BACS 371 Computer Forensics

1. BACS 371Computer Forensics Files, Partitions & File Systems

2. Data Hierarchy Computer Hard Disk Drive Partition File Physical File Logical File Cluster Sector Word Byte Bit

3. File • Collection of Information written to a disk • Generally created in an application-specific format • Occupies a fixed number of clusters • Each file’s cluster has a pointer to the next cluster in the file • The final cluster contains the End of File (EOF) marker

4. Files • Logical File Size • Exact size of contents of file in bytes • Physical File Size • Amount of space a file occupies on disc in bytes • Disk Slack • Unused space between logical end of file and physical end of a cluster • This is different from RAM slack which we will discuss next Physical File Size <- Logical File Size -> <- Disk Slack ->

5. Disk & Ram Slack Example File Contents: “Hello world!” 12 bytes 3rd Sector Disk Slack: 4096 Bytes – 512 Bytes = 3584 Bytes 1stSector • Assumptions: • Sector Size = 512 Bytes • Cluster Size = 4KB = 8 Sectors 2nd Sector RAM Slack: 512 bytes – 12 bytes = 500 bytes

6. Partitions • A partition is a logical volume within a physical volume (i.e., disk). • The Master Boot Record (MBR) of a disk defines the partitions found on the physical disk. • An MBR can define 4 primary partitions (max). • These partitions can be defined as “logical partitions.” • Logical partitions are capable of being further subdivided into smaller logical partitions.

7. Master Boot Record (MBR) • Executable Code • Machine Language Code • Processor Specific • Decodes Partition Table • 446 bytes long 446 • Partition Table • 4 Entries • First Entry Starts at offset 0x01BE • MBR “Signature” • 0x55AA

8. Disk Partitions (Physical and extended) Master Boot Record Unallocated space 3rd Partition (Extended) 2nd Partition (physical) PBR 1 1 2 2 3 3 4 4 Logical Partition #1 Logical Partition #2 Logical Partition #3 PBR Extended MBR PBR = Partition Boot Record 1st Partition (physical) PBR PBR PBR

9. Partition Layout http://www.microsoft.com/library/media/1033/technet/images/prodtechnol/winxppro/reskit/ch28/f28zs07_big.jpg

10. Extended Partition Layout http://www.microsoft.com/library/media/1033/technet/images/prodtechnol/winxppro/reskit/ch28/f28zs07_big.jpg

11. MBR with Extended Boot Record 446

12. Extended Boot Record 446

13. FAT File System

14. File Systems(See http://www.ntfs.com) • A method for storing and organizing computer files and the data they contain to make it easy to find and access them • File System Types • FAT (File Allocation Table) • FAT12 • FAT16 • FAT32 • exFAT • NTFS (New Technology File System) • Functions • Manage “free space” • Allocate clusters to file • Track time (MAC – Modify, Access, Create)

15. A FAT file system is composed of four different sections. • The Boot Sector (aka Partition Boot Record, BIOS Parameter Block, Drive Parameter Block or Reserved Sector). This is always the first sector of the partition and includes some basic file system information (in particular, its type), pointers to the location of the other sections and the operating system's boot loader code. • The FAT Region. This contains two copies of the File Allocation Table for the sake of redundancy, although the extra copy is rarely used, even by disk repair utilities. These are maps of the partition, indicating how the clusters are allocated. • The Root Directory Region. This is a Directory Table that stores information about the files and directories in the root directory. With FAT32 it can be stored anywhere in the partition, however with earlier versions it is always located immediately after the FAT Region. • The Data Region. This is where the actual file and directory data is stored and takes up most of the partition. The size of files and subdirectories can be increased arbitrarily (as long as there are free clusters) by simply adding more links to the file's chain in the FAT. Note however, that each cluster can be taken only by one file, and so if a 1KB file resides in a 32KB cluster, 31KB are wasted.

16. FAT File System This is all contained within a partition.

17. Partition Boot RecordAKA File System Boot Sector • The first physical sector in a logical volume • C 0, H 1, S 1 for first partition • Contains • Code • File System Specification Information

18. Partition Boot Record (PBR) BIOS Parameter Block • Executable Code • Machine Language Code • Processor Specific • Decodes BPB • Searches for OS • PBR “Signature” • 0x55AA

19. Partition Boot Record (FAT32 File system) • 010 - 210 Jump Instruction (3 bytes) • 310 - 1010 OEM ID (8 Bytes) • 1110 - 8310BIOS Parameter Block (BPB) (includes all below plus additional fields) all offsets in this section are from start of the BPB counting from 0 offset 1110 Bytes Per Sector 2 Bytes offset 1310 Sectors Per Cluster 1 Byte offset 2110 Media Descriptor 1 Byte offset 2410 Sectors Per Track 2 Bytes offset 2610 Number of Heads 2 Bytes offset 2810 Hidden Sectors 4 Bytes offset 3210 Total Sectors 4 Bytes • 6210 - 51110 Bootstrap Code (448 Bytes) • Ends with 55 AA NOTE: Offsets are from start of Partition, not start of Drive!

