Comunicació presentada a: PKC 2010. 13th International Conference on Practice and Theory in Public Key Cryptography, celebrada del 26 al 28 de maig de 2010 a París, França.

Attribute-based cryptography has emerged in the last years as a promising primitive for digital security. For instance, it provides good solutions to the problem of anonymous access control. In a ciphertext- policy attribute-based encryption scheme, the secret keys of the users de- pend on their attributes. When encrypting a message, the sender chooses which subset of attributes must be held by a receiver in order to be able to decrypt. All current attribute-based encryption schemes that admit reasonably expressive decryption policies produce ciphertexts whose size depends at least linearly on the number of attributes involved in the policy. In this paper we propose the first scheme whose ciphertexts have constant size. Our scheme works for the threshold case: users authorized to decrypt are those who hold at least t attributes among a certain universe of attributes, for some threshold t chosen by the sender. An extension to the case of weighted threshold decryption policies is possible. The security of the scheme against selective chosen plaintext attacks can be proven in the standard model by reduction to the augmented multi-sequence of exponents decisional Di e-Hellman (aMSE-DDH) problem.

The work of Javier Herranz is supported by a Ramón y Cajal grant, partially funded by the European Social Fund (ESF) of the Spanish MICINN Ministry. Carla Ràfols holds an FPI grant of the Spanish MICINN Ministry. The work of both these authors is partially supported by the Spanish MICINN Ministry under project MTM2009-07694. The work of Fabien Laguillaumie is supported by the French ANR-07-TCOM-013-04 PACE Project.