Evidence Acquisition, Handling, and Chain of Custody Questions

Intended answer (Digital Forensic Examiner perspective)

Authorized roles by stage

• Intake: evidence custodian or intake officer (forensically trained technician) signs and a supervising examiner or chain-of-custody (CoC) coordinator witnesses.
• Transfer (internal movements): transferring technician signs, receiving custodian signs; supervisor/coordinator witnesses.
• Analysis: examiner assigned to case documents start/finish of analysis; a different peer (or supervisor) witnesses critical transfers of custody (e.g., device to workstation, write-blocker removal).
• Release/Return: evidence custodian signs release, receiving agency/owner signs receipt; CoC coordinator or supervisor witnesses.

Segregation-of-duties principles

• Separate custody, analysis, and authorization: the person performing analysis should not be sole authorizer of evidence release.
• Require dual control for critical steps (signatory + witness).
• Role-based permissions in LIMS to enforce who can sign versus who can only witness.

Why limit signatory permissions

• Legal defensibility: shows independent checks, reduces risk of tampering allegations, and supports admissibility under chain-of-custody scrutiny.
• Internal control: minimizes fraud/error, provides audit trail, and preserves impartiality of examiners.

I would recommend documenting these rules in SOPs and enforcing via LIMS RBAC and periodic audits.

Situation / Goal
As a forensic examiner I must acquire a forensically sound copy of a production hardware RAID‑5 for offline analysis and reconstruction while preserving evidentiary integrity.

Immediate preservation steps

• Photograph chassis, controller model/firmware, cable layout, bay order, LED status.
• Place write‑blockers between any host and disks; if controller prevents write‑blocking, power down and remove drives.
• Log chain of custody, timestamps, serials, slot numbers and controller settings.

Imaging individual disks

• Image each physical disk end‑to‑end (including slack/unpartitioned areas) with forensically sound tools (dc3dd/Guymager/FTK Imager/EnCase).
• Calculate and record hashes (MD5/SHA256) before and after imaging.
• Preserve original drives when possible; work from bitstream images.

Documenting controller metadata

• Dump controller metadata/firmware settings (if accessible) — many hardware controllers store RAID maps or labels in disk headers or NVRAM.
• Record RAID ID, disk order, stripe (chunk) size, initialization time, parity algorithm, and any logical volume IDs.
• Tools: vendor management utility snapshots, controller export logs, or hex reads of disk headers to capture metadata signatures.

Controller‑level exports

• If controller supports “export”/“foreign import” or virtual disk export, export metadata but avoid operations that alter parity or sync; capture logs/screenshots of export operations.
• If controller can create a safe readonly image of the virtual disk, document steps and hash the result.

Reconstruction of logical volume

• Use documented disk order, stripe (chunk) size and parity rotation to map images back into the logical volume.
• Example: 4‑disk RAID‑5, chunk = 64 KB. Each stripe row holds 3 * 64KB data = 192KB (stripe width). Stripe_number = floor(logical_offset / stripe_width).
• Parity disk for a stripe (rotating parity) = (stripe_number mod N). Data disk index = compute offset within stripe and skip parity disk to locate which image contains the data.
• Practical tools: mdadm --build (Linux) with --chunk size and --assume-clean; or UFS Explorer / R‑Studio / RAID Reconstructor to assemble from individual images. For mdadm:

mdadm --build /dev/md0 --level=5 --raid-devices=4 --chunk=64K \
   --metadata=0.90 /path/disk0.img /path/disk1.img /path/disk2.img /path/disk3.img

• Mount read‑only and verify filesystem integrity, compare known file metadata, and rehash reconstructed image to confirm mappings.

Stripe size and offsets

• Stripe (chunk) size determines alignment: mis‑specifying chunk size or disk order shifts all data and breaks reconstruction.
• Calculation summary:

stripe_width = chunk_size * (N - 1)
stripe_number = floor(logical_offset / stripe_width)
parity_disk = stripe_number mod N
data_offset_within_stripe = logical_offset mod stripe_width

• Use these to determine which disk image and byte offset hold each logical block.

Validation and documentation

• After assembly, compute hashes of reconstructed volume and compare expected filesystem artifacts.
• Document every command, parameter, and rationale in the forensic report so findings are reproducible and defensible in court.

This approach preserves evidence, captures controller context, and allows accurate logical reconstruction by correctly handling stripe size, disk order and parity rotation.

