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One of the latest trends among cyber criminals is to encrypt files on a network, forcing the user to pay a ransom for the decryption of their data. Of course, criminals use cryptocurrencies such as Bitcoins, not your typical bank transfers. The attackers achieve their objectives via many routes. For example, a potential victim might be prepared to pay money for files because they have a certain emotional or economic value. An attacker will not be interested in why the owner wants to restore their files – as long as they are prepared to pay money for the decryption.

Based on current research, TeslaCrypt ransomware (which is also known as AlphaCrypt) seems to be backed by a fairly established team. Some years ago, CryptoLocker became famous for blackmailing its way to a $3 million fortune. TeslaCrypt is just one variant of many in the field of ransomware, including larger and more lucrative tools such as CryptoLocker, CryptoWall, and CryptoWall 2 and 3. The CryptoWall variants in particular are developing increasingly complex techniques to protect themselves against deletion or against decryption of the ransomed data.

TeslaCrypt Approach

In February 2015, the first successful attacks by TeslaCrypt were reported against both private users and large-scale enterprises, although it is unclear whether the enterprises were targeted actively. At first sight, the attacks seemed to be similar to those by CryptoLocker. Once TeslaCrypt infested a system, it performed a targeted search for photo and video files, as well as gaming files, including high score and activation keys for Valve's Steam gaming platform. This particular malware tool typically uses email attachments to invade a system or forwards the victim to a rogue website where their system is then infected. An exploit kit stored on the site then uses a Flash vulnerability (CVE-2015-0311) to run TeslaCrypt on the system.

A check is performed to see whether the URLReader2 COM interface has been installed correctly in the DirectShow Filter Graph List [1]. The malicious code files are then unpacked and executed after this check. The process launches autonomously, escalates its privileges, and copies the malicious files to a randomly chosen name in the user's home directory. A new process then launches, and the existing process is migrated to this process. Then, the original process is terminated and the original unpacked files are deleted. Now the new process runs the malicious code routines with escalated privileges and launches a window that is visible to the user with a prompt to pay up (Figure 1).

Figure 1: If you see this window, TeslaCrypt has encrypted your files and is asking for a ransom in Bitcoins.

Before the encryption process is run on the system, the following routines are run by the malware tool with the aid of five new process threads:

• Delete all Windows shadow copies
• Create and encrypt the key file (key.dat)
• Transfer the key file to the command and control server
• Implement a defense function every 200msec that prevents calls to the following processes, or keeps them from running: taskmgr, procexp, regedit, msconfig, and cmd.exe.

The user will be unable to launch other processes besides those listed at this point, because TeslaCrypt remembers the name of all processes launched on the system.

Solid AES Encryption

TeslaCrypt's main function is to encrypt the files that exist on the infected system. It is frightening to see that more than 170 million file types are potential encryption candidates. After TeslaCrypt has completed the init routine and deleted the Windows shadow copies, the key file key.dat is created, which contains all the encryption keys: primarily the payment key and the master encryption key. These keys are combined to encrypt files using a proprietary algorithm.

Other versions of TeslaCrypt create a third key known as the recovery key, which simply serves the purpose of allowing the victim to upload the file to a website before paying if no other actions are possible after infection with TeslaCrypt. This step is necessary because encryption differs from system to system. The file containing the recovery key is uploaded to a site on the deep web (a non-indexed part of the Internet). The typical payment method for this kind of site is the Bitcoin Internet currency. This means that the victim is forced to buy Bitcoins and then send them to the stated Bitcoin address. The keys are generated by reference to the following system information:

• Global LAN network statistic of the infected system
• 64 bytes generated randomly by the Windows cryptography function
• All descriptions of the active processes
• All modules loaded in each process
• The physical memory information of the infected system

Once all the data has been gathered, a very large array with many SHA1 hashes is generated. Each hash contains the first 20 bytes of the collected data. Then a global SHA1 hash is formed from all the arrays and stored temporarily. With the help of the globally generated SHA1, the master key and payment key are generated using a proprietary algorithm and SHA256 and written to the key.dat file. Each key has a length of 32 bytes (256 bits).

Encryption of the individual files occurs in a separate process. The routine goes through the following steps:

• Lists all the files that are candidates for encryption
• Generates a 16-byte initialization vector for AES
• Reads the target files
• Initializes the AES encryption algorithm
• Encrypts the target files with the help of a 256-bit AES CBC algorithm

Once this process is complete, all candidate files are encrypted with the stated algorithm. The user now has no chance of decrypting without a key. It is important to note that the user only has a certain time window in which to decrypt their data. If the user does not decrypt within a certain time, the key deposited on the C&C server is deleted.

By the way, the pop-up that prompts the user to decrypt shows misleading information. It claims to use RSA-2048 for the encryption, which, as you might know, is asymmetric encryption and would mean that it is impossible to decrypt the data without a backwards compatible private key.

As this article and other sources have stated, TeslaCrypt only uses symmetrical AES encryption. Of course, the matching key is required to decrypt the files because AES-256 has so far not been compromised.

Protections Against Infection

Full protection against this kind of malware is not possible. The attack and infection patterns can be fairly easily changed by the programmers so that signature and heuristic data required for detecting the malware by antivirus tools might be useless. Nevertheless, the following safeguards are meaningful to prevent infection by variants:

• Operate the system without permanent administrative privileges
• Patch software (in particular browser plugins) and the operating system with the latest updates
• Get the latest malware definitions
• Activate the local firewall
• Block known command and control target addresses to prevent communication with control servers
• Create valid and independent backups

These measures cannot completely prevent an infection.