Paper 2019/1409

Cloud-assisted Asynchronous Key Transport with Post-Quantum Security

Gareth T. Davies and Herman Galteland and Kristian Gjøsteen and Yao Jiang

Abstract

In cloud-based outsourced storage systems, many users wish to securely store their files for later retrieval, and additionally to share them with other users. These retrieving users may not be online at the point of the file upload, and in fact they may never come online at all. In this asynchoronous environment, key transport appears to be at odds with any demands for forward secrecy. Recently, Boyd et al. (ISC 2018) presented a protocol that allows an initiator to use a modified key encapsulation primitive, denoted a blinded KEM (BKEM), to transport a file encryption key to potentially many recipients via the (untrusted) storage server, in a way that gives some guarantees of forward secrecy. Until now all known constructions of BKEMs are built using RSA and DDH, and thus are only secure in the classical setting. We further the understanding of secure key transport protocols in two aspects. First, we show how to generically build blinded KEMs from homomorphic encryption schemes with certain prop- erties. Second, we construct the first post-quantum secure blinded KEMs, and the security of our constructions are based on hard lattice problems.