High Quality Content by WIKIPEDIA articles! Cryptovirology is a field that studies how to use cryptography to design powerful malicious software. The field was born with the observation that public-key cryptography can be used to break the symmetry between what an antivirus analyst sees regarding a virus and what the virus writer sees. The former only sees a public key whereas the latter sees a public key and corresponding private key. The first attack that was identified in the field is called "cryptoviral extortion". In this attack a virus, worm, or trojan hybrid encrypts the victim`s files and the user must pay the malware author to receive the needed session key (which is encrypted under the author`s public key that is contained in the malware) if the user does not have backups and needs the files back.