Overview
As a forensic examiner I’d implement a cryptographically verifiable redaction workflow so the third party can trust both the original evidence existed and the redacted artifact’s integrity without exposing irrelevant PII.

Workflow steps

1. Pre-redaction imaging & hashing

• Create a bit-for-bit forensic image (E01/RAW) and compute strong hashes (SHA-256 and SHA-512).
• Record: case ID, examiner, tool/version, host, UTC timestamp, device serial, and hash values in the custody log.
• Example log entry fields: { case_id, image_path, sha256, sha512, examiner, tool, host, ts }.

2. Produce a machine-readable redaction map

• Use a canonical JSON schema: { case_id, original_hashes:{sha256,sha512}, redaction_steps:[ { step_id, offset_start, offset_end, byte_range_hash, reason, classifier_rule_id } ], tool, operator, ts }.
• Include byte-range hashes (SHA-256 of each removed region) and human-friendly reason codes. Keep map immutable (signed).

3. Pre-redaction attestations (commitments)

• Sign the pre-redaction image hashes and the redaction map with the examiner’s private key (PKI). Produce detached signatures (e.g., CMS/PKCS#7 or OpenPGP). Store signatures in custody log.
• Optionally compute a Merkle tree over preserved vs removed segments and publish the root as a compact commitment.

4. Perform redaction (deterministic tool)

• Use a vetted deterministic redaction tool that records exact byte changes (zeroing, XOR mask, or replacement token). Generate the redacted artifact and log the tool commandline, version, configuration, and machine GUID.
• After redaction, compute post-redaction hashes (sha256/sha512).

5. Post-redaction attestations & artifacts package

• Produce signed artifacts: { redacted_file, redaction_map.json, pre_hash_signature, post_hash_signature, tool_logs } signed by examiner.
• Add a verification instruction file describing verification steps.

6. Chain-of-custody logging

• Each custody event (imaging, map creation, redaction, signing, release) gets an append-only log entry: { event_id, actor, action, artifact_refs, hash_before, hash_after, signature_ref, ts }.
• Store logs in WORM storage or append-only ledger (e.g., write-once S3 + object versioning, or blockchain-backed audit) and snapshot/export to case folder.

How third party verifies

• Validate signatures on pre- and post-hash and on the redaction_map using the examiner’s public key / CA chain.
• Confirm pre-image commitment: check original_hashes match the signed pre-hash (proves original existed).
• Apply redaction_map to the original image if granted access; if original not provided, they can verify consistency by checking: (a) byte-range hashes for removed regions and (b) post-redaction hash equals hash of redacted_file. If original withheld, the signed pre-hash + signed map + byte-range hashes provide verifiable commitment that redactions correspond to particular offsets.
• Verify custody log signatures/timestamps chain.

Notes, trade-offs, and best practices

• Use deterministic redaction (no metadata leaks), canonical JSON for maps, SHA-2 family, PKI-signed attestations, and WORM logging.
• Preserve original image offline; only share redacted artifact, redaction map, and signed attestations unless a court orders full disclosure.
• Retain tool versions and reproducible commands to support reproducibility and expert testimony.

Clarify requirements & constraints

• Input: acquired forensic images (E01, AFF, raw dd) from field kits or automated collectors.
• Must: verify integrity (whole-file hashes), extract metadata (host IDs, timestamps, tool/version), segment images and calculate per-segment hashes, store artifacts in evidence DB, alert SIEM on mismatches/anomalies, and maintain courtroom-quality chain of custody.

High-level architecture

• Ingest Gateway (SFTP/HTTPS/SMB/watch folder) → Validation & Parsing Engine → Segmenter & Hasher → Metadata Extractor → Evidence Database (WORM-backed) + Object Store (immutable blobs) → Audit Ledger (append-only) → Alerting/Rules Engine → SIEM/Case Mgmt UI

Core components & responsibilities

• Ingest Gateway: authenticate uploader (MFA + client certs); tag source agent ID and intake timestamp.
• Validation Engine: compute cryptographic hashes (SHA-256, SHA-512) of full image; compare against producer-supplied hash; verify image format signatures.
• Segmenter & Hasher: split into fixed-size segments (e.g., 10 MB) and compute segment hashes (Merkle tree optional for efficient verification).
• Metadata Extractor: parse examiner notes, imaging tool/version (e.g., FTK Imager vX), host identifiers (hostnames, MACs), acquisition timestamps, and capture provenance headers (E01 metadata).
• Evidence DB & Object Store: store metadata, hash values, and pointers to immutable blobs. Use write-once-read-many storage with object immutability and versioning.
• Audit Ledger: append-only ledger (blockchain-style or signed logs) recording every action (who, what, when, hash before/after, tool version) with digital signatures.
• Alerting/Rules Engine: detect full/segment hash mismatches, abnormal metadata (mismatched host IDs, inconsistent timestamps), unusual tool versions; forward alerts to SIEM via syslog/CEF/JSON API and create case tickets.
• Case Management UI / API: allow authorized access, export chain-of-custody reports, and produce court-ready attestations.

