The flag is in format dctf{7_4_2_3_7_4} where numbers represent number of characters between underscores. For clarification: you will get the whole flag as a result, including the dctf{} part.

Files

cfg.zip

What’s going on?

We have cfg.zip, which contains 2134 folders. An example of the structure is shown below:

Image not found!

  • Some folders contain only subfolders. Subfolders name is not random: subfolders have the same name of a folder from the upper layer.
  • Some folders contain only <letter>.txt files.
  • Some folders contain both txt files and subfolders.

Between challenge tags, we saw context-free-grammar. In order to make the solution more understandable, we briefly introduce what a context free grammar is.

Context free grammar

A context free grammar is a formal grammar which is used to generate all possible strings in a given formal language. Sentences are not influenced by context, but only by a set of rules, which define the syntax and the structure of sentences in these languages.

Context free grammars have the following components:

  • A set of terminal symbols which are the characters that appear in the language/strings generated by the grammar (e.g. characters, words etc). Terminal symbols never appear on the left-hand side of the production rule and are always on the right-hand side.

  • A set of non-terminal symbols (or variables), which are placeholders for patterns of terminal symbols that can be generated by the non-terminal symbols. These are the symbols that will always appear on the left-hand side of the production rules, though they can be included on the right-hand side. The strings that a CFG produces will contain only symbols from the set of nonterminal symbols.

  • A set of production rules which are the rules for replacing nonterminal symbols. Production rules have the following form: $variable \rightarrow string$ (of variables and terminals).

  • A start symbol $S$, which is a special non-terminal symbol that appears in the initial string generated by the grammar.

To create a string from a context-free grammar, we follow these steps:

  • Begin the string with a start symbol $S$.

  • Apply one of the production rules to the start symbol on the left-hand side, by replacing the start symbol with the right-hand side of the production.

  • Repeat the process of selecting nonterminal symbols in the string, and replacing them with the right-hand side of some corresponding production, until all non-terminals have been replaced by terminal symbols. Note, it could be that not all production rules are used.

Example

Given the following rules

S --> nounPhrase verbPhrase
nounPhrase --> adj noun
verbPhrase --> verb nounPhrase
adj  --> the
noun --> monkey | banana
verb --> ate

We consider:

  • Terminal symbols: the, monkey, banana, ate
  • Non terminal symbols: nounPhrase, verbPhrase, adj, noun, verb
  • Start symbol: S

We substitute in this way:

S --> adj noun verb nounPhrase 
	--> adj noun verb adj noun
  --> the <monkey | banana> ate the <monkey | banana>
  --> the monkey ate the banana

Solution

We found out folders were non-terminal symbols, and files with letters and numbers with .txt inside some folders were terminal symbols. We understood we had to replace all the folders with their content. This action could be recursive in case of more subdirectories in directory.

First, we got the cfg folder tree:

tree . > tree.txt

Applying what we understand from Context Free Grammar:

  • Start symbol is the S folder.
  • Terminal symbols are .txt files, which will compose our flag.
  • Non-terminal symbols are folders and subfolders.
  • Rules consist in substituting all folders with their content, recursively.

Image not found!

  • We can see S contains seven times ZC folders

  • Flag format is dctf{7_4_2_3_74}, which has seven symbols ({ } and five _)

  • ZC contains AMT/, HC/, XL/

    • AMT -> }.txt (the last ZC is })
    • HC -> {.txt (the first ZC is {)
    • XL -> _.txt (the middle ZC are _)

  • CZ will be dctf (warning: the characters are in the wrong order!)

And so on.

