Chapter 17 - Auditable Version Control Systems in Untrusted Public Clouds

Abstract

Software development usually relies on a Version Control System (VCS) to automate the management of source code, documentation, and configuration files. The VCS stores all the changes to the data in a repository, such that any version of the data is retrievable at any time in the future. Due to their potentially massive size, VCS repositories are often outsourced to public cloud providers which, unfortunately, are not necessarily trusted. Remote Data Integrity Checking (RDIC) can be used to address the concerns about the untrusted nature of the public clouds by allowing data owners to periodically and efficiently verify that the cloud servers continue to store their data. To reduce storage overhead, modern VCSes usually adopt “delta encoding”, in which only the differences between versions are recorded. As a particular type of delta encoding, skip delta encoding can optimize the combined cost of storage and retrieval. This chapter introduces Auditable Version Control Systems (AVCS), which are version control systems designed to function under an adversarial setting. We present the definition of AVCS and then introduce an AVCS construction for skip delta-based version control systems, which relies on RDIC mechanisms to ensure all the versions of a file are retrievable from the untrusted VCS server over time. In our construction, the cost of checking the integrity of all the versions of a file is the same as checking the integrity of one file version, and the client is only required to maintain the same amount of client storage like a regular (unsecure) version control system. We make the important observation that the only meaningful operation for real-world VCS systems which use delta encoding is append and leverage this observation to build the AVCS construction. We implemented AVCS on top of a popular open-source version control system, Apache Subversion (SVN). In addition to AVCS, we also summarize and compare other RDIC approaches for version control systems.