Gpcode is a trojan that encrypts files with certain extensions on local and remote drives and then asks a user to contact its author to buy a decryption solution.
F-Secure Anti-Virus is able to detect and decrypt files encrypted by the Gpcode trojan. To find and decrypt such files, please scan ALL files on the hard disk.
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Basically, the trojan takes the user's files as hostages and asks for a ransom to "free" them, making this a form of ransomware.
The trojan's file is a PE executable about 56 kilobytes long, packed with UPX file compressor.
After the trojan's file is run by a user it creates a startup key for its file in Windows Registry:
where [file name] is the name of the trojan's file.
The trojan starts to scan local and remote drives for files with the following extensions:
When a file with any of these extension is found, the trojan reads it to memory, encrypts file's data with a simple algorithm, saves encrypted data into a new file (the name of this file is 'coder' + original file's name: for example for FILE.PGP the trojan will create the CODERFILE.PGP file), deletes the original file and then renames the newly created file with the name of the original file.
After that the trojan creates a text file named ATTENTION!!!.TXT in the same folder where the encrypted file is located. This .txt file contains the following text:
All encrypted files have the following 21 byte text string in their beginning:
The encryption algorithm is quite simple - the trojan uses ADD operation on the original file's data with a single byte encryption key. The original value of the encryption key is 58 (0x3a) and it is modified using 2 fixed byte values which are 37 (0x25) and 92 (0x5c) after encryption of each next byte of the original file's data.
While the trojan scans local and remote drives, it keeps a track of all found folders and files in the AUTOSAVE.SIN file that is created in a temporary folder.
After all files are encrypted the trojan terminates its process, deletes its executable file, AUTOSAVE.SIN file and its startup key from the Registry.