Advances in File Carving

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  • Simple Carver Suite - www.simplecarver.comCarvFs - - - Rescue -


  • 1. Advances in File Carving
    Rob Zirnstein, President
    Forensic Innovations, Inc.
  • 2. Our Data is GONE!
    All of your servers have Crashed!
    Your customers’ Data is Lost!
    You backed up last week, but important business transactions have taken place since.
    70% of companies with devastating data loss go out of business.
    All it took was one employee writing a simple SQL database script after you fired them.
  • 3. We Didn’t Find The Evidence!
    What do you do when you’ve searched through all of the evidence and came up empty?
    When you know a suspect is hiding something, where do you look first?
    TrueCrypt Volumes & Unallocated Space
    Even good people shred data when faced with an investigation.
    The tools are easy to find.
  • 4. How They Hide the Evidence
    Deleting a file
    Sends the file to the Windows Recycle Bin
    Empty or bypass the Recycle Bin
    Undelete tools depend on the deleted directory entry
    That can be deleted or overwritten too
    Then there’s no undeleting possible
    Store files in a TrueCrypt Volume
    Undetectable as a file (except for my tools)
    Looks like random data in unallocated space (except for my tool)
  • 5. How To Get The Files Back
    File Carving
    Definition: “General term for extracting data (files) out of undifferentiated blocks (raw data), like "carving" a sculpture out of soap stone.”
    The sectors containing the files are orphaned
    Some of them may get overwritten
    They are like many jigsaw puzzles thrown into a trash bag, if they were fragmented.
    If some sectors were stored consecutively, then it’s like puzzle pieces that weren’t pulled apart before getting trashed.
  • 6. File Carving Assumptions
    No Files are Fragmented!?!
    All Files are stored in consecutive sectors
    Sector Size = 512 bytes
    May be detected through disk structure
    Cluster Size = 512 to 16,384 bytes
    May be detected through disk structure
    File Slack may be ignored
    RAM slack is ignored
    Or incorrectly bundled in with File Slack
    Isn’t it always zeroed out?
  • 7. File Carving Techniques
    Block Based Carving
    Statistical Carving
    Header/Footer Carving
    Header/Maximum File Size Carving
    Header/Embedded Length Carving
    File Structure Based Carving
    Semantic Carving
    Carving with Validation
    Fragment Recovery Carving
    Repackaging Carving
    Hash Carving
    Fuzzy Hash Carving
  • 8. Block Based Carving
    Analyze each sector on a block-by-block basis to determine if they belong together in the same file.
    Assuming that each sector can only be part of a single file
  • 9. Statistical Carving
    Use statistics or content characteristics to identify each sector.
    Entropy measurement
    Filter out blocks that clearly aren’t part of a desired file type.
  • 10. Header/Footer Carving
    Search for file header signature(s).
    Search for the matching file footer signatures.
    Capture the sectors in between.
  • 11. Header/Maximum File Size Carving
    Search for file header signature(s).
    Consult a list of maximum file lengths for each header type.
    Capture the sectors in between.
    Many file types do not detect the additional unrelated data that may get appended to the recovered file.
  • 12. Header/Embedded Length Carving
    Search for file header signature(s).
    Read the file length from one of the fields in the header.
  • 13. File Structure Based Carving
    Once a sector’s file type is identified
    Match to other sectors that contain similar data structures.
    Use knowledge of the file type’s data structures to search for structure parts expected to exist in later sectors.
  • 14. Semantic Carving
    Identify the language used in a sector.
    Identify the language used in each of the following sectors
    Collect the sectors that are written in the same language
  • 15. Carving with Validation
    Use a file interpreter or viewer to load each recovered file.
    If the interpreter encounters invalid data, assume that is the point where the carving method failed.
    Use on completed files.
    Use on each added sector.
  • 16. Fragment Recovery Carving
    Find two or more fragments that belong to the same file.
    Filter out the sectors between the fragments that don’t belong.
  • 17. Repackaging Carving
    Used on partially recovered files.
    Rebuild the parts of the file that were not able to be recovered.
    The result should be a file that can be opened with it’s native application or a standard viewer.
  • 18. SmartCarving
    Use knowledge of the file system’s typical fragmentation effects.
    Preprocess the source sectors.
    Decompress, decrypt or translate the data
    Collate the identified blocks.
    Sort by file type
    Reassemble the blocks in sequences that match their file type.
  • 19. Hash Carving
    Calculate a hash value for each sector
    MD5, SHA-1
    Compare the hash value to a list of known sector hash values
    This list can be of known Good and/or known Bad files.
    Filter out known Good files. (ex: Installed applications)
    Recover known Bad files. (ex: known illicit material)
  • 20. Fuzzy Hash Carving
    Calculate a fuzzy hash value for each sector.
    Compare the fuzzy hash values of sectors to determine which sectors are similar in content.
    Combine similar sectors into recovered files.
    Match raw data sectors together for object types that have no identifiable signatures or that extend beyond a single sector.
    Recover file types not previously encountered.
  • 21. Tools Today (1)
    Adroit Photo Recovery/Forensics
    combination of SmartCarving, header carving, structure based validation and validation of the entire file to determine if each new sector belongs; Repackaging Carving is also available;
    Supports JPEG, RAW camera images, PNG, BMP and GIF files
    header-footer carving; Supports 25 file types
    header-footer carving; Supports ~250 file types
    file structure based carving for avi, bmp, doc, gif, hmlt, jpg, mov, pdf, png, rar, wav and zip files.
    header-footer carving for art, asf, chm, cookie, cpp, dat, dbx, fws, idx, java, lnk, mail, mbx, mp3, mpg, ost, pgd, pgp, ppt, pst, ra, rdp, rpm, tif, txt, wma, wmv, wpc and xls files.
    Forensic Toolkit (FTK)
    internal techniques unknown; Supports abl, aol, asd, bmp, doc, dot, emf, gif, html, jpg, mpp, one, pdf, png, ppt, pub, puz, vsd, vss, vst, xla, xls and xlt files.
  • 22. Tools Today (2)
    HstEx / Netanalysis
    internal techniques unknown; Supports browser history formats
    NFI Defraser
    Fragment recovery carving & carving with validation; Supports MPEG, 3GPP, Quicktime & AVI files
    combination of file structure based carving and header-footer carving of 80 file formats
    appears to use a simple text search method, ignoring sector boundaries; Supports server log file formats
    Recover My Files
    internal techniques unknown; Supports 200 file types
    SmartCarving; Supported file types list not available
  • 23. Tools Today (3)
    combination of header-footer and header-maximum file size carving; Supports art, avi, dat, dbx, doc, fws, gif, htm, idx, java, jpg, mail, max, mbx, mov, mpg, ost, pdf, pgd, pgp, pins, png, pst, ra, rpm, tif, txt, wav, wpc and zip files.
    header-footer carving; unknown support list
  • 24. Tool Problems
    Few tools handle file fragmentation
    The tools that handle fragmentation support very few file types
    Most tools can not detect false positives
    Most tools hard code file type support
    Only 1 tool claims to rebuild partial files
    It only supports 5 file types (image files)
    Performance is a problem
    most tools utilize inefficient databases and scripting languages
  • 25. Future Tools
    Carver 2.0
    Open Source, in the early specification stages
    File Harvester
    Combination of multiple methods:
    Block Based Carving
    Statistical Carving
    Header/Footer Carving
    Header/Embedded Length Carving
    File Structure Based Carving
    Fragment Recovery Carving
    Repackaging Carving (Phase 3)
    Fuzzy Hash Carving
    (secret sauce)
  • 26. Thank you
    Rob Zirnstein
    (317) 430-6891