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Ransomware

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A cryptovirus, cryptotrojan or cryptoworm is a type of malware that encrypts the data belonging to an individual on a computer, demanding a ransom for its restoration. The term ransomware is commonly used to describe such software, although the field known as cryptovirology predates the term "ransomware".

This type of ransom attack can be accomplished by (for example) attaching a specially crafted file/program to an e-mail message and sending this to the victim. If the victim opens/executes the attachment, the program encrypts a number of files on the victim's computer. A ransom note is then left behind for the victim. The victim will be unable to open the encrypted files without the correct decryption key. Once the ransom demanded in the ransom note is paid, the cracker may (or may not) send the decryption key, enabling decryption of the "kidnapped" files.

The idea of maliciously encrypting plaintext is not new. The first example is probably the PC Cyborg Trojan that was found in 1989. It encrypted only filenames (using a very weak symmetric cipher) causing the file system to be corrupted. There have been other malware attacks that have maliciously encrypted plaintext since then. The 1996 IEEE paper by Young and Yung reviews the malware that has done this, and shows how public key cryptography may be used in such threats.

A cryptovirus, cryptotrojan, or cryptoworm is defined as malware that contains and uses the public key of its author. In cryptoviral extortion, the public key is used to hybrid encrypt the data of the victim and only the private key (which is not in the malware) can be used to recover the data. This is one of a myriad of attacks in the field known as cryptovirology.

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