In digital forensics, the evidence itself is fragile and easily altered. A single click can change a file's access timestamp. Booting a computer can overwrite thousands of deleted files. The value of your findings in an investigation—or a court of law—depends entirely on your ability to prove the evidence is authentic, unaltered, and handled with meticulous care from the moment it's found.

This process is known as preserving the Chain of Custody. It's the documented, unbroken timeline that tracks every person who handled the evidence, every action taken, and every location it was stored. Break this chain, and your evidence may be deemed inadmissible.

Here’s how to preserve digital evidence without breaking this critical chain.

The Four Golden Rules of Digital Evidence

Before you touch anything, remember these core principles:

1. Minimize Alteration: Take every possible step to avoid changing the original evidence. This is the foremost rule.

2. Document Everything: If it isn't documented, it didn't happen. Your notes and logs are your best defense in court.

3. Preserve Original Evidence: The original evidence device should be stored securely and never used for analysis. All work is done on a forensic copy.

4. Follow the Law and Policy: Always operate with proper legal authority (warrant, consent, etc.) and within your organization's policies.

Step-by-Step Guide to Preservation & Chain of Custody

Step 1: Preparation (Before You Arrive)

Goal: Avoid being unprepared at the scene.

• Get Legal Authority: Ensure you have the right to seize the evidence (e.g., a search warrant, court order, or written consent form). This is the most critical first step.

• Gear Up: Prepare your digital forensic toolkit:

  • Hardware Write-Blockers: For SATA, IDE, NVMe, and USB interfaces.

  • Forensic Imaging Equipment: Portable hard drives (new and sanitized), forensic duplicator (e.g., Tableau, WiebeTech), or software like FTK Imager.

  • Packing Materials: Anti-static bags, evidence tags, tamper-evident evidence bags, and sturdy boxes.

  • Camera: A digital camera for photographing the scene.

  • Toolkit: Screwdrivers, gloves, pens, and notebooks.

Step 2: Scene Documentation

Goal: Create a visual and written record of the evidence in situ (in its original state).

• Photograph and Video: Before touching anything, take wide-angle and close-up photos of:

  • The overall scene.

  • How the device is connected (power, network cables, peripherals).

  • The screen state (if it's on).

  • Serial numbers and model numbers.

• Take Notes: Document the date, time, location, and who is present. Describe the device's state (e.g., "Laptop is on and logged into a Windows desktop," "Phone is flashing a low battery warning").

Step 3: Evidence Collection & Isolation

Goal: Secure the evidence without altering it.

• Power Decision:

  • For Phones & Tablets: If on, isolate from networks (enable Airplane Mode, then turn off WiFi and Bluetooth manually). If off, leave it off. Connecting to a cell tower can change data.

  • For Computers: The debate between live acquisition and pulling the plug is complex. The safest, most common method for typical evidence is to perform a clean shutdown by pulling the power cord from the back of the desktop/tower or removing the battery from a laptop. This prevents disk encryption from locking the device. Note: For servers or active cyberattacks, a live memory capture may be preferred by experts.

• Bag and Tag:

  • Place devices in anti-static bags.

  • Fill out an evidence tag for each item. Include: Case #, Item #, Description, Serial #, Date/Time of Collection, Collector's Name and Signature.

  • Place the item and its tag into a tamper-evident evidence bag. Seal it and write your initials across the seal.

Step 4: Creating a Forensic Image (The "Preservation")

Goal: Create a bit-for-bit copy for analysis, leaving the original pristine.

1. In a Lab Setting, connect the original evidence drive to a write-blocker.

2. Connect the write-blocker to your forensic workstation.

3. Use a tool like Guymager, FTK Imager, or dd to create a forensic image (e.g., an .E01 or .dd file) onto your clean evidence drive.

4. Calculate a Hash Value: Generate cryptographic hash values (MD5, SHA-1, SHA-256) for the original drive and the image file. These values must match exactly to prove the copy is perfect and unaltered. Document these hashes.

Step 5: Maintaining the Chain of Custody Form

Goal: Create an unbroken log of every transfer.

The Chain of Custody form is a simple but critical document that travels with the evidence. Every time the evidence changes hands or location, it must be logged.

A sample CoC form includes:

If the form is blank for any period of time, the chain is broken.

Step 6: Secure Storage

Goal: Protect the evidence from tampering and environmental damage.

• Store original evidence in a locked, access-controlled secure facility (evidence locker or room).

• Control the environment to avoid extreme heat, cold, or moisture.

• The storage location should be documented on the Chain of Custody form.

What "Breaks" the Chain of Custody?

Be aware of these common pitfalls:

• Failing to Document a Transfer: An investigator takes evidence from storage to their desk without logging it.

• Leaving Evidence Unsecured: Leaving a seized laptop in an unlocked car or on a desk overnight.

• Poor Handling: Analyzing the original evidence instead of a forensic copy.

• Incomplete Documentation: Vague notes like "analyzed the drive" instead of "created forensic image case_001.e01 (SHA-256: abc123...) from evidence item #1 using a Tableau write-blocker."