Forensic Investigation: Not Exactly a Needle in a Haystack

These are some logical areas that may interest an investigator in locating digital evidence:

• File space. This refers to blocks on the drive that either are assigned to an active file or assigned to the file system depending on the structure such as FAT (Windows) or inode (UNIX). Of course viewing interesting files from file space is merely a matter of using a disk editor, locating the file, and copying the file to another media for viewing by the investigator. In this fashion, the original media does not suffer from being changed.

• Slack space. This is the space made up of the file system blocks that are partially used by the operating system. Slack space is prevalent in file systems that have written to a sector, then overwritten that space with the newly written information not occupying the entire sector creating a slack space containing data from the previous data. Tools like EnCase or a disk editor will allow investigators to see the "junk" contained in the slack space. Slack space seldom contains enough information to see the entire file, however there is often enough information to interest investigators. File names, file extensions, and pieces of text files are the usual finds.

• RAM space. RAM space is the term used to describe empty space between the data and the end of the sector. If there is an empty space, the operating system selects information from the data currently in RAM and writes it there. It can be similar to slack space in appearance.

  Experience Note

  An investigator conducting an analysis on a target hard drive was able to effectively refute allegations made by a defendant that he had never installed pirated software on his workstation. The defendant had installed a number of expensive applications on his workstation and deleted them and attempted to write over the disk space. However, there were enough data left in the slack space to demonstrate he had indeed installed these applications. The most incriminating evidence was the extensions of the application's files.

• Unallocated file space. Any unclaimed sector falling within an active partition or not.

• Unclaimed sectors can often be restored by Undelete utilities depending on the operating system and if the unallocated file space is partially overwritten or not.

Physical Level Search

Investigators should consider begin looking at the raw data contained on the target media. Often these analyses are performed with tools like a disk editor or EnCase. With the forensically correct duplicated software, many experienced investigators will perform these principle processes:

• String search

• Slack space

• Free space examination

All analysis operations must be performed on the forensic image or the restored image of the evidence. Never perform examinations on the original evidence.

There is a frequently pursued avenue in running string searches to produce lists of data; for example:

• All e-mail addresses

• All Web site URLs

• All gif and jpeg file extensions

• String searches matching specific words

• String search

Experience Note

There is a very handy DOS-based program called SearchString written by Dan Mares. It is available at www.maresware.com. This tool provides the context of the string search hit as well as the location being the byte offset from the beginning of the file. By inputting the specific string to be searched, this tool will scan the target media and produce the relative location of the item.

Also, most disk editors have well-developed string search capabilities. Many experienced investigators use disk editors to search for file extensions that are pertinent to the case, e.g., eml, png, gif, jpg, doc, txt, or exe.

File Slack and Free Space

Depending on the operating system's file system, there will be residue that can be located and examined when looking for evidence. File residue basically falls into two categories, file slack and free space.

Free space is that space located on a hard drive that is not allocated to a file. It can be space that has never been allocated to a file or space that is considered unallocated. This unallocated condition usually occurs after a file has been deleted. Unallocated file space occurring after a file has been deleted will often contain remnants of the deleted file. Fragmented data previously written could still reside in these areas and not be easily accessible to the everyday user. In order to gain visibility into these areas, it is necessary to work on the physical level.

In the case of slack space, this occurs when data is written to a storage medium in measures that fail to completely fill the block size as it is defined by the operating system. Investigators attempting to look into this area for evidence will also have to work beneath the operating system at the physical level of the medium.

Experience Note

An employee had been downloading obscene images to his work-station and subsequently deleting them. After a time, he performed word processing and other types of work thinking these had overwritten the images he had previously downloaded and would make viewing the images impossible. Fragments of these images and their file extensions were contained within the slack space and unallocated file space of his workstation hard drive. After forensically imaging the hard drive, investigators peered into slack areas using a disk editor. Investigators were aware that most photographic-quality image files have extensions such as .gif, .jpeg, and .png. They merely used the find function of the disk editor to perform a string search for these extensions. Experience and training taught them that deleted files in DOS-based operating systems are preceded by the σ character (lower-case sigma) and are listed with a hexadecimal value of E5h. They easily located the deleted files. After completing their search, they were able to identify the nature of the deleted files by their names and extensions and even recover some of the image fragments.

DOS-Based Operating Systems File Deletions

The file deletion process in DOS-based operating systems is a two-step process. In the first phase, the operating system marks the file entry with a lower-case sigma character× σ. This character has a hexadecimal value of E5h. In phase two, it clears the FAT chain marking all data blocks as empty. In principle, many operating systems handle file deletions in similar fashion.

Using an undelete utility, like Norton's Utility suite, the file recovery software searches the file directory tree for file names beginning with σ and labeled with the value of hexadecimal E5h. Once found, the utility starts at the file cluster offset that is specified in the directory entry. If the file cluster is not claimed by another file in the block allocation table (FAT), then the utility will indicate the file has a good chance of recovery. Many commercial file recovery utilities will reconstruct the deleted file by replacing the sigma character with another recognizable character and rebuild the FAT table. In processing, the utility looks to the file size specified in the directory entry and determines if that block is free. If it is possible, the program will advise that the file has a good chance of being recovered.

Reading E-Mail Headers

As it appears in your e-mail client, it seems that e-mail is passed directly from the sender to the recipient without any intermediate steps. Typically, an e-mail passes through at least four computers in its route. In the case of an ISP whose users connect via dial-up, DSL, Cable Internet, or T1, the client is the user's machine and the actual mail server belongs to the client's ISP. To review the process, when a user sends e-mail, she normally composes the message on her workstation and sends it off to either the mail server located within the company of the ISP. At this point, her workstation usually keeps a copy of the e-mail in the send folder. Even if she deletes the contents of the send folder, the e-mail will reside in the deleted folder until she deletes them from this folder.

Experience Note

It is possible that the e-mail client is configured to automatically empty the deleted folder, but as you have seen, there are ways to recover deleted files.

From her workstation, the e-mail server receives it and the server begins to look for the recipient's e-mail server, exchanging information packets with this server and eventually delivering the e-mail message. It does not really matter whether she is sending her e-mail through the Internet or merely within her own organization. For practical purposes, the process is basically the same. This e-mail will reside on this server until the recipient accesses his e-mail client and reads the e-mail. Of course, there are times depending on the type of e-mail configuration and the type of e-mail server, the e-mail server retains a copy of the e-mail or downloads the e-mail to the recipient's e-mail client located on the workstation. It is very possible that although the e-mail was downloaded to the recipient's workstation and the account emptied of the e-mail, there is a copy of the e-mail located on the e-mail server's backup storage. Tenacious investigators will pursue the chances of obtaining a copy of the e-mail from one of the many e-mail servers involved in the message transmission and receipt.