Cryptovirology

See also: the biological cryptovirus

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Cryptovirology

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", or ransomware. 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.

The field also encompasses covert attacks in which the attacker secretly steals private information such as private keys. An example of the latter type of attack are asymmetric backdoors. An asymmetric backdoor is a backdoor (e.g., in a cryptosystem) that can be used only by the attacker, even after it is found. This contrasts with the traditional backdoor that is symmetric, i.e., anyone that finds it can use it. Kleptography, a subfield of cryptovirology, is concerned with the study of asymmetric backdoors in key generation algorithms, digital signature algorithms, key exchanges, and so on.

Cryptovirology was born in academia. However, practitioners have recently expanded the scope of the field to include the analysis of cryptographic algorithms used by malware writers, attacks on these algorithms using automated methods (such as X-raying), and analysis of viruses' and packers' encryptors. Also included is the study of cryptography-based techniques (such as "delayed code") developed by malware writers to hamper malware analysis.

A "questionable encryption scheme", which was introduced by Young and Yung, is an attack tool in cryptovirology. Informally speaking, a questionable encryption scheme is a public key cryptosystem (3-tuple of algorithms) with two supplementary algorithms, forming a 5-tuple of algorithms. It includes a deliberately bogus yet carefully designed key pair generation algorithm that produces a "fake" public key. The corresponding private key cannot be used to decipher data "encrypted" using the fake public key. By supplying the key pair to an efficient verification predicate (the 5th algorithm in the 5-tuple) it is proven whether the public key is real or fake. When the public key is fake, it follows that no one can decipher data "enciphered" using the fake public key. A questionable encryption scheme has the property that real public keys are computationally indistinguishable from fake public keys when the private key is not available. The private key forms a poly-sized witness of encryption or decryption, whichever may be the case.

An application of a questionable encryption scheme is a trojan that gathers plaintext from the host, "encrypts" it using the trojan's own public key (which may be real or fake), and then exfiltrates the resulting "ciphertext". In this attack it is thoroughly intractable to prove that data theft has occurred. This holds even when all core dumps of the trojan and all the information that it broadcasts is entered into evidence. An analyst that jumps to the conclusion that the trojan "encrypts" data risks being proven wrong by the malware author (e.g., anonymously).

When the public key is fake, the attacker gets no plaintext from the trojan. So what's the use? A spoofing attack is possible in which some trojans steal data and some provably do not, thereby casting doubt on the true nature of future strains of malware. This attack implies a fundamental limitation on proving data theft.

There are many other attacks in the field of cryptovirology that are not mentioned here.

References

• A. Young, M. Yung. "Cryptovirology: Extortion-Based Security Threats and Countermeasures". IEEE Symposium on Security & Privacy, May 6-8, 1996. pp. 129–141. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help) [1]
• A. Young, M. Yung (1996). "The Dark Side of Black-Box Cryptography, or: Should we trust Capstone?". In Neal Koblitz (ed.). Proceedings of Crypto '96. Springer-Verlag. pp. 89–103. LNCS 1109. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help) [2]
• A. Young, M. Yung (1997). "Kleptography: Using Cryptography Against Cryptography". In W. Fumy (ed.). Proceedings of Eurocrypt '97. Springer-Verlag. pp. 62–74. LNCS 1233. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help) [3]
• A. Young, M. Yung (2004). Malicious Cryptography: Exposing Cryptovirology. Wiley. ISBN 0-7645-4975-8.
• A. Young, M. Yung (2006). "On Fundamental Limitations of Proving Data Theft". IEEE Transactions on Information Forensics and Security, 1(4). pp. 524–531. {{cite conference}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help) [4]

External links

• http://www.cryptovirology.com — site maintained by Adam Young and Moti Yung
• Cryptography and cryptovirology articles at VX Heavens
• Cryzip Trojan Encrypts Files, Demands Ransom
• Can a virus lead an enterprise to court?
• A student report entitled Superworms and Cryptovirology