A digital forensic image is a bit-by-bit copy of a storage device, including all data, deleted files, and unused portions, created for digital forensics purposes.
Understanding Digital Forensic Images
In the realm of digital forensics, preserving the original evidence is paramount. Simply copying files from a device isn't sufficient because it misses crucial information like deleted data, file system metadata, and the empty spaces on the drive that might contain residual information.
This is where a digital forensic image comes in. It's not just a file copy; it's an exact replica at the lowest level – the bits. Every single bit of data, from the very beginning to the very end of the storage medium, is copied. This ensures that the forensic examiner has a complete and unaltered snapshot of the device's state at the time of acquisition.
Why Are Forensic Images Used?
The primary reasons for creating a forensic image are centered around the integrity and authenticity of digital evidence:
- Preservation: The original device can be sealed and stored, allowing investigators to work on the copy (the image) without altering the original evidence.
- Completeness: It captures everything, including hidden files, deleted files, system structures, and residual data in unallocated space, which might be critical to an investigation.
- Integrity: Forensic imaging tools use hashing algorithms (like MD5 or SHA-256) during the imaging process. A hash value is calculated for the original device before imaging and for the completed image file after imaging. If these hash values match, it mathematically proves that the image is an exact, bit-for-bit copy of the original, ensuring its integrity.
- Admissibility: This rigorous process of creating a verifiable, complete copy makes the digital evidence derived from the image admissible in court or legal proceedings.
What Does a Forensic Image Capture?
Unlike a standard copy-and-paste or disk clone which might only duplicate active files, a forensic image captures:
- Active Data: All files and folders currently visible and accessible on the file system.
- Deleted Data: Information belonging to files that have been deleted but whose data hasn't yet been overwritten.
- File System Metadata: Data about the files and folders themselves (timestamps, permissions, locations, etc.) and the structure of the file system.
- Unallocated Space: The portions of the storage device marked as empty by the file system, which may still contain remnants of previously deleted files or other data.
- Slack Space: The unused space within the last block or cluster allocated to a file, which can sometimes contain residual data from previous files.
- Operating System Structures: Low-level data related to how the operating system interacts with the storage device.
Types of Storage Devices Imaged
Forensic images can be created from virtually any digital storage medium, including:
- Hard Disk Drives (HDDs)
- Solid State Drives (SSDs)
- USB Flash Drives
- SD Cards and other memory cards
- Mobile Phone Storage
- Server Storage Arrays
How Forensic Images are Created
Creating a forensic image typically involves:
- Connecting the storage device to a forensic workstation using a hardware write-blocker. The write-blocker prevents any modifications from being made to the original device.
- Using specialized forensic software (e.g., EnCase, FTK Imager, X-Ways Forensics, dd/dc3dd/dcfldd in Linux).
- Configuring the software to perform a bit-for-bit acquisition.
- Calculating and recording the initial hash value of the source device.
- Executing the imaging process, often saving the image to a separate destination drive in a specific forensic image format (like E01, DD, AFF).
- Calculating and recording the hash value of the resulting image file(s).
- Verifying that the source hash and image hash match.
- Maintaining a detailed chain of custody record documenting the entire process.
By creating this verifiable, complete copy, digital forensic investigators can analyze the image thoroughly using their tools without risking the integrity of the original evidence.
