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March 3, 2017

Gocryptfs Security Audit

I've completed a two-day audit of gocryptfs's cryptography design. You can download the audit report by clicking the link below. Thanks to 23andMe for funding this audit.

Abstract

This report documents a two-day audit of the gocryptfs encrypted filesystem. Unlike full-disk encryption systems, gocryptfs encrypts files individually using chunked AES-GCM (Galois Counter Mode) and encrypts filenames with AES-EME (ECB-Mix-ECB). Our audit focused on the crypgography design of gocryptfs's main file encryption features; it excluded its dependencies and its more complicated "Reverse Mode" feature that uses deterministic AES-SIV (Synthetic Initialization Vector) encryption. We did not look at the implementation code except when it was necessary to understand some aspect of the design.

We found that gocryptfs provides excellent confidentiality against a passive adversary, i.e. one that does not tamper with the encrypted files. On the other hand, we found that gocryptfs provides no security at all against an active adversary who can modify the ciphertexts while having read access to any subdirectory of the mounted filesystem. Against a less-powerful active adversary who can modify the ciphertexts but has no access to the mounted filesystem, gocryptfs keeps file contents secret and provides imperfect integrity protection. In at least one case, imperfections in the integrity protections lead to a break of confidentiality. It is possible that the integrity imperfections lead to further confidentiality breaks depending on which applications are using the filesystem.

We believe the reason these vulnerabilities exist is because gocryptfs doesn't have a clearly spelled-out threat model. Some of the attacks seem hard to avoid given gocryptfs's performance goals and may have been introduced "by design" to meet these goals. We suggest writing down an explicit threat model and updating the website to better communicate the security guarantees that gocryptfs provides. This way, users are less likely to rely on it in ways which would make them vulnerable.

Download

Security Audit of gocryptfs v1.2 (PDF)