arXiv subjects

Steven Keuchel

Publications and source records attributed to Steven Keuchel.

4 recordsLinked to original sources

arXiv2026

Cerisier: A Program Logic for Attestation in a Capability Machine

A key feature in trusted computing is attestation, which allows encapsulated components (enclaves) to prove their identity to (local or remote) distrusting components. Reasoning about software that uses the technique requires tracking how trust evolves after successful attestation. This process is security-critical and non-trivial, but no existing formal verification technique supports modular reasoning about attestation of enclaves and their clients, or proving end-to-end properties for systems combining trusted, untrusted and attested code. We contribute Cerisier, the first program logic for modular reasoning about trusted, untrusted and attested code, fully mechanized in the Iris separation logic and the Rocq Prover. We formalize a recent proposal, CHERI-TrEE, to extend capability machines with enclave primitives, as an extension to the Cerise capability machine and program logic. Our program logic comes with a universal contract for untrusted code, which captures both capability safety and local enclave attestation. Like Cerise, this universal contract is phrased in terms of a logical relation defining capabilities' authority. We demonstrate Cerisier by proving end-to-end properties for three representative applications of trusted computing: secure outsourced computation, mutual attestation and a modeled trusted sensor component.

cs.PL↗

arXiv2024

Formalizing, Verifying and Applying ISA Security Guarantees as Universal Contracts

Progress has recently been made on specifying instruction set architectures (ISAs) in executable formalisms rather than through prose. However, to date, those formal specifications are limited to the functional aspects of the ISA and do not cover its security guarantees. We present a novel, general method for formally specifying an ISAs security guarantees to (1) balance the needs of ISA implementations (hardware) and clients (software), (2) can be semi-automatically verified to hold for the ISA operational semantics, producing a high-assurance mechanically-verifiable proof, and (3) support informal and formal reasoning about security-critical software in the presence of adversarial code. Our method leverages universal contracts: software contracts that express bounds on the authority of arbitrary untrusted code. Universal contracts can be kept agnostic of software abstractions, and strike the right balance between requiring sufficient detail for reasoning about software and preserving implementation freedom of ISA designers and CPU implementers. We semi-automatically verify universal contracts against Sail implementations of ISA semantics using our Katamaran tool; a semi-automatic separation logic verifier for Sail which produces machine-checked proofs for successfully verified contracts. We demonstrate the generality of our method by applying it to two ISAs that offer very different security primitives: (1) MinimalCaps: a custom-built capability machine ISA and (2) a (somewhat simplified) version of RISC-V with PMP. We verify a femtokernel using the security guarantee we have formalized for RISC-V with PMP.

cs.PL↗

arXiv2020

Uninitialized Capabilities

This technical report describes a new extension to capability machines. Capability machines are a special type of processors that include better security primitives at the hardware level. In capability machines, every word has an associated tag bit that indicates whether the value it contains is a capability or a regular data value. Capabilities enable fine-grained control of the authority over memory that program components have. Conceptually, capabilities can be viewed as being an unforgeable token carrying authority over a resource. CHERI is a recently developed capability machine that aims to provide fine-grained memory protection, software compartmentalization and backwards compatibility. While our ideas are implemented on CHERI, they are not limited to it and should be applicable to other capability machines as well. In this technical report we propose a new type of capabilities, which represent the authority to access (read and write to) a block of memory but not view its initial contents. Our main goal is to use this new type of capability as part of a secure calling convention, but other applications may be possible too.

cs.PL↗

arXiv2017

Modular, Fully-abstract Compilation by Approximate Back-translation

A compiler is fully-abstract if the compilation from source language programs to target language programs reflects and preserves behavioural equivalence. Such compilers have important security benefits, as they limit the power of an attacker interacting with the program in the target language to that of an attacker interacting with the program in the source language. Proving compiler full-abstraction is, however, rather complicated. A common proof technique is based on the back-translation of target-level program contexts to behaviourally-equivalent source-level contexts. However, constructing such a back- translation is problematic when the source language is not strong enough to embed an encoding of the target language. For instance, when compiling from STLC to ULC, the lack of recursive types in the former prevents such a back-translation. We propose a general and elegant solution for this problem. The key insight is that it suffices to construct an approximate back-translation. The approximation is only accurate up to a certain number of steps and conservative beyond that, in the sense that the context generated by the back-translation may diverge when the original would not, but not vice versa. Based on this insight, we describe a general technique for proving compiler full-abstraction and demonstrate it on a compiler from STLC to ULC. The proof uses asymmetric cross-language logical relations and makes innovative use of step-indexing to express the relation between a context and its approximate back-translation. The proof extends easily to common compiler patterns such as modular compilation and it, to the best of our knowledge, it is the first compiler full abstraction proof to have been fully mechanised in Coq. We believe this proof technique can scale to challenging settings and enable simpler, more scalable proofs of compiler full-abstraction.

cs.PL↗