piece of paper and a pencil; access to one document from the library or the internet; to treat the problem like a puzzle as much as a code :)

He also writes that the process of breaking the code will require an intuitive leap. After that he suspects that decryption will be straightforward.

One person who has cracked it wrote: “it’s not cryptographically sophisticated. It’s more of a sudden realization. The encryption is lossy to a degree, but that doesn’t matter in a way. You could start by looking what’s there and what isn’t”

Based on all this I’d guess that the code is a book cipher that uses another book as the key (no idea what book this might be), where the first two digits of each quadruple are the page number and the second two numbers are the nth word (based on the repetition of some the quadruples e.g. 4427, possibly representing a repeated word).

The book contains several themes/repeating motifs (I’m unsure as to whether any of these may be relevant to cracking the code, but thought I would supply them to save you from having to read the book): consciousness, reality, non-linear narrative, computer attacks where the attack code is broken up into components and then reassembled, surveillance, fire judges, firespine, detectives/hunters, the number 4, sharks, the game “Go”, the chamber of isis, catabasis, alchemy and possibly a key word “Activation”.

There are more details on this Reddit page here. Along with screenshots of the author’s tweets.

Any help would be greatly appreciated— no idea how to crack this otherwise.

• I’m voting to close this question because there is "don't post the answer, just a hint" wording in a couple of the posted answers. The whole point of stackexchange is to post complete self-contained answers to questions. If the book's author doesn't want answers posted, this question should not be here.
– fljx
Commented Apr 20, 2023 at 10:14
• @fljx the author never stated anywhere that they don’t want the answer posted, and in fact said they would provide answers privately via Twitter at request. Stack exchange allows you to post answers with “spoiler sections” so I don’t see the issue with leaving the question as is. Commented Apr 20, 2023 at 13:37
• I know nothing about the source material or the author. I'm just going from the last paragraph in enkorvaks' answer below ("...please DO NOT post the answer..."), and a similar comment in another (now deleted) answer. If that isn't in line with the authors wishes, what's up with those comments?
– fljx
Commented Apr 20, 2023 at 14:39
• @fljx I’m not sure why other posters may have specified that they don’t want the full answer posted— you’ll note that they have no indication that their reasoning had anything to do with the author’s desires. Regardless, the question shouldn’t be penalized because some posters don’t want a full answer. If I had to speculate on their motives, maybe they enjoy the idea of continuing a mystery that has no publicly posted solution for another several years? Commented Apr 20, 2023 at 18:59

Partial answer. I don't think it is a book code, because it is too repetitive. In fact, if I take the difference between each adjacent pair of digits (modulo 7), it becomes extremely repetitive. Here, I've added spaces and line breaks to align the repetitions:

1101011055665060060060666330666
1101011055665060060060666330666
1101011055665060060060666330666
11010110556650600           666
11010110556650600           666
1101011055665060060060666330666
1101011055665060060060666330660
1101011055665060


When arranged in the same layout, the original numbers show ascending sequences going down the columns:

3455667116432776665554432147765
4566771227543117776665543251176
5677112331654221117776654362217
67112234427653322           217
67112234427653322           217
6711223442765332221117765473321
7122334553176443332221176514432
23445567753216655


...but I'm not sure where to look next, and I haven't even read the novel.

• Could this represent a musical fugue? (Just the fact that the pattern repeats itself but with an offset?) the book does make a reference to Bach’s fugue a “Musical Offering” Commented Mar 9, 2023 at 15:38
• I was thinking about how the information content of this string can't be very much. Since the following lines are only +0/+1/+2 shifts of the previous line, they can't encode more information than the shift amount. So there is the info in the digits of the first line, plus the shift amounts for each following line, plus the locations of the gaps in two of the lines. I suspect it isn't a cipher at all, and is more like an exercise in pattern recognition. There's no hidden message: either you know what the pattern refers to, or you don't. Commented Mar 12, 2023 at 23:17
• interesting! 2 thoughts: (1) the code could still work with another source (like a book cipher) provided the author had written the other book as well (since they could control the order of the word order). This would be supported by hints 1 and 4 since the author is an author, and he’s saying you need another book (or a documebt from a library) to crack it. (2) could the product of the modulo 7 output be the answer or the first part of the answer itself? Changing the 5’s and 6’s to 0’s and 1’s? Producing a binary string? Commented Mar 13, 2023 at 13:52
• I can only comment on the digit string itself. If it is in fact a book cipher, whatever you get after applying the book cipher will be just as repetitive as the digits themselves. If the differences modulo 7 are part of the solution, that reduces the amount of information, since it throws away the shifts. Converting to binary reduces it even more on top of that. Commented Mar 13, 2023 at 14:03

The answer to Q3 is Chess players, and (cryptic) crossword enthusiasts.

Does anyone have an idea how the code might be interpreted if it was describing a chess game?

I haven't read the book, so don't know what it is about, but the word "Gnomon" is the part of the sundial which casts the shadow. I am unaware if that has any relevance to the content of the book or the code.

Given Bletchly park hired chess players (part of the hint), and "Go" features, it might be a description of a "Go" game, rather than a chess game.

The comment about being lossy, though, means that something might be missing (or may be interpreted in one of several ways). The numbers range from 1 to 7, no 0, no 8, so chess may not be the key (since that goes up to 8 rows and columns).

And of course, please DO NOT post the answer if you find it. Just post a non-spoiler type hint, and let people work it out for themselves (like the author probably wanted).

• Excellent point, i had thought of crossword enthusiasts/code breakers, but not chess players. There’s no particular reference to chess in the book though, so I somewhat doubt it’ll be relevant. Yes the book goes into the meaning of the word “Gnomon” at length, apparently in addition to the piece of a sundial it broadly refers to something that sits perpendicular to the rest of a surface. Commented Mar 9, 2023 at 15:33
• I also haven't read the book, so this is a wild shot in the dark, but the grouping reminded me a bit of a puzzle I'd seen elsewhere. Treating each group of 4 as 2 groups of 2 and drawing lines to connect the relevant squares on a chess board revealed a key word in this other puzzle (meaning, for example, 3455 would mean drawing a line from square 34 to square 55). Is it possible something like that might work here? Commented Apr 1, 2023 at 20:53
• Can you clarify "please DO NOT post the answer". If this is the author's preference, the question should be closed. If not, then why? The whole point of stackexchange is to ask and answer questions.
– fljx
Commented Apr 20, 2023 at 14:48
• @fljx I assume that the author doesn't want the answer spoiled all over the internet. The question, however, isn't asking for the solution to the puzzle, but looking for hints as to how to proceed. As such, this question can be answered without spoilers. Commented Apr 21, 2023 at 0:47