**System-level non-interference for constant-time cryptography**

*Gilles Barthe and Gustavo Betarte and Juan Diego Campo and Carlos Luna and David Pichardie*

**Abstract: **Cache-based attacks are a class of side-channel attacks that are
particularly effective in virtualized or cloud-based environments,
where they have been used to recover secret keys from cryptographic
implementations. One common approach to thwart cache-based attacks is
to use \emph{constant-time} implementations, i.e.\, which do not
branch on secrets and do not perform memory accesses that depend on
secrets. However, there is no rigorous proof that constant-time
implementations are protected against concurrent cache-attacks in
virtualization platforms with shared cache; moreover, many prominent
implementations are not constant-time. An alternative approach is to
rely on system-level mechanisms. One recent such mechanism is stealth
memory, which provisions a small amount of private cache for programs
to carry potentially leaking computations securely. Stealth memory
induces a weak form of constant-time, called \emph{S-constant-time},
which encompasses some widely used cryptographic
implementations. However, there is no rigorous analysis of stealth
memory and S-constant-time, and no tool support for checking if
applications are S-constant-time.

We propose a new information-flow analysis that checks if an x86 application executes in constant-time, or in S-constant-time. Moreover, we prove that constant-time (resp. S-constant-time) programs do not leak confidential information through the cache to other operating systems executing concurrently on virtualization platforms (resp. platforms supporting stealth memory). The soundness proofs are based on new theorems of independent interest, including isolation theorems for virtualization platforms (resp. platforms supporting stealth memory), and proofs that constant-time implementations (resp. S-constant-time implementations) are non-interfering with respect to a strict information flow policy which disallows that control flow and memory accesses depend on secrets. We formalize our results using the \textsf{Coq} proof assistant and we demonstrate the effectiveness of our analyses on cryptographic implementations, including PolarSSL AES, DES and RC4, SHA256 and Salsa20.

**Category / Keywords: **non-interference, cache-based attacks, constant-time cryptography, stealth memory, Coq

**Date: **received 3 Jun 2014

**Contact author: **gjbarthe at gmail com

**Available format(s): **PDF | BibTeX Citation

**Version: **20140606:050829 (All versions of this report)

**Short URL: **ia.cr/2014/422

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