Dual System Encryption Framework in Prime-Order Groups via Computational Pair Encodings

We propose a new generic framework for achieving fully secure attribute based encryption (ABE) in prime-order bilinear groups. It is generic in the sense that it can be applied to ABE for arbitrary predicate. All previously available frameworks that are generic in this sense are given only in composite-order bilinear groups, of which operations are known to be much less efficient than in prime-order ones for the same security level. These consist of the frameworks by Wee (TCC’14) and Attrapadung (Eurocrypt’14). Both provide abstractions of dual-system encryption techniques introduced by Waters (Crypto’09). Our framework can be considered as a prime-order version of Attrapadung’s framework and works in a similar manner: it relies on a main component called pair encodings, and it generically compiles any secure pair encoding scheme for a predicate in consideration to a fully secure ABE scheme for that predicate. One feature of our new compiler is that although the resulting ABE schemes will be newly defined in prime-order groups, we require essentially the same security notions of pair encodings as before. Beside the security of pair encodings, our framework assumes only the Matrix Diffie-Hellman assumption (Escala et al., Crypto’13), which is a weak assumption that includes the Decisional Linear assumption as a special case. As for its applications, we can plug in available pair encoding schemes and automatically obtain the first fully secure ABE realizations in prime-order groups for predicates of which only fully secure schemes in composite-order groups were known. These include ABE for regular languages, ABE for monotone span programs (and hence Boolean formulae) with short ciphertexts or keys, and completely unbounded ABE for monotone span programs. As a side result, we establish the first generic implication from ABE for monotone span programs to ABE for branching programs. This implies fully-secure ABE for branching programs in some new variants, namely, unbounded, short-ciphertext, and short-key. Previous schemes are bounded and require linear-size ciphertexts and keys.