We just Ctrl + F on tree.txt file, ordered the characters, and got the flag:

dctf{c0nt3xt_fr33_15_n07_m34n1ng_fr33}

⠀⠀⠘⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ ⡜⠀⠀⠀ ⠀⠀⠀⠑⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡔⠁⠀⠀⠀ ⠀⠀⠀⠀⠈⠢⢄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⠴⠊⠀⠀⠀⠀⠀ ⠀⠀⠀⠀⠀⠀⠀⢸⠀⠀⠀⢀⣀⣀⣀⣀⣀⡀⠤⠄⠒⠈⠀⠀⠀⠀⠀⠀⠀⠀ ⠀⠀⠀⠀⠀⠀⠀⠘⣀⠄⠊⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ ⠀ ⣿⣿⣿⣿⣿⣿⣿⣿⡿⠿⠛⠛⠛⠋⠉⠈⠉⠉⠉⠉⠛⠻⢿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣿⡿⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠛⢿⣿⣿⣿⣿ ⣿⣿⣿⣿⡏⣀⠀⠀⠀⠀⠀⠀⠀⣀⣤⣤⣤⣄⡀⠀⠀⠀⠀⠀⠀⠀⠙⢿⣿⣿ ⣿⣿⣿⢏⣴⣿⣷⠀⠀⠀⠀⠀⢾⣿⣿⣿⣿⣿⣿⡆⠀⠀⠀⠀⠀⠀⠀⠈⣿⣿ ⣿⣿⣟⣾⣿⡟⠁⠀⠀⠀⠀⠀⢀⣾⣿⣿⣿⣿⣿⣷⢢⠀⠀⠀⠀⠀⠀⠀⢸⣿ ⣿⣿⣿⣿⣟⠀⡴⠄⠀⠀⠀⠀⠀⠀⠙⠻⣿⣿⣿⣿⣷⣄⠀⠀⠀⠀⠀⠀⠀⣿ ⣿⣿⣿⠟⠻⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠶⢴⣿⣿⣿⣿⣿⣧⠀⠀⠀⠀⠀⠀⣿ ⣿⣁⡀⠀⠀⢰⢠⣦⠀⠀⠀⠀⠀⠀⠀⠀⢀⣼⣿⣿⣿⣿⣿⡄⠀⣴⣶⣿⡄⣿ ⣿⡋⠀⠀⠀⠎⢸⣿⡆⠀⠀⠀⠀⠀⠀⣴⣿⣿⣿⣿⣿⣿⣿⠗⢘⣿⣟⠛⠿⣼ ⣿⣿⠋⢀⡌⢰⣿⡿⢿⡀⠀⠀⠀⠀⠀⠙⠿⣿⣿⣿⣿⣿⡇⠀⢸⣿⣿⣧⢀⣼ ⣿⣿⣷⢻⠄⠘⠛⠋⠛⠃⠀⠀⠀⠀⠀⢿⣧⠈⠉⠙⠛⠋⠀⠀⠀⣿⣿⣿⣿⣿ ⣿⣿⣧⠀⠈⢸⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠟⠀⠀⠀⠀⢀⢃⠀⠀⢸⣿⣿⣿⣿ ⣿⣿⡿⠀⠴⢗⣠⣤⣴⡶⠶⠖⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⡸⠀⣿⣿⣿⣿ ⣿⣿⣿⡀⢠⣾⣿⠏⠀⠠⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠛⠉⠀⣿⣿⣿⣿ ⣿⣿⣿⣧⠈⢹⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣰⣿⣿⣿⣿ ⣿⣿⣿⣿⡄⠈⠃⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣴⣾⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣧⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣾⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣷⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣴⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣿⣦⣄⣀⣀⣀⣀⠀⠀⠀⠀⠘⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣷⡄⠀⠀⠀⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣧⠀⠀⠀⠙⣿⣿⡟⢻⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⠇⠀⠁⠀⠀⠹⣿⠃⠀⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⣿⣿⣿⣿⡿⠛⣿⣿⠀⠀⠀⠀⠀⠀⠀⠀⢐⣿⣿⣿⣿⣿⣿⣿⣿⣿ ⣿⣿⣿⣿⠿⠛⠉⠉⠁⠀⢻⣿⡇⠀⠀⠀⠀⠀⠀⢀⠈⣿⣿⡿⠉⠛⠛⠛⠉⠉ ⣿⡿⠋⠁⠀⠀⢀⣀⣠⡴⣸⣿⣇⡄⠀⠀⠀⠀⢀⡿⠄⠙⠛⠀⣀⣠⣤⣤⠄

Learning sources

Thanks to the authors of these sources, who helped us understand better how context free grammar works: