A forensic image, also known as a bit-stream image or clone, is an exact replica of the contents of a digital storage device, such as a hard drive or cell phone. Forensic imaging is the process of creating this duplicate copy for investigative purposes in a forensically sound manner.
Why are forensic images used?
Forensic images are used by law enforcement, government agencies, legal teams, and forensic investigators to preserve digital evidence in its original state without altering the data. This allows the evidence to be analyzed without contaminating the original data storage device.
Preserving original evidence
Performing analysis directly on a device can alter metadata like access timestamps and potentially ruin the evidentiary value of the data. Imaging maintains data integrity by copying the data without making changes.
Allowing analysis of data
Investigators can work with the forensic image copy without worrying about tampering with the original evidence. This allows them to extract files, scan for deleted content, crack passwords, and perform data analytics.
Creating backups
The image acts as a backup of the data in case the original is lost or damaged. This backup can be preserved as evidence even if the original device no longer functions.
How are forensic images created?
Forensic imaging requires specialized tools and techniques to ensure the duplicate is an exact bit-for-bit clone of the original storage medium. There are two main approaches:
Hardware duplicators
These devices connect the evidence drive and a target drive to copy the contents at the physical level. They duplicate each bit from the source to destination drive.
Software tools
Forensic software is used to make a logical copy of the file system and contents of a drive. This involves reading the source drive and writing a file or disk image containing all the data from the evidence.
What are the key requirements of forensic imaging?
For a forensic image to have evidentiary value, there are several requirements that must be met:
Bit-stream copying
As mentioned above, hardware duplicators and proper software tools are necessary to perform low-level bit-stream copying. This duplicates both used and unused space on the drive.
Error checking
The integrity of the copy must be verified through hash values or other checksums. Any errors in duplication could impact the evidentiary value.
Chain of custody
Comprehensive documentation is required to track the handling and transfers of both the original device and forensic copies to prove no tampering occurred.
Preservation of metadata
File metadata like time/date stamps and access logs must be preserved during imaging to retain the full evidentiary context.
Authentication
The forensic image needs to be authenticated by the investigator via signing or encryption to confirm it has not been altered.
What types of storage can be forensically imaged?
The most common storage devices imaged in digital forensics include:
Hard drives
Both internal hard drives from computers and external USB hard drives can be forensically imaged.
Solid state drives (SSDs)
SSDs store data in flash memory chips rather than on spinning platters, but the contents can still be duplicated.
Optical discs
CDs, DVDs, and Blu-ray discs can all be copied sector-by-sector via forensic imaging tools.
Mobile devices
Smartphones and tablets can be imaged to copy data like contacts, text messages, call logs, and application data.
Cloud storage
Contents of cloud accounts may be retrievable by obtaining a forensic image of the account data from the service provider.
Remote servers
Network-based forensic imaging can acquire disk images of storage on remote servers as evidence.
What types of information are contained in a forensic image?
A forensic image contains an exact copy of all data on the source storage device. This includes:
Files and folders
All directories and files including documents, photos, application data, system settings, etc. are imaged.
Deleted content
Data residing in unallocated space after deletion is captured. This allows for recovery of deleted items.
Encrypted content
The raw encrypted data is imaged even if the keys needed to decrypt it are unavailable.
File slack space
Slack space refers to the unused space at the end of drive sectors. Even this can contain file remnants.
Metadata
Timestamps, access logs, ownership records, and other metadata are preserved.
Hidden data
Files marked hidden and other content not visible to the standard user is imaged.
What are some scenarios requiring a forensic image?
Some examples of cases where a forensic image would be used include:
Criminal investigations
Law enforcement would image devices like computers, phones, and hard drives to capture evidence for crimes that involve digital devices.
Corporate investigations
Companies may need to image employee computers and devices to investigate data theft, hacking, or policy violations.
eDiscovery
Civil litigation or regulatory investigations often require imaging storage devices to identify and collect relevant electronic documents and communications.
Data breaches
Forensic images help analyze compromised systems after a breach to determine root cause, impact, and recovery efforts.
Data recovery
Imaging failing drives preserves the data and creates a backup to allow for maximum recovery of files and information.
What are the steps to create a forensic image?
Performing a forensic imaging process involves several key steps:
1. Preparation
The investigator plans the imaging process, gathers required hardware/software tools, and documents the state of evidence devices.
2. Connecting devices
The evidence storage device is connected via write-blockers to prevent alteration. The destination device is attached.
3. Creating image
Using either hardware duplicators or software tools, a bit-stream copy is created of the evidence device to the destination.
4. Verification
Hash values are calculated on both devices to verify the forensic image is identical to the original.
5. Documentation
The investigator documents the process and maintains the chain of custody records.
What are best practices for forensic imaging?
Proper forensic imaging requires careful processes to ensure evidentiary standards. Best practices include:
- Using write-blocking to prevent alteration of the original data
- Verifying the integrity of the duplicate image
- Storing evidence securely with limited access
- Following chain of custody procedures carefully
- Documenting every step of the process thoroughly
- Repeating imaging when necessary to obtain optimal copies
- Using trained forensic specialists to handle imaging
What are the advantages of forensic imaging?
Key advantages include:
Preserves evidence integrity
Performing analysis on copies avoids contaminating the original evidence.
Allows in-depth analysis
Investigators can thoroughly examine a forensic image to extract all relevant case data.
Time savings
Multiple analysts can work on cloned data simultaneously to speed up investigations.
Insurance against data loss
The image acts as a reliable backup if the original is damaged or destroyed.
Standardizes evidence handling
Consistent, forensically sound imaging procedures increase credibility.
What are potential disadvantages or risks?
Some drawbacks include:
Requires specialized tools
The equipment for proper imaging can be expensive for organizations to obtain and maintain.
Large storage needs
Storing full forensic copies takes up considerable server space over time.
Time and labor intensive
Performing imaging thoroughly is a lengthy process requiring experienced personnel.
Challenges with proprietary devices
Some manufacturers use proprietary storage formats that complicate the imaging process.
Encryption can block access
Encrypted data cannot be accessed or imaged without decryption keys.
Can forensic images be authenticated in court?
Forensic images can serve as credible evidence in legal proceedings provided proper precautions are followed to document chain of custody and verify integrity. Some methods to authenticate images include:
Cryptographic hashing
Hash values can mathematically prove the copy is identical to the source evidence.
Investigator testimony
The specialist who performed imaging can attest to process and records.
Logs and documentation
Comprehensive notes validate imaging steps, device details, file metadata, etc.
Controlled access
Tracking who had access and under what conditions helps establish no tampering occurred.
Checksums
Complex checksums detect any changes that would invalidate the evidentiary value.
Example scenarios
Some examples help illustrate forensic imaging in practice:
Criminal investigation
Law enforcement seizes a suspect’s laptop computer. They create a forensic image of the laptop’s hard drive before reviewing any files to preserve the original state as evidence. They later find incriminating files through analyzing the forensic image.
Corporate investigation
A company suspects an employee of stealing trade secrets. The IT department makes a forensic image of the employee’s work desktop, company-issued laptop, and company cell phone during the investigation. This allows them to gather evidence from the images without tampering with the original devices.
Legal case
A law firm’s client is being sued for contract fraud. As part of legal discovery, the law firm obtains forensic images of the client’s email and document storage servers to preserve relevant information for the case. This allows them to thoroughly search the images for any emails, documents or other evidence related to the lawsuit.
Data breach
A retail company suffers a data breach exposing customer credit card data. Their incident response team forensically images affected servers and databases to create backups. This enables them to do forensic analysis on the images to determine how the attack occurred and what systems need to be rebuilt to close security gaps before bringing the original servers back online.
Conclusion
Forensic imaging creates an exact duplicate of digital storage media to enable investigation and analysis of the contents without altering the original data. When done properly following evidence handling standards, forensic images contain all data needed for a case and can be authenticated to serve as credible evidence. However, imaging does require specialized tools, training, and care to ensure the copies are true bit-stream duplicates that will stand up to legal scrutiny.