Overview of Recovering Data from SD Cards
SD cards, or secure digital cards, are removable flash memory cards used in devices like digital cameras, smartphones, and tablets to expand storage capacity (Cellebrite, 2020). SD cards can hold large amounts of data including photos, videos, text messages, call logs, browsing history, GPS information, and more. This makes them a crucial source of potential evidence in many criminal investigations.
When recovering data from SD cards in a criminal investigation, it is critical to preserve the integrity of the data and establish a strong chain of custody. Investigators must follow standard forensic procedures to create a bit-for-bit forensic image of the SD card before examining its contents, so they do not alter the original evidence. It is also important to recover deleted files and decrypt encrypted data where possible. The goal is to extract as much relevant evidence as possible from the SD card while avoiding any changes that could call the evidence into question.
There are several challenges that investigators face when recovering SD card data. Deleted files must be carefully reconstructed and sometimes data corruption occurs. Encryption can make data inaccessible and specialized techniques are required to attempt decryption. Proper validation of findings and documentation of procedures are vital. Overall, recovering all usable evidence from an SD card while ensuring its admissibility in court requires expertise in specialized forensic data recovery techniques.
Preserving the Integrity of the SD Card
When recovering data from an SD card found at a crime scene, it is critical to preserve the integrity of the data on the card. Mishandling the SD card can irreparably damage the data, rendering it unusable as evidence. Investigators should take great care when seizing the SD card to avoid any actions that could modify or delete data.
The SD card should be handled carefully by the edges to avoid scratching the metal contacts. The card should not be bent, folded, or touched on the metal areas. Static electricity can also destroy data, so grounding oneself first is advisable. As soon as the SD card is identified, it should be placed in a sealed Faraday bag or anti-static container to isolate it from electric signals. The bag should be immediately labeled with identifying information such as the case number, item number, date, time, and location. Securing the SD card in this manner preserves it in the state it was found and protects against damage during transport.
Proper handling and storage of the SD card allows investigators to create an exact forensic image for examination and maintain an evidence chain of custody. Following these best practices shows the integrity of the data was maintained when presented in court.
Creating a Bit-for-Bit Forensic Image
It is critical to create a forensic image of the SD card rather than accessing the card directly. A forensic image is an exact, bit-for-bit copy of the original media. This preserves the integrity of the evidence and ensures that analysis does not modify or corrupt the data on the card (Source). Once a forensic image is created, analysis and data extraction is performed on the image rather than the original.
Specialized hardware and software tools are used to create forensic images. Hardware write blockers prevent any writing or modification to the card during the imaging process. Software tools such as FTK Imager, EnCase, or dd can create a bitstream copy of the card, storing it as a digital forensic image file. FTK Imager provides an easy-to-use interface to calculate hash values and document details about the evidence (Source).
The resulting forensic image file is an exact duplicate of the SD card contents that can be preserved and analyzed without risk of altering the original evidence.
Examining the Forensic Image
Once a bit-for-bit forensic image of the SD card has been created, investigators can examine it using specialized forensic software tools. Some commonly used tools for analyzing SD card images include:
- FTK Imager – A free tool from AccessData that can view and extract data from forensic images. It can recover deleted files and analyze file system metadata (1).
- EnCase – A comprehensive forensic platform that enables end-to-end examination of digital evidence like SD cards (2).
- Autopsy – An open source digital forensics tool that can analyze images to extract photos, videos, documents and more.
When examining the forensic image, investigators will look to extract all available files including photos, videos, documents, chat logs and other data stored on the card. They will also analyze file metadata like time stamps, geo-location data and ownership details.
A key focus is recovering deleted and hidden files that the user may have attempted to erase. Even if a file is deleted, forensic tools can often recover it along with details like the original path, size and time it was created or modified. This provides critical insights into user activity.
By thoroughly examining the SD card image, investigators can reconstruct a detailed view of the card’s contents and user actions – including activity the user wanted to hide.
Extracting Photos and Videos
Once a forensic image of the SD card has been created, investigators can begin extracting photos and videos stored on the card. This allows them to review files that may contain evidence relevant to the case.
There are several forensic software tools that can be used to extract photos and videos from an SD card image, such as FTK Imager (source). The extracted files can then be viewed in their native format using image viewers and media players.
Analyzing the metadata of extracted photos and videos can provide useful information to investigators. Metadata includes details like date/time of capture, camera settings, and geolocation data. This can help establish a timeline of events or locate where a photo or video was taken. Some metadata forensics techniques include analyzing EXIF data, reading timestamp information, and correlating files based on metadata patterns (source).
It’s important that proper forensic procedures are followed when extracting photos and videos to ensure the data is unaltered. Investigators should use write-blocking tools to prevent modifying the SD card image during analysis. All extracted files must be validated against hash values and documented thoroughly.
