How Digital Evidence Is Acquired in Forensic Investigations | Chapter 3 from Learn Computer Forensics by William Oettinger

Acquiring digital evidence is one of the most delicate and technically demanding stages of any digital forensic investigation. In Chapter 3 of Learn Computer Forensics (Second Edition), William Oettinger outlines the key procedures, technologies, and best practices used to obtain digital evidence while preserving its integrity and admissibility.

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Why Evidence Acquisition Matters

Digital evidence is volatile. Mishandling during the acquisition phase can result in irreversible loss, contamination, or inadmissible findings. That’s why forensic investigators follow strict protocols to ensure that every byte of data is preserved with forensic precision.

Preparing the Forensic Environment

Before any data is acquired, investigators must:

• Prepare a forensically sterile environment
• Use validated tools like Autopsy, FTK Imager, X-Ways, EnCase
• Verify equipment with tool validation procedures to ensure repeatable, reliable results

Oettinger stresses the use of digital cameras, chain-of-custody documentation, and properly wiped storage media to ensure integrity and compliance with investigative standards.

Write Blocking: Hardware vs. Software

To protect the original evidence, write blockers are used to prevent data modification. These come in two forms:

• Hardware write blockers – e.g., Tableau T35u
• Software write blockers – e.g., PALADIN, WinFE

Write blockers are essential for preserving metadata, timestamps, and deleted files during acquisition.

Creating Forensic Images

The core of digital acquisition lies in imaging:

• Forensic images are bit-for-bit copies of the entire source drive, including unallocated space and deleted files
• Logical images capture specific files or directories of interest
• Special formats like DD, E01 (EnCase), and AFF (Advanced Forensics Format) are used to store image data

Oettinger explains the importance of sterile media and wiping drives before use to avoid evidence contamination. This ensures the forensic container itself does not corrupt or introduce foreign data into the investigation.

Imaging HDDs, SSDs, and Cloud Systems

Different types of storage present unique challenges:

• HDDs (Hard Disk Drives) allow traditional sector-based imaging
• SSDs complicate things with wear leveling, garbage collection, and TRIM commands that can erase data automatically
• Cloud environments require cooperation with service providers and adherence to legal requirements

Verifying Evidence with Hashes and File Signatures

After imaging, investigators validate their data using cryptographic hash functions:

• MD5 and SHA-1 hashes confirm that the image is identical to the source
• File signature verification helps detect hidden or altered files
• Checksum validation ensures ongoing data integrity throughout analysis

Maintaining Chain of Custody

Oettinger emphasizes that none of these steps matter unless the chain of custody is maintained. Proper logging, documentation, and sealed evidence bags ensure that evidence remains credible and legally defensible in court.

Conclusion

Acquiring digital evidence is not just about copying files—it’s a careful, methodical process that requires technical skill, legal knowledge, and investigative discipline. Chapter 3 serves as a comprehensive guide for forensic professionals who want to get acquisition right from the very beginning.

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