Graphic explanation of BitLocker attack guide process

Bkjia.com exclusive Article] A few days ago, Fraunhofer SIT security lab said they have successfully cracked Windows 7's disk encryption technology BitLocker. Fraunhofer SIT researchers say they can successfully crack data on disks even if BitLocker is used together with a hardware-based Trusted Platform Module (TPM. This article provides a detailed explanation of the complete process of cracking BitLocker through text and text.

1. Encryption and malicious waitress attacks
When a computer is lost or stolen, disk encryption can effectively protect the confidentiality of data and effectively prevent opportunistic attacks. However, encryption cannot block targeted attacks, especially when attackers can access the computer physically. The term "An evil waitress attack" describes a situation in which when a computer host leaves a hotel room and no one is watching the computer, an evil waitress or anyone else who can enter the room can beat the computer at will. In this way, they can break down machines with encryption and decryption functions to obtain the relevant keys and plaintext data. Attackers can modify the software on a computer and even its hardware. For example, they can install a hardware key recorder to steal the password. With this attack method, attackers can gain access to confidential data and even attack the entire operating system.

Ii. BitLocker driver Encryption
BitLocker drive encryption (BDE) is a feature provided by operating systems such as Windows Vista, Windows Server 2008, and Windows 7. When a computer has a trusted platform module, this feature allows you to use various features of the trusted computing technology platform. TPM allows software to lock data by encrypting the data with the key stored in TPM. to re-open the data, you need to unlock it, the method is to decrypt data using the same TPM and the specific status of the main component. During system boot, these components, such as BIOS and operating system boot programs, work together with TPM to establish a check tracking system that measures the current state of the system. If the current status of the system is different from the reference status specified when the data is locked, the TPM rejects the data.

Currently, the Trusted Computing Technology does not include keyboard measurement in the personal computer, nor does it establish a secure channel for the keyboard. Therefore, many hardware-based BitLocker attacks are still possible in the case of an evil waitress. At the same time, many people take it for granted that trusted computing technology can automatically protect the system from all software-based attacks targeting the guiding process. Specifically, they believe that machines with BitLocker and TPM can achieve this protection. Next, we will use a specific attack example to break this illusion: attackers can tamper with the boot program to access confidential data.

Iii. Attack Process demonstration

During system boot, BitLocker needs to interact with the user to obtain a password 1), or obtain a key file from the USB disk, or both. However, the program code that interacts with the user is located on an unencrypted disk. In this case, the problem arises: if an attacker can access a computer physically, he can modify the code freely, such as adding a function to store the user's key in the idle Disk Area. In this way, when the user directs the computer for the next time, The TPM will notice the modification and refuse to open all the keys bound to the unmodified status of the boot code. However, BitLocker does not use the measurement information provided by TPM to prevent the modified Code from running outside the encrypted partition.

 

Figure 1 BitLocker requires the user to submit a password

Therefore, the "evil waitress attack" can replace the original BitLocker Boot Code with her own boot code and spoof the user to interact with her boot code. After the modified Boot Code obtains the key from the user, it does not continue the subsequent boot process. However, it can restore the status of the original Bootstrap program and try to reboot the system in an imperceptible way. If the attacker was lucky enough to force the user to reboot the system, she could access the system again to steal computer data.
The following describes the attack process:
First, when the computer master is absent, the attacker guides the computer through a USB flash drive, modifies the boot program, and installs the trojan program. 2.

 

Figure 2 replace the boot program and implant Trojans

When the computer host starts the machine again, the trojan program displays a forged boot screen and tricks the user into entering the password, as shown in 3:
 

Figure 3 Trojan capture Password

The trojan program saves the password to the disk, restores the original boot program, and directs the system again, as shown in Figure 4:

 

Figure 4 Trojan logs the password and uses the original boot program to guide the machine

At this time, attackers can smoothly access the system, as if everything had never happened. When attackers access the machine again, they can start the machine from the USB flash drive in Linux, as shown in:

 

Figure 5 starting a machine from a USB flash drive in Linux

In the command line, read the password used to write the Trojan horse into the disk, reboot the Windows boot system, and enter the password you just obtained to log on to the system.

 

Figure 6 use the intercepted password to log on to the system

In this way, attackers can access all the data in the system.

Iv. Conclusion
Here we will end with the original words of the Fraunhofer SIT researchers: here we show attack methods, not suggesting that BitLocker has a vulnerability, or that trusted computing is useless. Like other disk encryption products, BitLocker is still effective, but not as secure as people think. By using TPM, BitLocker makes it easier for us to attack: attacks are easier to detect and many other attacks are rejected. Our attacks only apply to the specific combinations of platforms, applications, attack situations, and attack targets discussed here.

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