Full Report
I created a small crypto style CTF for Black Hat last year (we’re training again this year, check our courses out) and hid the starting point in an “easter egg” on a deck of cards. The deck of cards are a custom design by the SensePost training team, which were themed around hacking and were handed out during the conference. This post covers how we built it, and how to solve it.
Analysis Summary
# Best Practices: Cryptographic Implementation and CTF Scenario Hardening
## Overview
These practices are derived from the methods used to design and solve a multi-stage Capture The Flag (CTF) challenge, covering secure handling of information dissemination, cryptographic techniques (substitution ciphers and asymmetric encryption), and secure key management principles. While the context is a CTF, the extracted guidelines address fundamental security weaknesses exploited in the challenge.
## Key Recommendations
### Immediate Actions (Addressing Visible Information Leakage)
1. **Avoid Hardcoding Contextual Clues in Public-Facing Materials:** Immediately review all physical and digital promotional/giveaway materials (e.g., custom decks of cards, conference swag) for embedded, easily recognizable data points (like binary strings or easily guessable challenge keys).
2. **Scrutinize Link Shorteners for Initial Redirection:** Treat all shortened URLs (e.g., `t.ly/xxxxx`) as untrusted entry points. Before deploying, verify the final destination URL provided by the shortener service.
3. **Vary Encryption Keys Based on Context (Avoid Single-Use Keys):** Ensure that the key used for one level of decryption (e.g., the Vigenère key "WE HACK WE TRAIN YOU PWN") is not easily discoverable by analyzing branding or common phrases associated with the creator/event.
### Short-term Improvements (1-3 months)
1. **Enforce Polyalphabetic Substitution Best Practices:** If using substitution ciphers (like Vigenère) outside of controlled environments (CTFs), ensure the key selection mechanism is secure and not overtly related to the context of the encrypted data.
2. **Standardize Public Key Transmission Protocol:** When distributing public keys (e.g., for RSA), ensure they are always transmitted alongside established cryptographic metadata or directly within a standard format (like a PEM block), rather than alongside unrelated "garbage" in custom formats (like concatenated Base64 blocks) which invites guessing.
3. **Integrate Tools for Secure Key Generation:** Adopt industry-standard cryptographic libraries (e.g., OpenSSL, Python's `cryptography` module) for key generation, replacing custom scripts based on textbook mathematics for high-stakes environments.
### Long-term Strategy (3+ months)
1. **De-emphasize Mathematical Clues in Cryptographic Challenges:** For challenges that involve decrypting information based on public keys, ensure the modulus provided does **not** rely on easily predictable or small prime factors ($p$ and $q$). Large prime numbers must be used to ensure factorization is computationally infeasible for the target audience.
2. **Implement Strong Key Protection for Private Keys:** Formalize procedures ensuring that private keys are never stored alongside public keys, especially in environments shared with the public (even for testing/training setups).
3. **Review Encoding Obfuscation vs. Security:** If using layered encoding (like Base64 followed by encryption), ensure that the cleartext structure of the encrypted data is not trivially identifiable (e.g., "garbage looking string" that is obviously Base64 but decrypts to useless data).
## Implementation Guidance
### For Small Organizations
- **Implement MFA on all critical services immediately.** (Addressing general security posture, as CTF exercises often rely on simple entry points.)
- **Use vetted, pre-configured Vigenère/ROT implementations** if small-scale polyalphabetic substitution is required for legacy systems, rather than attempting custom implementation.
- **Utilize standard, regularly updated encryption tools** for data sharing instead of relying on manual RSA key component calculation.
### For Medium Organizations
- **Establish a Public Key Infrastructure (PKI) policy** for managing the distribution and revocation of public keys.
- **Mandate the use of strong, modern asymmetric algorithms (e.g., RSA 3072-bit minimum, or ECC)**. Avoid deploying systems where RSA modulus factorization is theoretically possible via brute force or simplified mathematics (as demonstrated by the $p, q$ selection in the example).
- **Conduct regular configuration reviews** of all tools utilizing shared secrets or cryptographic keys to ensure context-specific information leakage (like CTF keys) is excluded.
### For Large Enterprises
- **Adopt Hardware Security Modules (HSMs)** for the secure generation, storage, and management of private keys to prevent compromise via software-only attacks (like recovering keys from factorization efforts).
- **Formalize Code Review for Cryptography Libraries:** Any custom code utilizing cryptographic primitives (like generating primes or calculating modular inverses) must undergo specialized security review to identify vulnerabilities leading to key reuse or weak key generation.
- **Develop Incident Response Playbooks for Key Compromise:** Procedures must exist to immediately rotate credentials and revoke certificates if the underlying factors of a modulus ($p$ and $q$) are discovered (as was done in the challenge recreation).
## Configuration Examples
The exploit involved successfully recreating the private key ($d$) given the public key $(N, e)$ and factoring $N$ into $p$ and $q$.
**Vulnerable Configuration Scenario (To be avoided):**
If a system uses the Vigenère cipher, avoid using keys derived solely from readily available, public knowledge:
* **Key:** "WE HACK WE TRAIN YOU PWN"
* **Vulnerability:** High probability of success if this key is related to the organization's public motto or branding.
**Secure RSA Key Generation Principle (The Goal):**
Ensure that $p$ and $q$ are selected using robust, cryptographically secure random number generators and are sufficiently large (e.g., 2048 bits or greater for $N$) to make **factorint** (prime factorization) computationally infeasible within any reasonable timeframe.
## Compliance Alignment
The findings highlight risks related to:
* **ISO/IEC 27001 (A.14.2.1):** Application Development Security – The reliance on simple factorization methods demonstrates insufficient rigor in cryptographic implementation.
* **NIST SP 800-57 Part 1:** Recommendation for Key Establishment Techniques – Specifically concerning the selection criteria for key components (the selection of small, exploitable prime factors $p$ and $q$).
* **CIS Controls (Control 14):** Continuous Vulnerability Management – Failure to secure the initial distribution point (card easter egg) and weak cryptographic components represent unmanaged technical debt.
## Common Pitfalls to Avoid
1. **Assuming Obfuscation is Security:** Encoding data (Base64) without true encryption only hides the data temporarily; it does not protect confidentiality.
2. **Reusing Contextual Words as Keys:** Linking cryptographic keys directly to project names, slogans, or readily available text drastically increases the attack surface for substitution ciphers ($N$-gram frequency analysis or dictionary attacks).
3. **Developing Cryptography from Scratch (Textbook RSA):** Manually deriving prime factors ($p, q$) and calculating the modular inverse ($d$) without robust, audited library functions leads to potential catastrophic errors, such as choosing weak primes that allow for factorization.
## Resources
* **For Secure Key Generation:** Utilize libraries that abstract away manual prime selection and modular inverse calculation (e.g., Python's `cryptography` library, Java's `javax.crypto`).
* **For Cryptanalysis:** Resources like **CyberChef** are useful for prototyping (as noted in the article), but real-world threats require dedicated, high-performance factorization tools (e.g., Pollard's rho, ECM) running against very large key spaces.
* **Framework Knowledge:** NIST Special Publications on Cryptography (available at n a t s t d. g o v).