Full Report
We had published a network protocol analysis challenge for free entry to our BlackHat 2012 Vegas training courses and received seven correct answers. We’d like to thank those who attempted this challenge and hope that they find it useful. The winner, Peter Af Geijerstam managed to respond first, with the correct answer. As a result, he wins a free place on any of our Hacking By Numbers courses. Here is a brief solution for it:
Analysis Summary
# Main Topic
Analysis and solution summary for a network protocol challenge related to a simple challenge-response authentication mechanism, potentially mirroring a real-world protocol weakness exploitation scenario.
## Key Points
- The protocol uses a 3-digit random number (R) as a challenge and a 16-byte response (RESP).
- The response is calculated using the formula: `RESP = MD5(R + secret)`.
- The shared secret format was constrained to 7 alphanumeric characters, excluding uppercase letters.
- The solution involved capturing network traffic, identifying the challenge/response mechanism, and brute-forcing the shared secret based on known message formats and captured hashes.
- The recovered shared secret was `bm28lg1`.
- The session key (`kc`) was derived by setting R=448 (a specific challenge value observed in the server code) and recalculating the response using the recovered secret. The resulting session key was `07e0f7a7cbc2d8b3dba6b7d3b69c3236`.
## Threat Actors
- **Attacker/Winner:** Peter Af Geijerstam (identified as the winner who solved the challenge).
- No established threat groups or criminal actors are indicated; the context pertains to a security challenge/exercise.
## TTPs
- **Reconnaissance/Analysis:** Network protocol analysis using traffic capture tools.
- **Cryptographic Analysis:** Reverse engineering or source code review to determine the authentication formula.
- **Brute Force Attack:** Used on the resulting MD5 hash combined with known constraints (charset and message format) to recover the pre-shared key.
- **Tool Mentioned:** `hashcat` (specifically `cudaHashcat-plus32.exe`).
- **Crack Command Snippet:** `–attack-mode 3 –custom-charset1 abcdefghijklmnopqrstuvwxyz0123456789 hash[.]txt 448?1?1?1?1?1?1?1`
## Affected Systems
- The context describes a specific challenge implementation involving client and server binaries, using **Windows CryptoAPI** internally for MD5 hash generation.
- Systems designed around this specific *simple challenge-response authentication protocol* are conceptually vulnerable if they rely on weak secrets combined with MD5 hashing, especially in a known-answer attack scenario.
## Mitigations
- **Protocol Strength:** The core issue stems from using MD5 in a challenge-response authentication scheme where an attacker gains known R/RESP pairs. Stronger, modern cryptographic primitives (e.g., HMAC-SHA256) should be used instead of raw MD5 hashing for message integrity/authentication.
- **Secret Management:** Use secrets that are not easily guessable based on constrained character sets.
- **Implementation Note:** The provided challenge solution indicates that buffer sizes were arbitrarily set (128 bytes) to accommodate future unimplemented features (MSG messages), suggesting a potential implementation flaw in handling data boundaries, although this was not exploited in the primary challenge solution.
## Conclusion
The challenge illustrated a classic vulnerability where constraints on the secret and the exposure of input/output pairs (`R` and `MD5(R+secret)`) allowed for efficient offline brute-force recovery of the shared secret using specialized hash-cracking tools. This underscores the danger of relying on simple XOR/concatenation with weak hash functions like MD5 for critical authentication material.