Introduction to Android Live Imaging

In the realm of digital forensics, acquiring data from mobile devices presents a unique set of challenges. While physical acquisition methods aim to create a bit-for-bit copy of the entire storage medium, they often require advanced tools, device specific exploits, or even hardware modifications. Live imaging, on the other hand, focuses on acquiring data directly from a running device, often targeting specific partitions or volatile data that might be lost during a power-off.

This article delves into the methodology of performing live data acquisition on Android devices using the Android Debug Bridge (ADB) shell. This technique is particularly valuable when a device cannot be powered off (e.g., due to encryption keys residing only in volatile memory) or when a quick, targeted acquisition of specific partitions is necessary without altering the device’s state significantly.

Prerequisites for ADB Shell Live Acquisition

Before initiating any data acquisition, certain prerequisites must be met to ensure a smooth and forensically sound process.

Setting up ADB

ADB is a versatile command-line tool that allows communication with an Android device. It’s part of the Android SDK Platform-Tools package. Ensure it’s installed and accessible from your system’s PATH.

adb devices

This command lists connected devices. If your device appears with a unique identifier, ADB is configured correctly.

Enabling USB Debugging

On the Android device, navigate to ‘Settings’ > ‘About phone’ and tap ‘Build number’ seven times to enable Developer options. Then, go back to ‘Settings’ > ‘System’ > ‘Developer options’ and enable ‘USB debugging’. A prompt will appear on the device asking to authorize the host computer’s RSA key fingerprint; always accept this for forensic workstations.

Root Access (Crucial for Full Partition Imaging)

While some data can be accessed without root, acquiring a full image of system partitions (like `/system`, `/data`, or `/boot`) typically requires elevated privileges. Rooting a device can introduce changes, so it must be carefully considered within the forensic context. If the device is already rooted, this step is bypassed. If not, the process of rooting itself can alter the device’s state, which must be documented meticulously.

Identifying Device Partitions

Once ADB is set up and the device is ready, the next step is to identify the relevant partitions you wish to image. Android devices use block devices for storage, similar to Linux systems.

First, enter the ADB shell:

adb shell

Then, list the block devices. Often, partitions are named descriptively under `/dev/block/platform/*/by-name`:

su # Requires root to access many block devicesls -l /dev/block/platform/*/by-name

This command will output a list mapping partition names (e.g., `system`, `userdata`, `boot`) to their respective block device paths (e.g., `/dev/block/mmcblk0pXX` or `/dev/block/sdaXX`). Pay close attention to the `userdata` (or `data`) partition, as it typically contains user-generated content and application data.

You can also use `df -h` to see mounted filesystems and their sizes:

df -h

The `dd` Command for Data Acquisition

The `dd` command is a powerful, low-level utility commonly used in Linux for copying and converting files, especially raw data from block devices. It’s the primary tool for imaging partitions via ADB shell.

Understanding `dd` Syntax

The basic syntax for `dd` is:

dd if=<input_file> of=<output_file> bs=<block_size>

• `if`: Specifies the input file or device (e.g., `/dev/block/mmcblk0pXX`).
• `of`: Specifies the output file where the image will be written. This should be a writable location on the Android device’s storage, such as `/sdcard` or `/data/local/tmp`.
• `bs`: Defines the block size in bytes for input and output. A larger block size (e.g., `4096` or `8192`) can speed up the transfer.

Imaging a Specific Partition

Let’s assume we want to image the `userdata` partition, which we identified as `/dev/block/mmcblk0p35` (this path will vary by device). You’ll need to be in the `adb shell` and have root access (`su`).

adb shellsu # Grant root access if not already rooteddd if=/dev/block/mmcblk0p35 of=/sdcard/userdata.img bs=4096 # Replace mmcblk0p35 with your actual partition path

This command will copy the raw data from the `userdata` partition and write it to a file named `userdata.img` on the device’s internal storage (accessible via `/sdcard`). If `/sdcard` is not writable or has insufficient space, try `/data/local/tmp`.

Pulling the Image to Host Machine

Once the `dd` command completes on the device, the image file resides on the Android device itself. You must then transfer it to your forensic workstation using `adb pull`.

exit # Exit the su shell exit # Exit the adb shelladb pull /sdcard/userdata.img C:orensicsull_device_name
ecovered_images
eplace_with_date_time_userdata.img

Ensure you have sufficient free space on your forensic workstation to store the image.

Considerations and Best Practices for Forensic Integrity

Maintaining the integrity of evidence is paramount in digital forensics. Live imaging, by its nature, involves a running system, making it inherently more challenging than offline acquisition.

Write Protection and Device State

While `dd` is primarily a read operation, writing the image to the device’s internal storage (e.g., `/sdcard`) changes the device’s state. Always document these actions. The goal is to minimize modifications to the original evidence. Consider using `adb sideload` if a custom recovery is available to push directly to an external storage or network stream if possible.

Data Integrity and Hashing

After acquiring the image and transferring it, verify its integrity. Calculate cryptographic hashes (MD5 or SHA256) of the image both on the device (if possible) and on your forensic workstation.

On the device (before `adb pull`):

adb shellmd5sum /sdcard/userdata.img

On your forensic workstation (after `adb pull`):

md5sum C:orensicsull_device_name
ecovered_images
eplace_with_date_time_userdata.img

The hashes should match exactly. Any discrepancy indicates corruption or alteration during transfer.

Encryption (FDE/FBE)

If the device uses Full Disk Encryption (FDE) or File-Based Encryption (FBE), the acquired image will contain encrypted data. You will need the decryption key (usually the device passcode or pattern) to decrypt the image offline using tools like `tce-decrypt` or specialized forensic software.

Locked Bootloaders and Custom Recoveries

If the device has a locked bootloader and cannot be rooted, or if USB debugging is not enabled, ADB shell acquisition of protected partitions may not be possible. In such cases, commercial tools, physical acquisition methods, or exploiting vulnerabilities might be the only recourse.

Limitations and Alternatives

ADB shell live imaging is a powerful technique but has limitations. It’s not a true physical acquisition as it relies on the operating system’s kernel to read data. This means certain low-level data, such as unallocated space that the OS no longer tracks, might be inaccessible. Moreover, the process is slower than direct physical dumps.

For comprehensive forensic examinations, commercial tools (e.g., Cellebrite UFED, Oxygen Forensic Detective) offer more robust features, including deeper access, bypasses for locked devices, and automated data parsing.

Conclusion

Live imaging Android devices via ADB shell, particularly using the `dd` command, is an invaluable skill for mobile forensic investigators. It provides a means to acquire critical data from running devices, especially when dealing with encryption or volatile memory. While requiring careful execution and adherence to forensic best practices, this method allows for targeted, efficient data collection that can be crucial for uncovering digital evidence. Always prioritize documentation, integrity verification, and thorough understanding of the device’s state throughout the process.