1. Computer Forensics BACS 371
File Slack Summary

2. Places to hide evidence
4/14/2017
Places to hide evidence
Evidence can be hidden in many places within a disk.
The notion of “empty space” on a disk is more complicated than you might suspect.
The question becomes “what are the different types of empty space?”
Unused (bad blocks, deleted file space, network protocol fields)
Unallocated space (free space, unused sectors)
File slack (unused space at the end of each file)
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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 Physical File Size File Slack
Exact size of contents of file in bytes
Physical File Size
Amount of space a file occupies on disc in bytes
File Slack
Unused space between logical end of file and physical end of a cluster
Two types: RAM slack and Disk Slack
Physical File Size
<- Logical File Size ->
<- File Slack ->

5. File Slack What does File Slack Contain? Who knows??!!
Old data that was deleted but not overwritten yet
May contain remnants of older files, or other evidence including
Passwords
Old directory structures
Miscellaneous information ….

6. File Slack Example Hello World! Has 12 Characters in the file
But occupies 4096 bytes on the disk!

7. File Slack Example

8. File Slack Example File Contents: “Hello world!” 12 bytes 3rd Sector
Disk Slack:
4096 Bytes – 512 Bytes
= 3584 Bytes
RAM slack – to the end of the sector
DISK slack – to the end of the cluster
Assumptions:
Sector Size = 512 Bytes
Cluster Size = 4KB = 8 Sectors
2nd Sector
RAM Slack:
512 bytes – 12 bytes = 500 bytes

9. File Slack Summary RAM Slack Disk Slack
Unused space at the end of a sector. Contains information adjacent to the stored information from Main Memory (RAM).
Example: The file has only 12 characters, but must write a minimum 512-byte block to the disk – the other 500 characters are whatever happen to be in RAM at the time.
Disk Slack
Unused space at the end of the cluster. Contains information left over on the disk from prior files.
Example: The file system must always write in multiples of clusters (4096 bytes in this case.) The other 3584 bytes (7 sectors) are filled with whatever used to be in the clusters before they were marked for deletion.