Disk Imaging Techniques

Introduction

Forensic disk imaging is the foundation of digital forensics. Creating an exact bit-for-bit copy of storage media ensures that the original evidence remains pristine while allowing thorough analysis on the duplicate. This part covers the standards, tools, and techniques essential for forensically sound imaging.

Forensic Imaging Standards

Forensic imaging must meet strict standards to ensure evidence is admissible in court. The primary goal is to create an exact duplicate that can be verified as identical to the original.

Key Principles

• Bit-for-bit copy: Every sector must be copied, including unused space and slack
• No modification: The original evidence must not be altered in any way
• Verification: Hash values must confirm the copy is identical to the original
• Documentation: Every step must be thoroughly documented

Image Types

Forensic Imaging Workflow

A standardized workflow ensures consistency and thoroughness in every imaging operation.

Step 1: Document Evidence

Before touching the drive, document everything:

• Photograph the drive and any labels
• Record make, model, and serial number
• Note interface type and physical condition
• Document date, time, and examiner name

Step 2: Connect Through Write Blocker

Never connect evidence directly to a forensic workstation. Always use a verified write blocker.

Step 3: Calculate Source Hash

Before imaging, calculate hash values of the source drive. This establishes the baseline for verification.

Step 4: Create Image

Use appropriate forensic imaging software to create a bit-for-bit copy.

Step 5: Verify Image Hash

Calculate hash of the created image and compare to source hash. They must match exactly.

Step 6: Document Results

Record all hash values, image file details, any errors encountered, and total time taken.

Write Blocking

Write blockers prevent any data from being written to the evidence drive. This is essential for maintaining evidence integrity and is a fundamental requirement for forensic acquisition.

Types of Write Blockers

Hardware Write Blockers

Physical devices that intercept write commands at the hardware level:

• Most reliable and court-accepted method
• Available for SATA, SAS, USB, IDE, NVMe interfaces
• No software dependencies
• Examples: Tableau, WiebeTech, CRU

Software Write Blockers

Operating system utilities that prevent write operations:

• Lower cost alternative
• Depends on OS functioning correctly
• May not block all write paths
• Examples: Linux mount options, Windows Registry modifications

Write Blocker Verification

Before each use, verify the write blocker is functioning correctly:

1. Connect a test drive through the blocker
2. Attempt to write data to the drive
3. Verify the write operation failed or was blocked
4. Document the test results

Hash Verification

Cryptographic hash functions create a unique fingerprint of data. In forensics, hashes prove that evidence has not been modified.

Common Hash Algorithms

When to Hash

• Before imaging: Establish original state of evidence
• During imaging: Some tools calculate as they copy
• After imaging: Verify image matches source
• Before analysis: Confirm image hasn't changed
• After analysis: Prove image wasn't modified

Imaging with dd

The dd command is a powerful Unix/Linux utility for low-level data copying. It's commonly used in forensics for creating raw disk images.

Basic dd Syntax

# Basic forensic imaging command
dd if=/dev/sdb of=/evidence/case001/disk.raw bs=4M conv=noerror,sync status=progress

Key Parameters

• if= (input file): The source device to image
• of= (output file): The destination image file
• bs= (block size): Size of each read/write operation (4M is efficient)
• conv=noerror: Continue on read errors instead of stopping
• conv=sync: Pad error blocks with zeros to maintain alignment
• status=progress: Show progress during copying

dd with Hash Verification

# Calculate source hash first
sha256sum /dev/sdb > source_hash.txt

# Create image and calculate hash simultaneously
dd if=/dev/sdb bs=4M conv=noerror,sync | tee disk.raw | sha256sum > image_hash.txt

# Verify hashes match
diff source_hash.txt image_hash.txt

dc3dd: Forensic dd Alternative

dc3dd is a patched version of dd with forensic features built-in:

# dc3dd with automatic hashing and logging
dc3dd if=/dev/sdb of=disk.raw hash=md5 hash=sha256 log=imaging.log

FTK Imager

FTK Imager is a free forensic imaging tool from Exterro (formerly AccessData). It provides a graphical interface and supports multiple image formats.

Supported Image Formats

FTK Imager Features

• Create physical, logical, and memory images
• Mount images for preview without modification
• Export individual files from images
• Verify existing images
• Create file hashes
• Split large images into segments

Creating an Image with FTK Imager

1. Connect evidence through write blocker
2. Launch FTK Imager
3. File → Create Disk Image
4. Select source type (Physical Drive)
5. Select the evidence drive
6. Add image destination (choose format, e.g., E01)
7. Enter case information and evidence details
8. Select verification options (MD5, SHA1, SHA256)
9. Click Start to begin imaging
10. Review verification results upon completion

Other Imaging Tools

Various other tools are available for forensic imaging, each with specific strengths.

Image Format Considerations

Choosing the right image format depends on your workflow, tools, and storage constraints.

Raw vs Forensic Formats