Not TRUe

Challenge Description

no. no. that’s Not TRUe. that’s impossible!

We are given a remote service (nc) that implements a custom authentication scheme. The server uses a truncated hash/MAC to verify messages. The name “Not TRUe” hints at a truncation attack — where a cryptographic hash or MAC is truncated to fewer bits, making it feasible to brute-force a valid tag.

Approach

The challenge name “Not TRUe” is a play on “TRU” / truncation. In proper cryptographic implementations, MACs and hashes use their full output length (e.g., 256 bits for SHA-256). However, when the output is truncated to a small number of bits (e.g., 16-32 bits), it becomes computationally trivial to forge a valid tag through brute force.

Vulnerability Analysis

1. Truncated MAC/Hash: The server computes a MAC or hash over user-supplied data but only checks a truncated portion (e.g., the first 2-4 bytes). This drastically reduces the search space from 2^256 to 2^16 or 2^32.

2. Attack Strategy: We can forge a message with a valid truncated tag by:

   • Observing the server’s behavior and understanding the truncation length
   • Brute-forcing the truncated portion by trying random inputs until one produces a matching truncated hash
   • Alternatively, if the server uses HMAC-SHA256 truncated to N bits, we generate candidate messages and check if the first N bits of their hash match what the server expects

3. Birthday-style collision: With a 16-bit truncation, we only need ~2^8 = 256 attempts on average (birthday bound) or at most 2^16 = 65536 attempts for a guaranteed match against a fixed target.

Server Interaction Pattern

The typical flow is:

1. Connect to the server
2. Server provides a challenge or expects a message + tag pair
3. We craft a message and brute-force the truncated tag
4. Submit the forged message to retrieve the flag

Solution

1. Connect to the challenge server using netcat or pwntools
2. Parse the server’s response to understand the expected format
3. Brute-force the truncated hash/MAC by iterating through possible values
4. Send the forged message + valid truncated tag
5. Receive the flag

Solution Script

python3 solve.py

Flag

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