Custom / Homegrown Crypto

← Insufficient Cryptography

custom_crypto_implementation   MEDIUM

Description

A homegrown XOR-based ’encryption’ scheme trivially reversible.

How it works

This encryption is repeating-key XOR with a 4-byte constant, base64-wrapped to look opaque. XOR is its own inverse, so the ciphertext is reversed with no real key and leaks structure to frequency analysis. The app decrypts its own output immediately, which shows it protects nothing.

How to exercise it. DVMA is the harness - open this module from the home index and tap the demo action. The screen ships the malicious input and simulates the attacker (e.g. the companion app, crafted intent, or scanned payload) in-process, and the evidence panel prints the proof. The Tools (optional) and Attack inputs below are only needed to reproduce the exploit end-to-end on a real device.

Exploit steps

1. Set up. Build DVMA with a flavor that enables the Insufficient Cryptography category (e.g. --dart-define-from-file=config/flavors/dev.json) and run on an emulator/simulator you control. The demo needs no external tooling; for the optional on-device reproduction the relevant tools are: r2, r2ghidra, Ghidra, JEB, r2frida, frequency analysis.
2. Locate the target. From the home index, open Custom / Homegrown Crypto (custom_crypto_implementation). The How it works section above describes this module’s specific weakness; the screen states the intended-secure behavior and exposes the vulnerable action.
3. Exploit. Capture the algorithm/key/IV the module uses (the evidence panel prints it), then reproduce the weakness offline - e.g. recompute the digest, brute the short key, or decrypt a captured blob - to show the protection is ineffective.
4. Observe the evidence. Trigger the vulnerable action and read the evidence panel - it prints the concrete proof (leaked value, accepted replay, executed payload, or unauthorized result).
5. Contrast with the secure path. Run the module’s secure/hardened action (where provided) and confirm the same attack is rejected - this is what a correct implementation should do.
6. Map it back. Cross-reference the MASTG v2 test(s) MASTG-TEST-0210, MASTG-TEST-0221 for the canonical procedure and remediation.

Tools (optional)

• r2
• r2ghidra
• Ghidra
• JEB
• r2frida
• frequency analysis