We present an approach for verifying systems at runtime. Our approach targets distributed systems whose components communicate with monitors over unreliable channels, where messages can be delayed, reordered, or even lost. Furthermore, our approach handles an expressive specification language that extends the real-time logic MTL with freeze quantifiers for reasoning about data values. The logic’s main novelty is a new three-valued semantics that is well suited for runtime verification as it accounts for partial knowledge about a system’s behavior. Based on this semantics, we present online algorithms that reason soundly and completely about streams where events can occur out of order. We also evaluate our algorithms experimentally. Depending on the specification, our prototype implementation scales to out-of-order streams with hundreds to thousands of events per second.

The first international Competition on Runtime Verification (CRV) was held in September 2014, in Toronto, Canada, as a satellite event of the 14th international conference on Runtime Verification (RV’14). The event was organized in three tracks: (1) offline monitoring, (2) online monitoring of C programs, and (3) online monitoring of Java programs. In this paper, we report on the phases and rules, a description of the participating teams and their submitted benchmark, the (full) results, as well as the lessons learned from the competition.

Our goal is to certify absence of information leaks in multi-agent workflows, such as conference management systems like EasyChair. These workflows can be executed by any number of agents some of which may form coalitions against the system. Therefore, checking noninterference is a challenging problem. Our paper offers two main contributions: First, a technique is provided to translate noninterference (in presence of various agent capabilities and declassification conditions) into universally quantified invariants of an instrumented new workflow program. Second, general techniques are developed for checking and inferring universally quantified inductive invariants for workflow programs. In particular, a large class of workflows is identified where inductiveness of invariants is decidable, as well as a smaller, still useful class of workflows where the weakest inductive universal invariant implying the desired invariant, is effectively computable. The new algorithms are implemented and applied to certify noninterference for workflows arising from conference management systems.

We consider the automatic verification of information flow security policies of web-based workflows, such as conference submission systems like EasyChair. Our workflow description language allows for loops, non-deterministic choice, and an unbounded number of participating agents. The information flow policies are specified in a temporal logic for hyperproperties. We show that the verification problem can be reduced to the satisfiability of a formula of first-order linear-time temporal logic, and provide decidability results for relevant classes of workflows and specifications. We report on experimental results obtained with an implementation of our approach on a series of benchmarks.

Prior runtime-verification approaches for distributed systems are limited as they do not account for network failures and they assume that system messages are received in the order they are sent. To overcome these limitations, we present an online algorithm for verifying observed system behavior at runtime with respect to specifications written in the real-time logic MTL that efficiently handles out-of-order message deliveries and operates in the presence of failures. Our algorithm uses a three-valued semantics for MTL, where the third truth value models knowledge gaps, and it resolves knowledge gaps as it propagates Boolean values through the formula structure. We establish the algorithm's soundness and provide completeness guarantees. We also show that it supports distributed system monitoring, where multiple monitors cooperate and exchange their observations and conclusions.

Runtime monitoring is a general approach to verifying system properties at runtime by comparing system events against a specification formalizing which event sequences are allowed. We present a runtime monitoring algorithm for a safety fragment of metric first-order temporal logic that overcomes the limitations of prior monitoring algorithms with respect to the expressiveness of their property specification languages. Our approach, based on automatic structures, allows the unrestricted use of negation, universal and existential quantification over infinite domains, and the arbitrary nesting of both past and bounded future operators. Furthermore, we show how to use and optimize our approach for the common case where structures consist of only finite relations, over possibly infinite domains. We also report on case studies from the domain of security and compliance in which we empirically evaluate the presented algorithms. Taken together, our results show that metric first-order temporal logic can serve as an effective specification language for expressing and monitoring a wide variety of practically relevant system properties.

We revisit Schneider’s work on policy enforcement by execution monitoring. We overcome limitations of Schneider’s setting by distinguishing between system actions that are controllable by an enforcement mechanism and those actions that are only observable, that is, the enforcement mechanism sees them but cannot prevent their execution. For this refined setting, we give necessary and sufficient conditions on when a security policy is enforceable. To state these conditions, we generalize the standard notion of safety properties. Our classification of system actions also allows one, for example, to reason about the enforceability of policies that involve timing constraints. Furthermore, for different specification languages, we investigate the decision problem of whether a given policy is enforceable. We provide complexity results and show how to synthesize an enforcement mechanism from an enforceable policy.

