Meaning
An exact sector-by-sector duplication of a physical storage medium captures the entire state of a system at a specific moment. This replica, known as a bit stream image, includes all active files, deleted data, and unallocated space. Forensic analysts require this complete copy to perform investigations without altering the original drive.
Acquisition Process
Creating a replica of a drive requires a hardware write-blocker to prevent any modification to the source media. The bit stream image is generated by reading every sector sequentially and writing the data to a destination file. This procedure ensures that even transient system files are preserved for future investigation.
Forensic Soundness
The validity of digital evidence relies on proving that no changes occurred during collection. Generating a bit stream image allows investigators to verify the reproduction through cryptographic hash algorithms. Any discrepancy between the original drive hash and the copy hash reveals tampering.
Data Integrity
Preserving metadata is an essential step in maintaining evidence. While a standard file copy alters access dates, a bit stream image keeps all creation dates, modification logs, and system artifacts intact. This completeness is necessary when disputes arise regarding the timing of file deletions or unauthorized access.
Proving that files were not modified after collection prevents challenges to the evidence in subsequent reviews.
