TL;DR
Yes, there are standards for creating forensic disk images to ensure they’re legally admissible and reliable. This guide covers the key ones – focusing on bit-by-bit copies (imaging), write-blockers, hashing, and documentation.
1. Why Standards Matter in cyber security Forensics
When you need a disk image for legal reasons or an investigation, it must be done right. If not, the evidence could be thrown out of court. Standards ensure:
- Integrity: The image is a complete and accurate copy of the original drive.
- Authenticity: You can prove the image hasn’t been altered.
- Admissibility: It’s accepted as evidence in court.
2. Key Standards & Guidelines
There isn’t one single ‘law’, but these are widely accepted:
- ISO 17024: Accreditation for forensic examination bodies – shows competence.
- NIST Special Publication 800-86 (Guide to Digital Evidence Authentication and Preservation): A US standard, very influential globally.
- ACPO Guidelines on Computer Evidence (UK): Specifically for UK law enforcement but good practice generally.
3. The Imaging Process: Step-by-Step
- Identify the Drive: Carefully label the source drive with its physical location and any identifying information. Photograph it!
- Use a Write-Blocker: Crucially important. This prevents any changes being written to the original drive during imaging. Hardware write-blockers are best, but software ones can be used carefully (see section 6).
- Choose Your Imaging Tool: Popular options include:
- dd: A command-line tool available on most Linux distributions. Powerful but requires careful use.
- FTK Imager: Free, widely used GUI tool.
- EnCase Forensic Imager: Commercial, feature-rich.
- Create the Image: Perform a bit-by-bit (sector-by-sector) copy of the drive. This captures everything, including deleted files and unallocated space.
dd if=/dev/sdX of=image.img bs=512 conv=sync,noerror status=progress(Replace /dev/sdX with the correct drive identifier – be very careful!)
- Verify the Image: Calculate a cryptographic hash (MD5, SHA-1, or preferably SHA-256) of both the original drive and the image. They *must* match.
sha256sum image.img(This will output the hash value.)
- Document Everything: Detailed notes are vital! Record:
- Date and time of imaging
- Hardware/software used (including write-blocker serial number)
- Hash values for both source drive and image
- Chain of custody information (who handled the drive, when, and where)
4. Imaging Formats
- Raw (.img): A sector-by-sector copy. Large file size.
- EnCase Image File Format (.E01): Compressed, supports metadata and fragmentation.
- AFF4: Modern format designed for flexibility and scalability.
Raw images are often preferred for simplicity and compatibility.
5. Hash Values Explained
A hash function creates a unique ‘fingerprint’ of the data. Even a tiny change to the drive will result in a different hash value. SHA-256 is now recommended over MD5 or SHA-1 as they are considered less secure.
6. Software Write-Blockers: Use with Caution
Software write-blockers can be used if hardware isn’t available, but they’re less reliable. Ensure the software is trusted and properly configured. Always verify the image thoroughly after using a software write-blocker.
7. Chain of Custody
This is a record of who has handled the evidence from seizure to presentation in court. It’s essential for maintaining integrity and authenticity. Include dates, times, signatures, and reasons for handling.