Kleptography

Kleptography is the study of stealing information securely and subliminally. Kleptography is a subfield of cryptovirology and is a natural extension of the theory of subliminal channels that was pioneered by Gus Simmons while at Sandia National Laboratory. Kleptography is also related to steganography. The concept of an asymmetric backdoor, synonymously referred to as kleptography, was introduced by Adam Young and Moti Yung in the Proceedings of Advances in Cryptology—Crypto '96.

A kleptographic attack is an attack which uses asymmetric cryptography to implement a cryptographic backdoor. For example, one such attack could be to subtly modify how the public and private key pairs are generated by the cryptosystem so that the private key could be derived from the public key using the attacker's private key. In a well-designed attack, the outputs of the infected cryptosystem would be computationally indistinguishable from the outputs of the corresponding uninfected cryptosystem.If the infected cryptosystem is a black-box implementation such as a hardware security module, a smartcard, or a Trusted Platform Module, a successful attack could go completely unnoticed.

A reverse engineer might be able to uncover a backdoor inserted by an attacker, and when it is a symmetric backdoor, even use it himself. However, by definition a kleptographic backdoor is asymmetric and the reverse-engineer cannot use it. A kleptographic attack (asymmetric backdoor) requires a private key known only to the attacker in order to use the backdoor. In this case, even if the reverse engineer was well-funded and gained complete knowledge of the backdoor, it would remain useless for him to extract the plaintext without the attacker's private key.

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