## Cryptology ePrint Archive: Report 2012/136

Adaptive Key Protection in Complex Cryptosystems with Attributes

Zilong Wang and Danfeng (Daphne) Yao and Rongquan Feng

Abstract: In the attribute-based encryption (ABE) model, attributes (as opposed to identities) are used to encrypt messages, and all the receivers with qualifying attributes can decrypt the ciphertext. However, compromised attribute keys may affect the communications of many users who share the same access control policies. We present the notion of forward-secure attribute-based encryption (fs-ABE) and give a concrete construction based on bilinear map and decisional bilinear Diffie-Hellman assumption. Forward security means that a compromised private key by an adversary at time $t$ does not break the confidentiality of the communication that took place prior to $t$. We describe how to achieve both forward security and encryption with attributes, and formally prove our security against the adaptive chosen-ciphertext adversaries. Our scheme is non-trivial, and the key size only grows polynomially with $\log N$ (where $N$ is the number of time periods). We further generalize our scheme to support the individualized key-updating schedule for each attribute, which provides a finer granularity for key management. Our insights on the required properties that an ABE scheme needs to possess in order to be forward-secure compatible are useful beyond the specific fs-ABE construction given. We raise an open question at the end of the paper on the escrow problem of the master key in ABE schemes.

Category / Keywords: public-key cryptography / Attribute-based encryption, forward security, key update