The primary function of this option is to enable communication between the Android device and the workstation on which the Android SDK is installed. On a Samsung phone, you can access this under Settings | Developer Options, as shown in the following screenshot:
Other Android phones may have different environments and configuration features. You may have to force the Developer Options option by accessing build mode.
However, starting from Android 4.2, the Developer Options menu is hidden to make sure that users do not enable it by accident. To enable it, go to Settings | About Phone and then tap the Build Number field seven times. After this, Developer Options will be available in the Settings menu. Prior to Android 4.2.2, enabling this option was the only requirement for communicating with the device over ADB; however, starting from Android 4.2.2, Google has introduced the secure USB debugging option. This feature only allows hosts that are explicitly authorized by the user to connect to the device using ADB.
Thus, when you connect the device to a new workstation via USB in order to access ADB, you need to first unlock the device and authorize access by pressing OK in the confirmation window, as shown in the following screenshot. If Always allow from this computer is checked, the device will not prompt for authorization in the future:
When the USB debugging option is selected, the device will run the adb daemon (adbd) in the background and will continuously look for a USB connection. The daemon will usually run under a nonprivileged shell user account and thus will not provide access to the complete data; however, on rooted phones, adbd will run under the root account and therefore provide access to all the data. It is not recommended that you root a device to gain full access unless all other forensic methods fail. Should you elect to root an Android device, the methods must be well-documented and tested prior to attempting it on real evidence. Rooting will be discussed at the end of this chapter.
On the workstation where the Android SDK is installed, adbd will run as a background process. Also, on the same workstation, a client program, which can be invoked from a shell by issuing the adb command, will run. When the adb client is started, it first checks whether an adb daemon is already running. If the response is negative, it initiates a new process to start the adb daemon. The adb client program communicates with local adbd over port 5037.