COEN 252 Computer Forensics

Forensics Process for Hard Drive Examination

Best Evidence

Best evidence is original evidence. FRE: multiple copies of electronic

files are considered “original”. Evidence need to be protected

against Normal accidents. Accidents in the analysis process. Tampering.

Best Evidence

Computer data are “writings” and “records”.

Need to be authenticated. Whoever collected them should

testify during direct examination that the information is what the proponents claim.

Best Evidence

If not authenticated, documents and writings are usually inadmissible.

Hence, careful record keeping needed.

In addition to careful handling of evidence.

Best Evidence

Chain of Custody Protects evidence against

advertent or inadvertent tampering.

Evidence needs to be traced from the moment it was collected to the moment it was presented in judicial proceedings.

Best Evidence

Hard drive yields a byte stream. Protect with cryptographically

secure checksum A.k.a. hash A.k.a. signature

Best Evidence

Cryptographically secure checksum Small byte string calculated from

byte stream X as f (X). Given c, “computationally

impossible” to find a byte stream X with f (X) = c.

I.e. nothing better than brute force, which takes too long.

Change even a bit in X and f (X) changes.

Best Evidence

Collect c =f (X) from original byte stream.

Maintain c securely, e.g. with evidence log.

Prove that X has not changed by recalculating c = f (X ).

Best Evidence

Cryptographically secure checksums MD5 (a classic, 16B checksum)

Considered broken SHA1 (a classic 20B checksum) SHA256 etc. (much longer

checksum)

What is computationally possible, changes with progress.

Best Evidence

Chain of custody for physical objects Inventoried and labeled by

evidence custodians. Booked into evidence locker. Access documents by evidence

technician.

Evidence Collection from Computer Media Identify computer media. Detailed report of situation.

Evidence custodian inventories best evidence and logs it in the Evidence log.

Perform a forensic duplication of the original media to storage media.

If the original can be kept, use it as the best evidence. If not, duplicate the data immediately, and use the media as the best evidence. Label the best evidence and store it in evidence locker.

Make a duplicate of the best evidence and use it for forensic analysis.

Make more working copies as necessary, e.g. in order to mount the file system for quasi-life investigations.

Make backup of best evidence.

Evidence Handling Procedures Before examining the contents of a hard

drive, record information about the computer system.

Take digital photographs of the system and the media that is being duplicated.

Fill out an evidence tag for the original media and / or for the forensic duplicate.

Label all media appropriately with an evidence label.

Evidence Handling Procedures Store the best evidence copy in the

evidence locker. An evidence custodian enters a record

of the best evidence into the evidence log. Each access to the best evidence is also entered into the log.

All examinations on the forensics copy are performed on a forensic copy, the working copy.

Evidence Handling Procedures An evidence custodian ensures that

backup copies of the best evidence are created.

An evidence custodian ensures that all disposition dates are met. The dates are assigned by the principal investigator.

An evidence custodian performs a monthly audit to ensure all of the best evidence is present, properly stored, and labeled.

Evidence Handling ProceduresEvidence System Description

Describe Location Individuals

Who occupy the room or office where the original evidence was found.

Have access to it. Who can actually use it. Who are present.

Evidence Handling ProceduresEvidence System Description Location of system in room. State of the system

Powered on/off. Data on screen. Time/date of system BIOS Network connections. Serial numbers, make etc. of hard drives

and peripherals. Peripherals attached to the system

Evidence Handling ProceduresDigital Photos Used

For protection against unwarranted claims.

To ensure returning the system to the exact state prior to forensic duplication.

To capture current configuration. For investigative hints.

Label photos clearly, make log entries for pictures taken.

Evidence Handling ProceduresEvidence Tags

Needs to satisfy federal and state guidelines.

Contains info on Place, Person from whom item was received. If item requires consent to search. Description of item. If the item is a storage device, what is contained in

it. Date / time when taken. Full name and signature of individual initially

receiving the evidence. Case and tag number related to the evidence.

Evidence Handling ProceduresEvidence Labels

Label physical items, i.e. suspect hard drive With case number and evidence tag

number. Date and time evidence was

collected. A brief description of the items

contained within the envelope.

Evidence Handling ProceduresEvidence Pouch

Evidence Handling Procedures

Evidence Handling Procedures

Evidence Handling Procedures

Evidence Handling ProceduresEvidence Storage

Investigator needs to maintain positive control of the evidence at all times.

Evidence protection includes protection against the environment such as electromagnetic fields.

Evidence Handling ProceduresEvidence Log

Every time evidence is accessed, log Evidence tag number Date Action taken Consultant performing the action Identify information on the action Audits

Evidence Handling ProceduresEvidence Disposition

Initial disposition: Analysis and case finished. Working copies are no longer needed. Only backup might be needed.

Final disposition: Backups are no longer needed.

Evidence Handling ProceduresAudits Labs should get certified for their

procedures. Internal audit to

Ensure compliance with internal standards by reviewing evidence locker access log forms.

Perform an inventory. Check disposition requirements. Perform checks for needed backups. Review case folders.

Develop goal of investigation and stick to it.

Analyst should make and state findings.

An analyst that offers opinions, is an expert witness.

Data Analysis

More art than science. Feed-back with investigator. Needs accurate documentation. Analyst needs to be able to testify

months after analysis. Findings need to be repeatable.

Data Analysis

Report

Report all steps in the investigation.

Immediately Clearly

Testify

Report

Accurately describe details of an incident.

Be understandable to decision makers.

Be able to withstand a barrage of legal scrutiny.

Be unambiguous and not open to misunderstanding.

Be easily referenced (Bates numbers)

Contain all information required to draw conclusions.

Report