CTFtime.org

In a pickle

by b3zt3m0r

Tags: python pickle ascii 

Rating:

In a pickle

Problem description

We managed to intercept communication between und3rm4t3r and his hacker friends. However it is obfuscated using something. We just can't figure out what it is. Maybe you can help us find the flag?

data

Category: misc. Points: 200. Tag: easy.

data

(dp0 I1 S'D' p1 sI2 S'UCTF' p2 sI3 S'{' p3 sI4 I112 sI5 I49 sI6 I99 sI7 I107 sI8 I108 sI9 I51 sI10 I95 sI11 I121 sI12 I48 sI13 I117 sI14 I82 sI15 I95 sI16 I109 sI17 I51 sI18 I53 sI19 I53 sI20 I52 sI21 I103 sI22 I51 sI23 S'}' p4 sI24 S"I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is " p5 s.

Concepts

Solving the challenge

The data file contains a pickled (serialized) Python object. Pickling converts a Python object hierarchy into a byte stream, while unpickling is the reverse process. To solve the challenge we need to unpickle (deserialize) the object, using the pickle.load(file, *, fix_imports=True, encoding='ASCII', errors='strict') method.

Unpickle data file into a variable content:

import pickle

with open('./data', 'rb') as pickle_file:
    content = pickle.load(pickle_file)

Examining the content variable:

>>> print(content)
{1: 'D', 2: 'UCTF', 3: '{', 4: 112, 5: 49, 6: 99, 7: 107, 8: 108, 9: 51, 10: 95,
 11: 121, 12: 48, 13: 117, 14: 82, 15: 95, 16: 109, 17: 51, 18: 53, 19: 53, 20: 52,
 21: 103, 22: 51, 23: '}', 24: "I know that the intelligence agency's are onto
 me so now i'm using ways to evade them: I am just glad that you know how to use
 pickle. Anyway the flag is "}

>>> type(content)
<class 'dict'>

The content variable is a dict. Looking at the values, we can identify that all values belonging to key 4-22 are stored as ASCII decimal. Thus, we need to convert the ASCII decimals to character. In this case (since there are no other integer values in the dict), we can convert all int values to their corresponding character using the chr(i) function.

Storing the flag in a flag variable:

flag = ""
for key in sorted(content)[:-1]: # Sorting dict to ensure correct ordering. Omitting the (string) value from the last key (24)
    val = content[key]
    if isinstance(val, int):
        val = chr(val)  # Transforming ASCII decimal to character
    flag += val

Now we can output the flag:

>>> print(flag)
DUCTF{p1ckl3_y0uR_m3554g3}
Original writeup (https://github.com/jeanettesa/ctf-writeups/blob/master/2020/DownUnderCTF/in_a_pickle/in_a_pickle.md).

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