Abstract. We present a general method to prove security properties of cryptographic protocols against active adversaries, when the messages exchanged by the honest parties are arbitrary expressions built using encryption and concatenation operations. The method allows to express security properties and carry out proofs using a simple logic based language, where messages are represented by syntactic expressions, and does not require dealing with probability distributions or asymptotic notation explicitly. Still, we show that the method is sound, meaning that logic statements can be naturally interpreted in the computational setting in such a way that if a statement holds true for any abstract (symbolic) execution of the protocol in the presence of a Dolev-Yao adversary, then its computational interpretation is also correct in the standard computational model where the adversary is an arbitrary probabilistic polynomial time program. This is the first paper providing a simple framework for translating security proofs from the logic setting to the standard computational setting for the case of powerful active adversaries that have total control of the communication network. 1

protocol analysis

The history of the application of formal methods to cryptographic protocol analysis spans nearly twenty years, and recently has been showing signs of new maturity and consolidation. A number of specialized tools have been developed, and others have effectively demonstrated that existing general-purpose tools can also be applied to these problems with good results. However, with this better understanding of the field comes new problems that strain against the limits of the existing tools. In this paper we will outline some of these new problem areas, and describe what new research needs to be done to to meet the challenges posed.

We provide the first computational analysis of the well known Needham-Schröeder(-Lowe) protocol. We show that Lowe's attack to the original protocol can naturally be cast to the computational framework. Then we prove that chosen-plaintext security for encryption schemes is not sufficient to ensure soundness of formal proofs with respect to the computational setting, by exhibiting an attack against the corrected version of the protocol implemented using an ElGamal encryption scheme. Our main result is a proof that, when implemented using an encryption scheme that satisfies indistinguishability under chosen-ciphertext attack, the Needham-Schröeder-Lowe protocol is indeed a secure mutual authentication protocol. The technicalities of our proof reveal new insights regarding the relation between formal and computational models for system security.

Copyright c

Security protocols intend to give their parties reasonable assurance that certain security properties will protect their communication session. However, the literature confirms that the protocols may suffer subtle and hidden attacks. Flawed protocols are customarily sent back to the design process, but the costs of reengineering a deployed protocol may be prohibitive. This paper outlines the concept of retaliation: who would steal a sum of money today, should this pose significant risks of having twice as much stolen back tomorrow? Attacks are always balanced decisions: if an attack can be retaliated, the economics of security may convince us to live with a flawed protocol. This new perspective requires a new threat model where any party may decide to subvert the protocol for his own sake, depending on the risks of retaliation. This threat model, which for example is also suitable to studying non-repudiation protocols, seems more appropriate than the Dolev-Yao model to the present technological/social setting.

Abstract: Intrusion Detection Systems (IDS) are responsible for monitoring and analyzing host or network activity to detect intrusions in order to protect information from unauthorized access or manipulation. There are two main approaches for intrusion detection: signature-based and anomaly-based. Signature-based detection employs pattern matching to match attack signatures with observed data making it ideal for detecting known attacks. However, it cannot detect unknown attacks for which there is no signature available. Anomaly-based detection uses machine-learning techniques to create a profile of normal system behavior and uses this profile to detect deviations from the normal behavior. Although this technique is effective in detecting unknown attacks, it has a drawback of a high false alarm rate. In this paper, we describe our anomaly-based IDS designed for detecting malicious use of cryptographic and application-level protocols. Our system has several unique characteristics and benefits, such as the ability to monitor cryptographic protocols and application-level protocols embedded in encrypted sessions, a very lightweight monitoring process, and the ability to react to protocol misuse by modifying protocol response directly.

In this paper we report on an analysis for finding known-pair and chosen-text attacks in protocols. As these attacks are at the level of blocks, we extend the attacker by special capabilities related to block chaining techniques. The analysis is automated using Blanchet’s protocol verifier and illustrated on two well-known protocols, the Needham-Schroeder-Lowe public-key protocol as well as the Needham-Schroeder symmetric-key protocol. On the first protocol, we show how the special intruder capabilities related to chaining may compromise the secrecy of nonces and that chosenciphertext attacks are possible. We propose two modified versions of the protocol which strengthen its security. We then illustrate known-pair and chosen-plaintext attacks on the second protocol.

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Abstract As more resources are added to computer networks, and as more vendors look to the world wide web as a viable marketplace, the importance of being able to restrict access and to ensure some kind of acceptable behavior, even in the presence of malicious adversaries, becomes paramount. Many researchers have proposed the use of security protocols to provide these security guarantees. In this thesis, I describe a method of verifying these protocols using a special purpose model checker, Brutus, which performs an exhaustive state space search of a protocol model. This tool also includes a natural deduction style derivation engine which models the capabilities of an adversary trying to attack the protocol. Since the models are necessarily abstractions, one cannot prove a protocol correct. However, the tool is extremely useful as a debugger. I have used this tool to analyze fifteen different security protocols, and have found the previously reported attacks for them.