Digital Evidence Testimony and Chain of Custody for Electronic Items

Why Digital Evidence Testimony Demands Precision

Digital evidence—smartphones, laptops, USB drives, surveillance hard drives—appears in an increasing share of criminal cases. As a crime scene technician, you may be the person who collected, packaged, and transferred that device. On the witness stand, opposing counsel will probe every gap in your documentation. A single vague answer about how you handled a USB drive can raise enough doubt to suppress the evidence entirely. Your testimony must be technically accurate, consistent with your written records, and free of overstatement.

Hash Values: The Digital Fingerprint

A hash value is a fixed-length alphanumeric string produced by running a file or drive's contents through a mathematical algorithm. Common algorithms used in forensic work include MD5 and SHA-256. The algorithm reads every bit on the storage medium and outputs a unique string. If even one bit changes—a single character edited, a file renamed, a timestamp updated—the hash output changes completely. This property makes hash values the primary method for proving that a forensic image is identical to the original evidence.

In practice, you compute a hash of the original device immediately after seizing it (before any examination), then compute a second hash of the forensic image after copying is complete. If the two values match, you can testify that the image is a bit-for-bit duplicate and that no data was altered during the process. Both hash values must be recorded in your case notes and chain-of-custody documentation the moment they are generated. Do not rely on memory.

Write-Blocking: Protecting the Original

A write-blocker is a hardware device or software setting that allows a forensic tool to read every bit from a storage device while preventing any write commands from reaching it. Connecting a drive directly to a standard computer—even to view its contents—can automatically modify timestamps, log files, and other metadata. These modifications are enough to compromise the integrity of the evidence and your credibility on the stand.

Always attach a hardware write-blocker between the evidence device and your forensic workstation before any connection is made. Confirm the write-blocker is active before proceeding. Note the make, model, and serial number of the write-blocker in your case documentation. During testimony you will likely be asked: Was a write-blocker used? What kind? Did you verify it was functioning? You must be able to answer all three from your written record, not from general recollection.

Forensic Imaging

A forensic image is a complete, sector-by-sector copy of a storage device, including deleted files, unallocated space, and file-system metadata. It is distinct from a simple file copy or backup, which captures only visible files. Forensic imaging software—such as tools validated by the National Institute of Justice or similar bodies—creates this exact duplicate and simultaneously records hash values at the start and end of the process.

Once imaging is complete, the original device is sealed, labeled, and returned to secure storage. All subsequent examination is performed on the forensic image, not the original. This practice preserves the original for court and allows multiple examiners to work independently from copies without touching the source evidence.

Chain of Custody for Electronic Items

The chain of custody for digital evidence follows the same core principles as physical evidence, with additional technical entries. Your chain-of-custody form for an electronic item should record:

• Collection: date, time, location, collector's name, device description (make, model, serial number, condition)
• Packaging: how the device was protected from electrostatic discharge, whether it was placed in airplane mode or a Faraday bag to prevent remote wiping
• Transfer: every person who received or relinquished custody, with signatures and timestamps
• Examination: write-blocker used, forensic imaging software used, hash values generated, examiner name
• Storage: location and conditions of secure storage between each step

A Faraday bag blocks radio signals and prevents a remote-wipe command from reaching a seized phone or tablet. Note its use on the form. Any gap in this record is a gap that cross-examination will exploit.

Testifying Accurately: What You Can and Cannot Say

On the stand, speak only to what you personally did and documented. You may testify that you computed a hash value and that it matched the image hash because you recorded both values at the time. You may not testify about what a detective found on the image if you did not conduct that examination yourself. Stay within your role.

If asked whether the evidence was altered, the correct answer is grounded in the hash comparison: "The hash value of the forensic image matched the hash value of the original device, which indicates no data changed during the imaging process." Do not say the evidence is "impossible" to alter in any absolute sense; say the hash comparison shows no alteration occurred in your handling.

Safety and Handling Reminders

Electronic devices can pose physical hazards. Swollen or punctured lithium-ion batteries may vent toxic gases or ignite. Do not force open damaged devices in the field. Use appropriate PPE (nitrile gloves minimum) when handling unknown devices to avoid fingerprint contamination and chemical exposure. Follow your agency's protocols for damaged electronics, which may require a hazmat consultation before handling.