Cryptology ePrint Archive: Report 2016/691

Targeted Homomorphic Attribute Based Encryption

Zvika Brakerski and David Cash and Rotem Tsabary and Hoeteck Wee

Abstract: In (key-policy) attribute based encryption (ABE), messages are encrypted respective to attributes $x$, and keys are generated respective to policy functions $f$. The ciphertext is decryptable by a key only if $f(x)=0$. Adding homomorphic capabilities to ABE is a long standing open problem, with current techniques only allowing compact homomorphic evaluation on ciphertext respective to the same $x$. Recent advances in the study of multi-key FHE also allow cross-attribute homomorphism with ciphertext size growing (quadratically) with the number of input ciphertexts.

We present an ABE scheme where homomorphic operations can be performed compactly across attributes. Of course, decrypting the resulting ciphertext needs to be done with a key respective to a policy $f$ with $f(x_i)=0$ for all attributes involved in the computation. In our scheme, the target policy $f$ needs to be known to the evaluator, we call this targeted homomorphism. Our scheme is secure under the polynomial hardness of learning with errors (LWE) with sub-exponential modulus-to-noise ratio.

We present a second scheme where there needs not be a single target policy. Instead, the decryptor only needs a set of keys representing policies $f_j$ s.t.\ for any attribute $x_i$ there exists $f_j$ with $f_j(x_i)=0$. In this scheme, the ciphertext size grows (quadratically) with the size of the set of policies (and is still independent of the number of inputs or attributes). Again, the target set of policies needs to be known at evaluation time. This latter scheme is secure in the random oracle model under the polynomial hardness of LWE with sub-exponential noise ratio.

Category / Keywords: public-key cryptography / attribute-based encryption, fully homomorphic encryption, attribute-based FHE, homomorphic ABE

Date: received 11 Jul 2016, last revised 11 Jul 2016

Contact author: rotem ts0 at gmail com

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Version: 20160713:134219 (All versions of this report)

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