Towards Fresh Re-Keying with Leakage-Resilient PRFs: Cipher Design Principles and Analysis

Leakage-resilient cryptography aims at developing new algorithms for which physical security against side-channel attacks can be formally analyzed. Following the work of Dziembowski and Pietrzak at FOCS 2008, several symmetric cryptographic primitives have been investigated in this setting. Most of them can be instantiated with a block cipher as underlying component. Such an approach naturally raises the question whether certain block ciphers are better suited for this purpose. In order to answer this question, we consider a leakage-resilient re-keying function, and evaluate its security at different abstraction levels. That is, we study possible attacks exploiting specific features of the algorithmic description, hardware architecture and physical implementation of this construction. These evaluations lead to two main outcomes. First, we complement previous works on leakage-resilient cryptography and further specify the conditions under which they actually provide physical security. Second, we take advantage of our analysis to extract new design principles for block ciphers to be used in leakage-resilient primitives. While our investigations focus on side-channel attacks in the first place, we hope these new design principles will trigger the interest of symmetric cryptographers to design new block ciphers combining good properties for secure implementations and security against black box (mathematical) cryptanalysis

Leakage-Resilient and Misuse-Resistant Authenticated Encryption

Leakage-resilience and misuse-resistance are two important properties for the deployment of authenticated encryption schemes. They aim at mitigating the impact of implementation flaws due to side-channel leakages and misused randomness. In this paper, we discuss their interactions and incompatibilities. For this purpose, we first show a generic composition mode of a MAC with an encryption scheme that leads to a misuse-resistant authenticated encryption scheme, and also show that misuse-resistance does not hold anymore in the presence of leakages, even when relying on leakage-resilient MACs and encryption schemes. Next, we argue that full misuse-resistance with leakage may be impossible to achieve with simple primitives such as hash functions and block ciphers. As a result, we formalize a new security notion of ciphertext integrity with misuse and leakage, which seems to be the best that can be achieved in a symmetric cryptographic setting, and describe first efficient constructions satisfying it

Leakage and Tamper Resilient Permutation-Based Cryptography

Implementation attacks such as power analysis and fault attacks have shown that, if potential attackers have physical access to a cryptographic device, achieving practical security requires more considerations apart from just cryptanalytic security. In recent years, and with the advent of micro-architectural or hardware-oriented attacks, it became more and more clear that similar attack vectors can also be exploited on larger computing platforms and without the requirement of physical proximity of an attacker. While newly discovered attacks typically come with implementation recommendations that help counteract a specific attack vector, the process of constantly patching cryptographic code is quite time consuming in some cases, and simply not possible in other cases. What adds up to the problem is that the popular approach of leakage resilient cryptography only provably solves part of the problem: it discards the threat of faults. Therefore, we put forward the usage of leakage and tamper resilient cryptographic algorithms, as they can offer built-in protection against various types of physical and hardware oriented attacks, likely including attack vectors that will only be discovered in the future. In detail, we present the - to the best of our knowledge - first framework for proving the security of permutation-based symmetric cryptographic constructions in the leakage and tamper resilient setting. As a proof of concept, we apply the framework to a sponge-based stream encryption scheme called asakey and provide a practical analysis of its resistance against side channel and fault attacks

On the Security of Fresh Re-keying to Counteract Side-Channel and Fault Attacks

At AFRICACRYPT 2010 and CARDIS 2011, fresh re-keying schemes to counter side-channel and fault attacks were introduced. The idea behind those schemes is to shift the main burden of side-channel protection to a re-keying function $g$ that is easier to protect than the main block cipher. This function produces new session keys based on the secret master key and random nonces for every block of message that is encrypted. In this paper, we present a generic chosen-plaintext key-recovery attack on both fresh re-keying schemes. The attack is based on two observations: Since session key collisions for the same message are easy to detect, it is possible to recover one session key with a simple time-memory trade-off strategy; and if the re-keying function is easy to invert (such as the suggested multiplication constructions), the attacker can use the session key to recover the master key. The attack has a complexity of about $2 \cdot 2^{n/2}$ (instead of the expected $2^n$) for an $n$-bit key. For the typically employed block cipher AES-128, this would result in a key-recovery attack complexity of only $2^{65}$. If weaker primitives like 80-bit PRESENT are used, even lower attack complexities are possible

