Protecting Encrypted Cookies from Compression Side-Channel Attacks

Compression is desirable for network applications as it saves bandwidth; however, when data is compressed before being encrypted, the amount of compression leaks information about the amount of redundancy in the plaintext. This side channel has led to successful CRIME and BREACH attacks on web traffic protected by the Transport Layer Security (TLS) protocol. The general guidance in light of these attacks has been to disable compression, preserving confidentiality but sacrificing bandwidth. In this paper, we examine two techniques—heuristic separation of secrets and fixed-dictionary compression—for enabling compression while protecting high-value secrets, such as cookies, from attack. We model the security offered by these techniques and report on the amount of compressibility that they can achieve. This is the full version of a paper published in the Proceedings of the 19th International Conference on Financial Cryptography and Data Security (FC 2015) in San Juan, Puerto Rico, USA, January 26–30, 2015, organized by the International Financial Cryptography Association in cooperation with IACR.