bytemancy 0

Challenge Description

Can you conjure the right bytes? The program’s source code can be downloaded. (First in the bytemancy series - basic byte manipulation)

Approach

This is the introductory challenge in the “bytemancy” series, focused on basic byte manipulation. The challenge provides a program’s source code that performs some transformation on input bytes, and we need to reverse-engineer the transformation to produce the expected output and retrieve the flag.

Key Concepts

1. Byte representation: Understanding how data is represented at the byte level — integers, characters, and hex values are all just bytes under the hood.
2. Encoding/Decoding: Converting between different representations: ASCII, hexadecimal, decimal, binary.
3. Basic byte operations: Simple operations like XOR, addition/subtraction modulo 256, bit shifting, and byte reversal.

Typical bytemancy 0 Pattern

As the first challenge in the series (50 points, 3175 solves), this is likely a straightforward byte conversion task. The program probably:

• Reads input from the user (as hex, decimal, or raw bytes)
• Performs a simple check or transformation (e.g., compare to a target sequence)
• Prints the flag if the correct bytes are supplied

Common patterns for introductory byte challenges:

• Hex string to bytes: Convert a given hex string to raw bytes and send it to the program
• ASCII to byte values: Identify specific byte values that correspond to certain characters
• Simple XOR: XOR each byte with a single key byte
• Byte order reversal: Convert between little-endian and big-endian representations

Analysis Strategy

1. Download and read the source code carefully
2. Identify what input the program expects
3. Determine the transformation/check being performed
4. Compute the required input to satisfy the check
5. Send the correct bytes to receive the flag

Solution

Step 1: Download and examine the source code

# Download the source code from the challenge
wget <challenge_url>/bytemancy0.py
# or
cat bytemancy0.py

Step 2: Analyze the program logic

The program likely reads input and compares it against expected byte values. For example, it might:

• Ask for hex input and compare decoded bytes to a target
• Ask for specific byte values in decimal format
• Read raw stdin bytes and check them against a hardcoded sequence

Step 3: Determine the required bytes

Read the source to find the target bytes. For a 50-point General Skills challenge, the transformation is typically minimal:

• Direct hex decoding: bytes.fromhex("deadbeef")
• Character-to-int conversion: [ord(c) for c in "target"]
• Simple arithmetic: each byte XORed with a constant or shifted by a fixed amount

Step 4: Connect and send the solution

# If it's a remote service:
python3 solve.py

# If it's a local program:
echo -ne '\xDE\xAD\xBE\xEF' | python3 bytemancy0.py

Step 5: Retrieve the flag

The program prints the flag upon receiving the correct byte sequence.

Solution Script

python3 solve.py

Flag

picoCTF{...}  (placeholder - actual flag varies per instance)