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Contextuality offers security

The discovery of quantum key distribution by Bennett and Brassard (BB84) bases on the fundamental quantum feature: incompatibility of measurements of quantum non-commuting observables. In 1991 Ekert showed that cryptographic key can be generated at a distance with help of entangled (correlated) quantum particles. Recently Barrett, Hardy and Kent showed that the non-locality used by Ekert is itself a good resource of cryptographic key even beyond quantum mechanics. Their result paved the way to new generation of quantum cryptographic protocols - secure even if the devices are built by the very eavesdropper. However, there is a question, which is fundamental from both practical and philosophical point of view: does Nature offer security on operational level based on the original concept behind quantum cryptography - that information gain about one bservables must cause disturbance to another, incompatible one? Here we resolve this problem by using in place of non-locality another striking feature of quantum world - contextuality. It is a strong version of incompatibility manifested in the famous Kochen-Specker paradox. The crucial concept is the use of a new class of families of bipa