convertme.py Beginner picoMini 2022 Solution

Description

1. Step 1

   Run the script

   Observation

   I noticed the challenge provided a Python script named convertme.py and described a conversion task, which suggested running it first to see what input it expects and what number it generates.

   Execute the script. It generates a random decimal number and asks you to type its binary representation.

   python

   python3 convertme.py

   Copied!

   What didn't work first

   Tried: Running the script with python instead of python3

   On systems where python points to Python 2, the script may fail with a syntax error or behave differently because print is a statement in Python 2, not a function. The challenge uses Python 3 syntax, so python3 is the correct interpreter to invoke.

   Tried: Opening convertme.py in a text editor to find a hardcoded answer

   The script generates a random decimal number each run using a random number generator, so no hardcoded answer exists in the source. Reading the file reveals the generation logic but not the answer - you must actually perform the conversion for whichever number the script produces.

   Learn more

   Binary (base-2) is the foundational number system of all digital computing. Every value a computer stores or processes is ultimately represented as a sequence of 0s and 1s - each called a bit. Understanding binary is essential for understanding how computers handle integers, memory addresses, network masks, and file permissions.

   The script generates a random decimal number so you cannot simply memorize the answer - you need to perform the conversion yourself or use a tool. This is intentional: the challenge teaches the concept rather than a specific value.

2. Step 2

   Convert the decimal number to binary

   Observation

   I noticed the script prompted for the binary representation of a randomly generated decimal number, which suggested using Python's built-in bin() function to perform the conversion quickly and accurately without manual long division.

   Use Python's built-in bin() function, which returns a string prefixed with '0b'. Slice off the prefix with [2:] to get just the binary digits.

   python

   python3 -c "print(bin(25)[2:])"

   Copied!

   Expected output

   11001

   What didn't work first

   Tried: Typing the bin() output including the 0b prefix into the script prompt

   The script expects only the raw binary digit string such as 11001, not the Python literal form 0b11001. Submitting with the prefix causes the answer check to fail and the script reports an incorrect answer. The [2:] slice is what removes the prefix before entering the value.

   Tried: Using hex(n) or oct(n) instead of bin(n) to convert the number

   hex() returns a hexadecimal string (e.g., 0x19) and oct() returns an octal string (e.g., 0o31), neither of which is the binary representation the script expects. The prompt specifically asks for binary - base-2 digits only - so bin() is the correct built-in to use.

   Learn more

   Python's bin() built-in converts any integer to its binary string representation. The return value is always prefixed with "0b" - Python's notation for binary literals - so slicing with [2:] strips that prefix and leaves only the digit string (e.g., "11001" for 25).

   The manual conversion algorithm works by repeatedly dividing the number by 2 and recording the remainders from bottom to top. For example, 25 ÷ 2 = 12 remainder 1, 12 ÷ 2 = 6 remainder 0, 6 ÷ 2 = 3 remainder 0, 3 ÷ 2 = 1 remainder 1, 1 ÷ 2 = 0 remainder 1 - reading the remainders upward gives 11001.

   Python also provides hex() and oct() for hexadecimal and octal conversions, both using the same [2:] prefix-stripping trick. These number bases appear constantly in CTF challenges: hex in memory dumps and shellcode, octal in Unix file permissions.

3. Step 3

   Enter the binary value

   Observation

   I noticed the script was waiting for input after displaying the decimal number, which indicated that typing the computed binary string into the prompt would trigger the flag to be printed if the answer was correct.

   Type the binary answer into the script prompt. If correct, the script prints the flag.

   Learn more

   This step reinforces that the flag is gated behind a correct answer - a common CTF pattern where knowledge or computation is required before a secret is revealed. In real systems, this mirrors challenge-response authentication: the server poses a problem, and only a client with the correct knowledge can respond.

   For harder variants of this type of challenge (where the server gives you very little time to respond), you would automate the response using pwntools: receive the number, compute bin(n)[2:] programmatically, and send it back - all within milliseconds. Building that habit now prepares you for timed challenges later.

Flag

Binary (base-2) has only digits 0 and 1; each position is a power of 2. Python's bin() prefixes with '0b' - slicing [2:] removes it to give just the binary digits.