Interfaces

• Ingest: REST API and secure file transfer endpoints; mutual TLS + RBAC.
• SIEM: CEF/LEEF over TLS, with structured JSON payloads for automated ingestion.
• Forensic Tools: standard E01/AFF parsers and YARA/regex hooks; plugins for EnCase/FTK exports.
• Audit Access: signed logs accessible via API; cryptographic verification endpoints to re-hash and validate stored blobs.

Audit, logging & chain-of-custody

• Log every event with: timestamp (UTC), actor ID (human or agent), action, original and computed hashes, tool/version, source IP, and digital signature.
• Store logs in immutable WORM storage and replicate to secondary region.
• Maintain cryptographic chain: hash(image) → store blob → record hash in ledger signed by ingestion key. Any subsequent access or modification generates a new signed ledger entry that references prior entry.
• Retention & legal: configurable retention, exportable COC report with signed timestamps, audit trail, and verification instructions for third parties.

Security & compliance

• Keys stored in HSM; strong role separation; tamper-evident hardware for field acquisitions; periodic revalidation jobs that re-hash segments and compare to stored segment hashes; SIEM escalations for divergences.

Operational notes

• Use Merkle trees for efficient partial revalidation during triage.
• Alert thresholds: immediate for full-hash mismatch; contextual scoring for metadata anomalies.
• Test with controlled tamper and chain-of-custody exercises; document SOPs for court testimony.

Overview
Below is the per-step chain-of-custody documentation I would produce when imaging a suspect workstation. Each entry is dated, timestamped, signed/initialed by the operator.

1. Pre-imaging checks (document)

• Case ID, investigator name & badge/ID number, contact info, and receiving agency.
• Location, room, and owner/possessor present (if any).
• Device power state, plugged peripherals, network connections, battery level (if laptop).
• Reason for search/seizure, legal authorization reference (warrant number/consent).
• Observed condition and any visible damage.

2. Device identification

• Manufacturer/make, model, full serial number, asset/tag numbers.
• MAC addresses (if visible), installed OS version (if accessible).
• Photograph label/serial and overall device from multiple angles.

3. Write-blocker usage

• Write-blocker brand/model and serial number.
• Attach time, disconnect time, and operator initials.
• Photo of write-blocker connected showing cable orientation and device LED status.

4. Imaging command, tool & version

• Tool name/version (e.g., FTK Imager 4.5.0.123, dd 1.12).
• Exact imaging command or GUI steps used (copy full dd command with flags or FTK log).
• Destination media ID, capacity, serial number, and storage chain (e.g., evidence drive E01 path).

5. Hash values pre/post-imaging

• Acquisition (source) hash(s) if possible (MD5/SHA1/SHA256) calculated before attaching write-blocker or via hardware hash device.
• Image file hash(s) immediately after imaging and after transfer/verification.
• Hash algorithm(s) used, timestamp, and tool/version that computed hashes.

6. Photographic evidence

• Wide shots of workstation in situ, close-ups of ports/labels, write-blocker connections, evidence drive labels, and imaging workstation screen showing tool/command output.
• Photos labeled with date/time and operator initials.

7. Operator identity & handling log

• Continuous chain-of-custody log entries: who handled evidence, when, why, and signature.
• Storage location and access conditions (e.g., evidence locker number).

8. Deviations from procedure

• Any unexpected behavior (e.g., powered-on device, encryption, hardware failure) documented with time, actions taken, authorization for deviation, and supervisory approval/signature.
• Mitigations performed and impact assessment on evidence integrity.

9. Final packaging & transfer

• Sealing method, seal ID, packer’s name, date/time, transport method, and recipient signature upon transfer.

Each document is attached to the case file (digital and printed) and signed. This provides auditable proof of integrity and handling suitable for legal proceedings.