“To share or not to share” in client-side encrypted clouds

With the advent of cloud computing, a number of cloud providers have arisen to provide Storage-as-a-Service (SaaS) offerings to both regular consumers and business organizations. SaaS (different than Software-as-a-Service in this context) refers to an architectural model in which a cloud provider provides digital storage on their own infrastructure. Three models exist amongst SaaS providers for protecting the confidentiality of data stored in the cloud: 1) no encryption (data is stored in plain text), 2) server-side encryption (data is encrypted once uploaded), and 3) client-side encryption (data is encrypted prior to upload). Through a combination of a Network and Source Code Analysis, this paper seeks to identify weaknesses in the third model, as it claims to offer 100% user data confidentiality throughout all data transactions. The weaknesses we uncovered primarily center around the fact that the cloud providers we evaluated (Wuala, Tresorit, and Spider Oak) were each operating in a Certificate Authority capacity to facilitate data sharing. In this capacity, they assume the role of both certificate issuer and certificate authorizer as denoted in a Public-Key Infrastructure (PKI) scheme - which gives them the ability to view user data contradicting their claims of 100% data confidentiality. We have collated our analysis and findings in this paper and explore some potential solutions to address these weaknesses in these sharing methods. The solutions proposed are a combination of best practices associated with the use of PKI and other cryptographic primitives generally accepted for protecting the confidentiality of shared information.