UK student develops antidote for first Android file-encrypting ransomware

A hardcoded encryption key allows recovering files held hostage by the Simplocker Android malware

Victims of Simplocker, the first file-encrypting ransomware threat for Android devices, can recover files without paying cybercriminals because the malicious program uses a hardcoded encryption key.

Simon Bell, a computer science student in his final year at the University of Sussex in the U.K., recently analyzed the threat in detail and was the first to reveal the existence of the key.

Simplocker was identified by security researchers from antivirus vendor ESET at the beginning of June and is the first ransomware application for Android to use encryption as an extortion method.

The malware scans the SD memory cards of infected devices for files with the jpeg, jpg, png, bmp, gif, pdf, doc, docx, txt, avi, mkv, 3gp and mp4 extensions and encrypts them using the AES (Advanced Encryption Standard) algorithm. Another interesting aspect of the threat is that it communicates with a command-and-control server hosted on the Tor anonymity network.

The ESET researchers said that Simplocker is most likely a proof-of-concept or a work in progress, an idea further enforced by Bell's discovery that the malware uses a master encryption key included in its code instead of separate unique keys for each infected device obtained from the C&C server.

A Java class called AesCrypt that's used by Simplocker "contains a method called encrypt() which uses AES encryption and cipher password 'jndlasf074hr'," Bell said Monday in a blog post.

He used that information and the malware's own decrypt() method to create a separate Java program that can recover files encrypted by the threat.

"The antidote for this ransomware was incredibly easy to create because the ransomware came with both the decryption method and the decryption password," Bell said in a separate blog post Tuesday. "Therefore producing an antidote was more of a copy-and-paste job than anything."

Future versions of Simplocker will probably better protect the decryption keys and will likely obtain them from the C&C server, he said.

"Since the Simplelocker ransomware is a proof-of-concept, the antidote provided here is simply a solution to this proof-of-concept," Bell said. "Future versions of advanced smartphone ransomware will likely prove significantly harder to reverse engineer."

Antivirus vendor Avast also released a free Simplocker removal tool Tuesday that's capable of decrypting the files affected by the malware. The tool is called AVAST Ransomware Removal and is easier to use than Bell's antidote because it's packaged as an app that can be downloaded from Google Play.

Encrypting files proved a profitable technique for ransomware programs on Windows, so security researchers expect Android ransomware to take a similar approach. Users are advised to install apps only from trusted sources such as Google Play in order to limit the chances of becoming victims of such threats.

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