20. Decoding a Partition Boot Record (BIOS Parameter Block – BPB) • Jump Instruction • Offset 0 10 • 3 bytes • OEM Name • Offset 3 10 • 8 bytes • Decode as ASCII • “MSDOS5.0” • Bytes Per Sector • Offset 11 10 • 2 bytes • Decode as Number • (Swap “endian”) • 0x0200 = 512 • Sectors Per Cluster • Offset 13 10 • 1 byte • Decode as Number • 0x08 = 8 • 8 * 512 = 4096 bytes/cluster • Media Type • Offset 21 10 • 1 byte • Decode from Table • 0xF8 means HD • Heads • Offset 26 10 • 2 bytes • Decode as Number • (Swap “endian”) • 0x00FF = 255 • Sectors per Track • Offset 24 10 • 2 bytes • Decode as Number • (Swap “endian”) • 0x003F = 63 • Total Sectors • Offset 32 10 • 4 bytes • Decode as Number • (Swap “endian”) • 0x000E37BA = 931,770 • 477,066,240 Bytes • FAT Size (Sectors) • Offset 36 10 • 4 bytes • Decode as Number • (Swap “endian”) • 0x0000038D = 909 • 465,408 Bytes (*512) • 58,176 Entries (/4) • 238,288,896 bytes addressed • (*4096) • File System Type • Offset 82 10 • 8 bytes • Decode as ASCII • “FAT32 ”

21. Partition Boot Sector Decoded

22. File Allocation Table • A partition is divided up into identically sized clusters, small blocks of contiguous space. Cluster sizes vary depending on the type of FAT file system being used and the size of the partition, typically cluster sizes lie somewhere between 2KB and 32KB. Each file may occupy one or more of these clusters depending on its size; thus, a file is represented by a chain of these clusters (referred to as a singly linked list). However these chains are not necessarily stored adjacently on the disk's surface but are often instead fragmented throughout the Data Region. • The File Allocation Table (FAT) is a list of entries that map to each cluster on the partition. Each entry records one of five things: • the address of the next cluster in a chain • a special end of file (EOF) character that indicates the end of a chain • a special character to mark a bad cluster • a special character to mark a reserved cluster • a zero to note that that cluster is unused

23. FAT Content • Database of • File names • Directory names • Date and time stamps (MAC) • Modify • Access • Create • Starting cluster number • Attributes • Archive • Hidden • System • Read Only • Located on outermost track of disk

24. File Allocation Table (FAT)

25. Directory table • A directory table is a special type of file that represents a directory (nowadays commonly known as a folder). Each file or directory stored within it is represented by a 32 byte entry in the table. Each entry records the name, extension, attributes (archive, directory, hidden, read-only, system and volume), the date and time of creation, the address of the first cluster of the file/directory's data and finally the size of the file/directory. • Aside from the Root Directory Table in FAT12 and FAT16 file systems which occupies the special Root Directory Region location, all Directory Tables are stored in the Data Region. • Legal characters for DOS file names include the following: • Upper case letters A-Z • Numbers 0-9 • Space (though trailing spaces are considered to be padding and not a part of the file name) •  ! # $ % & ( ) - @ ^ _ ` { } ~ ' • Values 128-255

26. Directory to FAT interaction

27. Root Directory

28. FAT32 • 32-bit Cluster Numbers • Only 28 bits actually used • Addresses 228 Clusters (~ 268,435,438) • Drive sizes ~ 1TB (228 clusters * 4096 Bytes per cluster ~ 1.1TB) • WinXP limited to 32GB using FAT32 • Max file size in FAT32 is 232-1 bytes ~ 4GB

29. Advantages of FAT32 over FAT16 • FAT32 offers smaller cluster sizes -> less wasted space • FAT32 systems can reallocate and change the size of the root directory • FAT32 drives can contain a copy of the boot record(s) –> less prone to failure • Allow for long file names

30. Long File Names “Trick” • Phony entries are added to the Directory Tables • Entries are marked with a volume label attribute • Each phony entry can contain up to 13 UTF-16 characters (26 bytes)

31. Long File Names Entries Red entries are short file name entries. Blue are for a long file name. Read the long filename entries from the bottom to the top. Note that first byte in each group of long filenames are 01, 02, 03, 04, 05, and 06 (or’ed with 40 to indicate the last segment). Filename entries have 0F in 12th byte. Directory entries have a 10 in this position (indicating a directory).

32. FAT Root Directory Volume ID Directory Entry Single Directory Entry for a file with a “short” filename. Multiple Directory Entries for a file with a “long” filename. There are 4 entries to contain the long file name, and 1 entry to contain the complete set of file information including the “short” file name. • Designates Attribute Bits • 0x08 = Volume Label • 0x20 = Archive • 0x0F = Long File Name

33. File System Comparisons

34. File System Features

35. Overall Performance Criteria NTFS5 NTFS FAT32 FAT16