Guess Mode

For decoding multiple layers of codecs, codext features a guess mode relying on an Artificial Intelligence algorithm, the Breadth-First tree Search (BFS). For many cases, the default parameters are sufficient for guess-decoding things. But it may require parameters tuning.


Parameters

BFS stops when a given condition, in the form of a function applied to the decoded string at the current depth, is met. It returns two results: the decoded string and a tuple with the related encoding names in order of application.

The following parameters are tunable:

  • stop_func: can be a function or a regular expression to be matched (automatically converted to a function that uses the re module) ; by default, checks if all input characters are printable.
  • min_depth: the minimum depth for the tree search (allows to avoid a bit of overhead while checking the current decoded output at a depth with the stop function when we are sure it should not be the right result) ; by default 0.
  • max_depth: the maximum depth for the tree search ; by default 5.
  • codec_categories: a string indicating a codec category or a list of category strings ; by default, None, meaning the whole categories (very slow).
  • found: a list or tuple of currently found encodings that can be used to save time if the first decoding steps are known ; by default, an empty tuple.

A simple example for a 1-stage base64-encoded string:

>>> codext.guess("VGhpcyBpcyBhIHRlc3Q=")
{('base64',): 'This is a test'}

An example of a 2-stages base64- then base62-encoded string:

>>> codext.guess("CJG3Ix8bVcSRMLOqwDUg28aDsT7")
{('base62',): 'VGhpcyBpcyBhIHRlc3Q='}

In the second example, we can see that the given encoded string is not decoded as expected. This is the case because the (default) stop condition is too broad and stops if all the characters of the output are printable. If we have a prior knowledge on what we should expect, we can input a simple string or a regex:

Default stop function

:::python

codext.stopfunc.default.name '...'

The output depends on whether you have a language detection backend library installed ; see section Natural Language Detection. If no such library is installed, the default function is "text".

>>> codext.guess("CJG3Ix8bVcSRMLOqwDUg28aDsT7", "test")
{('base62', 'base64'): 'This is a test'}

In this example, the string "test" is converted to a function that uses this string as regular expression. Instead of a string, we can also pass a function. For this purpose, standard stop functions are predefined. So, we can for instance use stopfunc.lang_en to stop when we find something that is English. Note that working this way gives lots of false positives if the text is very short like in the example case. That's why the codec_categories argument is used to only consider baseX codecs. This is also demonstrated in the next examples.

>>> codext.stopfunc._reload_lang("langdetect")
>>> codext.guess("CJG3Ix8bVcSRMLOqwDUg28aDsT7", codext.stopfunc.lang_en, codec_categories="base")
('This is a test', ('base62', 'base64'))

If we know the first encoding, we can set this in the found parameter to save time:

>>> codext.guess("CJG3Ix8bVcSRMLOqwDUg28aDsT7", "test", found=["base62"])
('This is a test', ('base62', 'base64'))

If we are sure that only base (which is a valid category) encodings are used, we can restrict the tree search using the codec_categories parameter to save time:

>>> codext.guess("CJG3Ix8bVcSRMLOqwDUg28aDsT7", "test", codec_categories="base")
('This is a test', ('base62', 'base64'))

Another example of 2-stages encoded string:

>>> codext.guess("LSAuLi4uIC4uIC4uLiAvIC4uIC4uLiAvIC4tIC8gLSAuIC4uLiAt", "test")
('this is a test', ('base64', 'morse'))
>>> codext.guess("LSAuLi4uIC4uIC4uLiAvIC4uIC4uLiAvIC4tIC8gLSAuIC4uLiAt", "test", codec_categories=["base", "language"])
('this is a test', ('base64', 'morse'))

When multiple results are expected, stop and show arguments can be used respectively to avoid stopping while finding a result and to display the intermediate result.

Computation time

Note that, in the very last examples, the first call takes much longer than the second one but requires no knowledge about the possible categories of encodings.


Available Stop Functions

A few stop functions are predefined in the stopfunc submodule.

>>> import codext
>>> dir(codext.stopfunc)
['LANG_BACKEND', 'LANG_BACKENDS', ..., '_reload_lang', 'default', 'flag', ..., 'printables', 'regex', 'text']

Currently, the following stop functions are provided:

  • flag: searches for the pattern "[Ff][Ll1][Aa4@][Gg9]" (either UTF-8 or UTF-16)
  • lang_**: checks if the given lang is detected (note that it first checks if all characters are text ; see text hereafter)
  • printables: checks that every output character is in the set of printables
  • regex(pattern): takes one argument, the regular expression, for checking a string against the given pattern
  • text: checks for printables and an entropy less than 4.6 (empirically determined)

A stop function can be used as the second argument of the guess function or as a keyword-argument, as shown in the following examples:

>>> codext.guess("...", codext.stopfunc.text)
[...]
>>> codext.guess("...", [...], stop_func=codext.stopfunc.text)
[...]

When a string is given, it is automatically converted to a regex stop function.

>>> s = codext.encode("pattern testing", "leetspeak")
>>> s
'p4773rn 73571n9'
>>> stop_func = codext.stopfunc.regex("p[a4@][t7]{2}[e3]rn")
>>> stop_func(s)
True
>>> codext.guess(s, stop_func)
[...]

Additionally, a simple stop function is predefined for CTF players, matching various declinations of the word flag. Alternatively, a pattern can always be used when flags have a particular format.

>>> codext.stopfunc.flag("test string")
False
>>> codext.stopfunc.flag("test f1@9")
True
>>> codext.stopfunc.regex(r"^CTF\{.*?\}$")("CTF{098f6bcd4621d373cade4e832627b4f6}")
True

The particular type of stop function lang_** is explained in the next section.


Natural Language Detection

As in many cases, we are trying to decode inputs to readable text, it is necessary to narrow the scope while searching for valid decoded outputs. As matching printables and even text (as defined here before as printables with an entropy of less than 4.6) is too broad for many cases, it may be very useful to apply natural language detection. In codext, this is done by relying on Natural Language Processing (NLP) backend libraries, loaded only if they were separately installed.

Currently, the following backends are supported, in order of precedence (this order was empirically determined by testing):

  • langid: Standalone Language Identification (LangID) tool.
  • langdetect: Port of Nakatani Shuyo's language-detection library (version from 03/03/2014) to Python.
  • pycld2: Python bindings for the Compact Langauge Detect 2 (CLD2).
  • cld3: Python bindings to the Compact Language Detector v3 (CLD3).
  • textblob: Python (2 and 3) library for processing textual data.

The way NLP is used is to check that these libraries exist and to take the first one by default. This sets up the stopfunc.default for the guess mode. This behavior aims to keep language detection as optional and to avoid multiple specific requirements having the same purpose.

While loaded, the default backend can be switched to another one by using the _reload_lang function:

>>> codext.stopfunc._reload_lang("pycld2")  # this loads pycld2 and attaches lang_** functions to the stopfunc submodule
>>> codext.stopfunc._reload_lang()          # this unloads any loaded backend

Each time a backend is loaded, it gets lang_** stop functions attached to the stopfunc submodule for each supported language.


Ranking Heuristic

Work in progress

This part is still in progress and shall be improved with better features and/or using machine learning.