Revocation in Publicly Verifiable Outsourced Computation

Abstract

Abstract. The combination of software-as-a-service and the increasing use of mobile devices gives rise to a considerable difference in computational power between servers and clients. Thus, there is a desire for clients to outsource the evaluation of complex functions to an external server. Servers providing such a service may be rewarded per computation, and as such have an incentive to cheat by returning garbage rather than devoting resources and time to compute a valid result. In this work, we introduce the notion of Revocable Publicly Verifiable Computation (RPVC), where a cheating server is revoked and may not perform future computations (thus incurring a financial penalty). We introduce a Key Distribution Center (KDC) to efficiently handle the generation and distribution of the keys required to support RPVC. The KDC is an authority over entities in the system and enables revocation. We also introduce a notion of blind verification such that results are verifiable (and hence servers can be rewarded or punished) without learning the value. We present a rigorous definitional framework, define a number of new security models and present a construction of such a scheme built upon Key-Policy Attribute-based Encryption.

Access Control in Publicly Verifiable Outsourced Computation

“…Recent work has proposed extensions to VC where there exist large pools of delegators, known as Publicly Verifiable Outsourced Computation (PVC) [20], and servers [2]. As with any multi-user setting, we may wish to control access to resources.…”

“…Some VC settings distinguish between delegators and verifiers [2]. Delegators (or distinguished verifiers) may learn the result of the computation, whereas standard verifiers may only confirm that the result was computed correctly.…”

“…We instead use a cryptographic enforcement mechanism for access control policies where cryptographic keys are used to protect objects and restrict access -the access control mechanism reduces to the appropriate distribution of keys to authorized entities. We use the trusted Key Distribution Center (KDC) introduced in the Revocable Publicly Verifiable Outsourced Computation model (RPVC) [2] and extend its duties to instantiate the access control mechanism. In RPVC, the KDC issued keys that enable the delegation and evaluation of particular functions; we additionally require it to issue keys appropriate to the access control policy that enables read and write access to certain components of the PVC system.…”

“…We formulate policies in a generic fashion (irrespective of the cryptographic enforcement mechanism employed) in terms of authorization labels and graph-based policies over entities and computations. Then, in Section 4, we extend prior literature [16,20,2] to formally define a framework for PVC-AC. This results in novel security notions that we introduce in Section 5.…”

“…Our construction generically extends any RPVC scheme to use a symmetric Key Assignment Scheme (KAS). When instantiated with a PVC scheme built from Key-policy Attribute-based Encryption [2,20], this provides a pragmatic blend of symmetric and asymmetric primitives -the symmetric KAS enables the efficient derivation of keys, each associated to a security label, while policies are specified in terms of these labels. A trusted authority on entities, the KDC, issues keys to each entity corresponding to the highest security label for which they are authorized, and they may derive "lower" keys as required.…”