Abstract
We propose a new identity-based cryptographic primitive which we call graded encryption. In a graded encryption scheme, there is one central (mostly offline) authority and a number of sub-authorities holding master keys that correspond to different levels. As in identity-based encryption, a sender can encrypt a message using only the identity of the receiver (plus public parameters) but it may also specify a numerical grade i. Users may decrypt messages directed to their identity at grade i as long as they have executed a key-upgrade protocol with sub-authorities 1,…,i. We require a grade i ciphertext to be secure in a strong sense: as long as there is one sub-authority with index j ≤ i that is not corrupted, the plaintext should be hidden from any recipient that has not properly upgraded her identity.
Graded encryption is motivated by multi-stage games (e.g., “who wants to be a millionaire”) played in a distributed fashion. Players unlock ciphertexts that belong to a certain stage i only if they have met all challenges posed by sub-authorities {1,…,i }. This holds true even if players collaborate with some of the sub-authorities. We give an efficient construction that has secret key and ciphertext size of a constant number of group elements. We also demonstrate further applications of graded encryption such as proving that a certain path was followed in a graph.
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Kiayias, A., Osmanoglu, M., Tang, Q. (2014). Graded Encryption, or How to Play “Who Wants To Be A Millionaire?” Distributively. In: Chow, S.S.M., Camenisch, J., Hui, L.C.K., Yiu, S.M. (eds) Information Security. ISC 2014. Lecture Notes in Computer Science, vol 8783. Springer, Cham. https://doi.org/10.1007/978-3-319-13257-0_22
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DOI: https://doi.org/10.1007/978-3-319-13257-0_22
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