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Study of stealing information securely and subliminally
Kleptography is the study of stealing information securely and subliminally. The term was introduced by Adam Young and Moti Yung in the Proceedings of Advances in Cryptology – Crypto '96. 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. A kleptographic backdoor is synonymously referred to as an asymmetric backdoor. Kleptography encompasses secure and covert communications through cryptosystems and cryptographic protocols. This is reminiscent of, but not the same as steganography that studies covert communications through graphics, video, digital audio data, and so forth.

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 themself.

Kleptographic attacks have been designed for RSA key generation, the Diffie–Hellman key exchange, the Digital Signature Algorithm, and other cryptographic algorithms and protocols. In each case, the attacker is able to compromise the particular cryptographic algorithm or protocol by inspecting the information that the backdoor information is encoded in (e. g., the public key, the digital signature, the key exchange messages, etc.) and then exploiting the logic of the asymmetric backdoor using their secret key (usually a private key). A. Juels and J. Guajardo proposed a method (KEGVER) through which a third party can verify RSA key generation. This is devised as a form of distributed key generation in which the secret key is only known to the black box itself. This assures that the key generation process was not modified and that the private key cannot be reproduced through a kleptographic attack. the platform independent version of the open source CrypTool project. A demonstration of the prevention of kleptographic attacks by means of the KEGVER method is also implemented in JCrypTool. The Dual EC DRBG cryptographic pseudo random number generator from the NIST SP 800 90A is thought to contain a kleptographic backdoor. Dual EC DRBG utilizes elliptic curve cryptography, and NSA is thought to hold a private key which, together with bias flaws in Dual EC DRBG, allows NSA to decrypt SSL traffic between computers using Dual EC DRBG for example. The algebraic nature of the attack follows the structure of the repeated Dlog Kleptogram in the work of Young and Yung.