Biblio

Remote Attestation ( RA) is a security service that enables a trusted verifier ( Vrf) to measure current memory state of an untrusted remote prover ( Prv). If correctly implemented, RA allows Vrf to remotely detect if Prv's memory reflects a compromised state. However, RA by itself offers no means of remedying the situation once P rv is determined to be compromised. In this work we show how a secure RA architecture can be extended to enable important and useful security services for low-end embedded devices. In particular, we extend the formally verified RA architecture, VRASED, to implement provably secure software update, erasure, and system-wide resets. When (serially) composed, these features guarantee to Vrf that a remote Prv has been updated to a functional and malware-free state, and was properly initialized after such process. These services are provably secure against an adversary (represented by malware) that compromises Prv and exerts full control of its software state. Our results demonstrate that such services incur minimal additional overhead (0.4% extra hardware footprint, and 100-s milliseconds to generate combined proofs of update, erasure, and reset), making them practical even for the lowest-end embedded devices, e.g., those based on MSP430 or AVR ATMega micro-controller units (MCUs). All changes introduced by our new services to VRASED trusted components are also formally verified.

Assuring integrity of information (e.g., data and/or software) is usually accomplished by cryptographic means, such as hash functions or message authentication codes (MACs). Computing such integrity-ensuring functions can be time-consuming if the amount of input data is large and/or the computing platform is weak. At the same time, in real-time or safety-critical settings, it is often impractical or even undesirable to guarantee atomicity of computing a time-consuming integrity-ensuring function. Meanwhile, standard correctness and security definitions of such functions assume that input data (regardless of its size) remains consistent throughout computation. However, temporal consistency may be lost if another process interrupts execution of an integrity-ensuring function and modifies portions of input that either or both: (1) were already processed, or (2) were not processed yet. Lack of temporal consistency might yield an integrity result that is non-sensical or simply incorrect. Such subtleties and discrepancies between (implicit) assumptions in definitions and implementations can be a source of inconsistenceies, which might lead to vulnerabilities. In this paper, we systematically explore the notion of temporal consistency of cryptographic integrity-ensuring functions. We show that its lack in implementations of such functions can lead to inconsistent results and security violations in protocols and systems using them, e.g., remote attestation, remote updates and secure resets. We consider several mechanisms that guarantee temporal consistency of implementations of integrity-ensuring functions in embedded systems with a focus on remote attestation. We also assess performance of proposed mechanisms on two commodity hardware platforms: I.MX6-SabreLite and ODROID-XU4.

In a secret sharing scheme a dealer shares a secret s among n parties such that an adversary corrupting up to t parties does not learn s, while any t+1 parties can efficiently recover s. Over a long period of time all parties may be corrupted thus violating the threshold, which is accounted for in Proactive Secret Sharing (PSS). PSS schemes periodically rerandomize (refresh) the shares of the secret and invalidate old ones. PSS retains confidentiality even when all parties are corrupted over the lifetime of the secret, but no more than t during a certain window of time, called the refresh period. Existing PSS schemes only guarantee secrecy in the presence of an honest majority with less than n2 total corruptions during a refresh period; an adversary corrupting a single additional party, even if only passively, obtains the secret. This work is the first feasibility result demonstrating PSS tolerating a dishonest majority, it introduces the first PSS scheme secure against t passive adversaries without recovery of lost shares, it can also recover from honest faulty parties losing their shares, and when tolerating e faults the scheme tolerates t passive corruptions. A non-robust version of the scheme can tolerate t active adversaries, and mixed adversaries that control a combination of passively and actively corrupted parties that are a majority, but where less than n/2-e of such corruptions are active. We achieve these high thresholds with O(n4) communication when sharing a single secret, and O(n3) communication when sharing multiple secrets in batches.