Recovering Deleted Files
When files are deleted from an SD card, they are not actually erased from the storage device immediately. Instead, the file system simply marks the space those files occupied as available for new data to be written. Until that space is overwritten, forensic analysts can often recover deleted files from the unallocated space on the SD card using data recovery tools.
One technique for recovering deleted files is called file carving. File carvers scan the raw data on a storage device looking for the binary signatures that indicate the start and end of certain types of files, such as JPEGs or MP3s. When a complete file is reconstructed by the carver, it can be extracted and recovered.
File carvers may also use metadata such as file headers, footers, and directory entries to help reconstruct fragmented files. More advanced carvers can detect and recover files based on internal file structures and heuristics. However, recovering files through carving can be an imperfect process, and files may not be fully intact if sectors containing their data were partially overwritten.
By thoroughly searching unallocated and slack space on storage devices, analysts can often recover a great deal of deleted data using carving techniques. When combined with scanning for known file signatures, file carving provides an effective means for recovering evidence that a suspect may have attempted to destroy.
Sources:
https://www.ibeesoft.com/recover-data/best-free-sd-card-data-recovery-software.html
Analyzing File System Data
Examining the file system structures on an SD card can provide valuable clues for investigators. The file allocation table (FAT) or NTFS file system contains metadata about each file such as creation/modification times, file sizes, folder locations, etc. Analyzing this metadata can help determine when files were created/accessed and establish a timeline of usage.
Investigators can view data usage and look for clusters of activity or files accessed around a certain timeframe which may be relevant to the investigation. They can also look for traces of deleted files based on file system artifacts. The Cellebrite article highlights the importance of recovering deleted files/photos from unallocated space on SD cards.
File access times can provide clues about what the user was doing – for example, if a number of files were accessed shortly before deletion, it could indicate attempted concealment of evidence. Analyzing this kind of file system metadata provides insights that may not be available from just looking at active files.
Decrypting Encrypted Data
Decrypting encrypted data from SD cards found at crime scenes can be challenging for forensic investigators. Many Android devices encrypt SD cards by default, using cryptographic keys tied to the specific device. Once removed from the device, the encrypted card will appear blank and inaccessible on another system.
To decrypt the data, investigators need access to the cryptographic keys stored on the original device. This may require exploiting vulnerabilities in the device’s encryption implementation to extract the keys. Some forensic tools like Cellebrite offer decryption capabilities by manually entering the encryption keys from the original device.
If the original device is not available, brute force decryption is theoretically possible but extremely difficult given the strength of modern encryption algorithms. Rainbow tables and decryption dictionaries have limited effectiveness. In many cases, encrypted SD card data may remain inaccessible without the correct cryptographic keys from the associated device.
Proper forensic procedures like imaging the encrypted card before decryption attempts are important to preserve data integrity. Documentation of decryption efforts and results provides a clear record for evidence presentation. While challenging, decryption provides invaluable data that may help investigations.
Validating and Documenting Findings
When recovering data from an SD card as part of a criminal investigation, it is crucial to verify the integrity of any files extracted from the card. This involves validating that the recovered data matches the original data bit-for-bit through cryptographic hashing. Comparing the hash values of the original forensic image to the extracted files will confirm nothing was altered and the chain of custody remains intact.
Detailed documentation also plays a vital role in maintaining evidentiary standards. All steps taken during the data extraction process should be thoroughly documented, including notes on extraction methods used, specific files recovered, and any encryption encountered. Dates, times, software versions, and equipment used should also be recorded.
Comprehensive documentation allows investigators to retrace the process if any issues arise. It also demonstrates the recovered data has not been manipulated or contaminated since acquisition. Together with cryptographic validation, documentation helps establish the authenticity and reliability of digital evidence extracted from an SD card.
Adhering to Standard Procedures
When recovering data from SD cards found at a crime scene, it is crucial to follow standard forensic protocols and maintain proper chain of custody records. According to the Interpol Guidelines for Digital Forensics First Responders, adhering to forensic best practices preserves the integrity of the evidence and ensures it can be relied upon in legal proceedings.
Some key steps to follow proper forensic procedures include:
- Documenting how the SD card was discovered and handled at the crime scene.
- Placing the SD card in a protective enclosure to avoid further modification.
- Maintaining a chain of custody log that details every individual who handled the evidence.
- Using write-blocking hardware and validated forensic software to create a bit-for-bit image of the SD card.
- Performing all analysis on working copies of evidence to avoid altering the original data.
- Adhering to jurisdictional laws and policies regarding handling of electronic evidence.
Following these and other standard digital forensics protocols demonstrates professional conduct and helps establish that the recovered data can be considered accurate and reliable. Maintaining meticulous chain of custody and documenting each step taken preserves the evidentiary value of the SD card. By adhering to these best practices, forensic examiners can recover, analyze, and present data from SD cards in a court-admissible manner.