IT systems manage increasing amounts of sensitive data and there is a growing concern that they comply with policies that regulate data usage. In this paper, we use temporal logic to express policies and runtime monitoring to check system compliance. While well-established methods for monitoring linearly ordered system behavior exist, a major challenge is monitoring distributed and concurrent systems where actions are locally observed in the different system parts. These observations can only be partially ordered, while policy compliance may depend on the actions' actual order of appearance. Technically speaking, it is in general intractable to check compliance of partially ordered traces. We identify fragments of our policy specification language for which compliance can be checked efficiently, namely, by monitoring a single representative trace in which the observed actions are totally ordered. Through a case study we show that the fragments are capable of expressing nontrivial policies and that monitoring representative traces is feasible on real-world data.

We narrow the gap between concrete implementations of cryptographic protocols and their verified models. We develop and verify a small functional implementation of the Transport Layer Security protocol (TLS 1.0). We make use of the same executable code for interoperability testing against mainstream implementations for automated symbolic cryptographic verification and automated computational cryptographic verification. We rely on a combination of recent tools and also develop a new tool for extracting computational models from executable code. We obtain strong security guarantees for TLS as used in typical deployments.

We have previously presented a monitoring algorithm for compliance checking of policies formalized in an expressive metric first-order temporal logic. We explain here the steps required to go from the original algorithm to a working infrastructure capable of monitoring an existing distributed application producing millions of log entries per day. The main challenge is to correctly and efficiently monitor the trace interleavings obtained by totally ordering actions that happen at the same time. We provide solutions based on formula transformations and monitoring representative traces. We also report, for the first time, on statistics on the performance of our monitor on real-world data, providing evidence of its suitability for nontrivial applications.

There is a large amount of work dedicated to the formal verification of security protocols. In this article, we revisit and extend the NP-complete decision procedure for a bounded number of sessions. We use a, now standard, deducibility constraint formalism for modeling security protocols. Our first contribution is to give a simple set of constraint simplification rules, that allows to reduce any deducibility constraint to a set of solved forms , representing all solutions (within the bound on sessions). As a consequence, we prove that deciding the existence of key cycles is NP-complete for a bounded number of sessions. The problem of key-cycles has been put forward by recent works relating computational and symbolic models. The so-called soundness of the symbolic model requires indeed that no key cycle (e.g., enc(k, k)) ever occurs in the execution of the protocol. Otherwise, stronger security assumptions (such as KDM-security) are required. We show that our decision procedure can also be applied to prove again the decidability of authentication-like properties and the decidability of a significant fragment of protocols with timestamps.

We intend to narrow the gap between concrete implementations of cryptographic protocols and their verified models. We develop and verify a small functional implementation of the Transport Layer Security protocol (TLS 1.0). We make use of the same executable code for interoperability testing against mainstream implementations, for automated symbolic cryptographic verification, and for automated computational cryptographic verification. We rely on a combination of recent tools, and we also develop a new tool for extracting computational models from executable code. We obtain strong security guarantees for TLS as used in typical deployments.

Two styles of definitions are usually considered to express that a security protocol preserves the confidentiality of a data s. Reachability-based secrecy means that s should never be disclosed while equivalence-based secrecy states that two executions of a protocol with distinct instances for s should be indistinguishable to an attacker. Although the second formulation ensures a higher level of security and is closer to cryptographic notions of secrecy, decidability results and automatic tools have mainly focused on the first definition so far. This paper initiates a systematic investigation of the situations where syntactic secrecy entails strong secrecy. We show that in the passive case, reachability-based secrecy actually implies equivalence-based secrecy for digital signatures, symmetric and asymmetric encryption provided that the primitives are probabilistic. For active adversaries, we provide sufficient (and rather tight) conditions on the protocol for this implication to hold.

Formal methods have proved to be very useful for analyzing cryptographic protocols. However, most existing techniques apply to the case of abstract encryption schemes and pairing. In this paper, we consider more complex, less studied cryptographic primitives like CBC encryption and blind signatures. This leads us to introduce a new fragment of Horn clauses. We show decidability of this fragment using a combination of several resolution strategies.As a consequence, we obtain a new decidability result for a class of cryptographic protocols (with an unbounded number of sessions and a bounded number of nonces) that may use for example CBC encryption and blind signatures. We apply this result to fix the Needham-Schroeder symmetric key authentication protocol, which is known to be flawed when CBC mode is used.