Related-key attack

In cryptography, a related-key attack is any form of cryptanalysis which presumes that the attacker has the capability to consider the operation of a cipher under several different keys. Before the attack, the values of the keys are unknown, but some relationship concerning them is available (for example, a fixed difference between two keys).

This appears, at first glance, to be an unrealistic model; it would certainly be unlikely that an attacker could persuade a human cryptographer to encrypt plaintexts under numerous secret keys unknown to the attacker, and add to that the fact that they have to be related in some way. However, modern cryptography is implemented in software or hardware and is used for a diverse range of applications; for many cases, a related-key attack is often made very feasible, such as in some key-exchange protocols. In addition, any cipher that can prevent related-key attacks means that it has a strong key-schedule design; this is a conservative approach to security.