Analyzing encrypted firmware

Analyzing encrypted firmware involves identifying and decrypting its contents before performing further analysis. This requires an understanding of the encryption methods, keys, and algorithms used, as well as the tools to analyze the structure. Here’s a structured approach:

1. Understand the Context

• Firmware Source: Know where the firmware came from (e.g., an IoT device, a router, etc.).
• Encryption Method Used: Check for information about the encryption type in documentation or through reverse engineering.
• Legal Aspects: Ensure you have the legal right to analyze the firmware.

2. Initial Inspection of the Firmware File

• File Type and Structure:
  • Use tools like file, binwalk, or hexdump to identify the file format and detect encrypted sections. binwalk firmware.bin
• Entropy Analysis:
  • High entropy (random-looking data) suggests encryption or compression. Use binwalk -E or tools like entropy-analyzer.

3. Gather Encryption Information

• Look for Keys and Hints:
  • Examine the device for stored encryption keys in its memory or firmware header.
  • Reverse-engineer the firmware loader or bootloader for clues about encryption algorithms and key storage.
• Debugging and Monitoring:
  • Use hardware debugging tools (e.g., JTAG, UART) to analyze the firmware decryption process during boot.
• Look for Obvious Patterns:
  • Common encryption standards (e.g., AES, DES) often have identifiable patterns in the encrypted payload.

4. Extract Encrypted Sections

• Dump the encrypted portion of the firmware using tools like:
  • dd for specific offsets.
  • binwalk with extraction flags.

5. Identify the Encryption Scheme

• Common encryption methods include AES, RSA, and proprietary algorithms. You can:
  • Look for common headers (e.g., PKCS#7 for RSA, IV for AES).
  • Use tools like CyberChef, hashcat, or custom scripts to test for known encryption schemes.

6. Locate or Retrieve Encryption Keys

• On the Device:
  • Keys may be stored in firmware or hardware modules like TPM (Trusted Platform Module) or secure elements.
  • Analyze the PCB for EEPROM or other storage chips that might hold keys.
• Via Debugging:
  • Use JTAG/SWD or UART interfaces to monitor the firmware decryption process.
  • Capture memory dumps or key exchanges during device initialization.

7. Decrypt the Firmware

• Use the identified algorithm and keys with appropriate tools:
  • For AES: openssl enc -d -aes-128-cbc -in encrypted.bin -out decrypted.bin -K <key> -iv <iv>
  • For RSA: Use openssl rsautl.

8. Analyze the Decrypted Firmware

• Tools like Binwalk, Ghidra, or IDA Pro can be used to analyze the now-readable firmware. Look for:
  • Code sections and logic.
  • Strings and configurations.
  • Vulnerabilities and exploits.

9. Handling Proprietary or Complex Encryption

If the firmware uses proprietary encryption:

• Reverse Engineer the Decryption Code:
  • Analyze the firmware loader or bootloader (using Ghidra/IDA Pro) for decryption routines.
• Side-Channel Analysis:
  • Techniques like timing analysis or power consumption monitoring may reveal key information.

10. Tools for Encrypted Firmware Analysis

• Static Analysis:
  • Binwalk, Radare2, Ghidra
• Dynamic Analysis:
  • Debuggers (e.g., OpenOCD, JTAG tools)
• Encryption Tools:
  • CyberChef, openssl, custom Python scripts.