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Imaging & Verification

Disk imaging and verification are fundamental steps in digital forensics. Before investigators analyze any storage device, they must create an exact, bit-for-bit copy of the original media. This ensures the original evidence remains untouched while all examination occurs on a verified forensic image. Proper imaging protects the integrity of evidence and maintains legal admissibility.

This chapter explains the imaging process, common tools such as dd and FTK Imager, and how verification using hash values ensures authenticity.

What Is Disk Imaging?

Disk imaging is the process of creating a complete replica of a storage device, including:

  • Files and folders

  • Hidden system files

  • Unallocated space

  • Slack space

  • Deleted data

  • Boot sectors

  • File system structures

The goal is to preserve every bit of the source drive, not just visible user data. This replicated file is called a forensic image.

Once the image is created, investigators perform all analysis on the copy—not on the original device.

Why Disk Imaging Is Important

  • Preserves original evidence

  • Prevents accidental modification

  • Enables repeatable analysis

  • Allows investigators to take snapshots of evidence at a specific time

  • Satisfies legal requirements for digital evidence handling

A proper forensic image captures both allocated and unallocated data, ensuring nothing is missed.

Types of Forensic Images

1. Bit-for-Bit Image (Raw Image)

Includes every sector of the drive.
File extension: .img or .dd

2. E01 Format (EnCase Format)

Compressed and includes metadata such as notes and hashes.

3. AFF (Advanced Forensics Format)

Open-source format with metadata support.

Disk Imaging Process

  1. Prepare hardware and write blockers to prevent modification.

  2. Calculate a hash of the original drive (before imaging).

  3. Acquire the image using a forensic tool.

  4. Calculate a hash of the image file.

  5. Compare both hashes to verify the image is identical.

  6. Document every action and store the original securely.

Using "dd" for Forensic Imaging

The dd tool is a command-line utility used on Linux and macOS. It can create exact disk copies with precise control.

Basic Imaging Command

dd if=/dev/sdX of=/mnt/evidence/disk.img bs=4M conv=noerror,sync

Explanation:

  • if= input file (source disk)

  • of= output file (forensic image)

  • bs=4M block size for speed

  • noerror continue even if errors occur

  • sync keeps output aligned even with errors

Hashing with dd and sha256sum

Before and after imaging:

sha256sum /dev/sdX
sha256sum disk.img

If both hashes match, the image is verified.

Advantages of dd

  • Very reliable

  • Fully raw imaging

  • Works on almost any system

Disadvantages of dd

  • No built-in compression

  • No metadata storage

  • Slower compared to advanced tools

  • Easy to make mistakes if commands are incorrect

FTK Imager for Disk Imaging

FTK Imager is a widely used GUI tool for imaging and previewing drives. It is preferred in many professional labs due to its ease of use and robust features.

Key Features

  • Creates raw (DD), E01, and AFF images

  • Generates MD5/SHA1/SHA256 hashes automatically

  • Allows previewing files before imaging

  • Supports both physical and logical acquisition

  • Offers write-blocked imaging through hardware support

Steps to Create an Image with FTK Imager

  1. Launch FTK Imager.

  2. Click “Create Disk Image.”

  3. Select the source (physical drive, partition, or image).

  4. Choose the destination image format (Raw or E01).

  5. Enter case details and metadata.

  6. Set the path and filename for the image.

  7. Enable hash verification (MD5/SHA).

  8. Start the imaging process.

  9. Compare computed hashes to confirm integrity.

FTK Imager also logs the entire imaging process automatically.

Advantages of FTK Imager

  • Easy graphical interface

  • Built-in hashing and verification

  • Supports multiple formats

  • Generates detailed audit logs

  • Less prone to user mistakes

Disadvantages

  • Windows-based

  • Limited automation for large-scale imaging

Verification of Forensic Images

Verification is essential to prove that the image is an exact copy of the original device.

Hash Algorithms Commonly Used

  • MD5

  • SHA-1

  • SHA-256

  • SHA-512

The hash value is a digital fingerprint.
If the hash of the original drive matches the hash of the forensic image, the copy is valid.

Verification Steps

  1. Hash original media.

  2. Hash the forensic image.

  3. Compare values.

  4. Document everything in case logs.

If the values differ, the image is invalid and must be recreated.

Best Practices for Disk Imaging

  • Use hardware write blockers at all times.

  • Document device details, serial numbers, and timestamps.

  • Use reliable tools like dd, FTK Imager, or commercial suites.

  • Always verify using hashes.

  • Store original evidence securely and work only on copies.

  • Maintain clear chain of custody records.

Summary

Disk imaging is a foundational forensic procedure used to preserve digital evidence in its exact state. Tools like dd and FTK Imager allow investigators to create complete sector-by-sector copies of storage devices. Verification through hashing ensures the integrity and admissibility of evidence. Proper imaging and verification guarantee that analysts can examine data safely without risking alteration of the original evidence.

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