Security of Ubiquitous Computing Systems

The chapters in this open access book arise out of the EU Cost Action project Cryptacus, the objective of which was to improve and adapt existent cryptanalysis methodologies and tools to the ubiquitous computing framework. The cryptanalysis implemented lies along four axes: cryptographic models, cryptanalysis of building blocks, hardware and software security engineering, and security assessment of real-world systems. The authors are top-class researchers in security and cryptography, and the contributions are of value to researchers and practitioners in these domains. This book is open access under a CC BY license

Cryptographic Analysis of Secure Messaging Protocols

Instant messaging applications promise their users a secure and private way to communicate. The validity of these promises rests on the design of the underlying protocol, the cryptographic primitives used and the quality of the implementation. Though secure messaging designs exist in the literature, for various reasons developers of messaging applications often opt to design their own protocols, creating a gap between cryptography as understood by academic research and cryptography as implemented in practice. This thesis contributes to bridging this gap by approaching it from both sides: by looking for flaws in the protocols underlying real-world messaging applications, as well as by performing a rigorous analysis of their security guarantees in a provable security model.Secure messaging can provide a host of different, sometimes conflicting, security and privacy guarantees. It is thus important to judge applications based on the concrete security expectations of their users. This is particularly significant for higher-risk users such as activists or civil rights protesters. To position our work, we first studied the security practices of protesters in the context of the 2019 Anti-ELAB protests in Hong Kong using in-depth, semi-structured interviews with participants of these protests. We report how they organised on different chat platforms based on their perceived security, and how they developed tactics and strategies to enable pseudonymity and detect compromise.Then, we analysed two messaging applications relevant in the protest context: Bridgefy and Telegram. Bridgefy is a mobile mesh messaging application, allowing users in relative proximity to communicate without the Internet. It was being promoted as a secure communication tool for use in areas experiencing large-scale protests. We showed that Bridgefy permitted its users to be tracked, offered no authenticity, no effective confidentiality protections and lacked resilience against adversarially crafted messages. We verified these vulnerabilities by demonstrating a series of practical attacks.Telegram is a messaging platform with over 500 million users, yet prior to this work its bespoke protocol, MTProto, had received little attention from the cryptographic community. We provided the first comprehensive study of the MTProto symmetric channel as implemented in cloud chats. We gave both positive and negative results. First, we found two attacks on the existing protocol, and two attacks on its implementation in official clients which exploit timing side channels and uncover a vulnerability in the key exchange protocol. Second, we proved that a fixed version of the symmetric MTProto protocol achieves security in a suitable bidirectional secure channel model, albeit under unstudied assumptions. Our model itself advances the state-of-the-art for secure channels

Combining Leakage-Resilient PRFs and Shuffling Towards Bounded Security for Small Embedded Devices

Combining countermeasures is usually assumed to be the best way to protect embedded devices against side-channel attacks. These combinations are at least expected to increase the number of measurements of successful attacks to some reasonable extent, and at best to guarantee a bounded time complexity independent of the number of measurements. This latter guarantee, only possible in the context of leakage resilient constructions, was only reached either for stateful (pseudo-random generator) constructions, or large parallel implementations so far. In this paper, we describe a first proposal of stateless (pseudo-random function) construction, for which we have strong hints that security bounded implementations are reachable under the constraints of small embedded devices. Our proposal essentially combines the well-known shuffling countermeasure with a tweaked pseudo-random function introduced at CHES 2012.We rst detail is performances. Then we analyze it against standard differential power analysis and discuss the different parameters influencing its security bounds. Finally, we put forward that its implementation in 8-bit microcontrollers can provide a better security vs. performance tradeo than state-of-the art (combinations of) countermeasures

Security of Ubiquitous Computing Systems

The chapters in this open access book arise out of the EU Cost Action project Cryptacus, the objective of which was to improve and adapt existent cryptanalysis methodologies and tools to the ubiquitous computing framework. The cryptanalysis implemented lies along four axes: cryptographic models, cryptanalysis of building blocks, hardware and software security engineering, and security assessment of real-world systems. The authors are top-class researchers in security and cryptography, and the contributions are of value to researchers and practitioners in these domains. This book is open access under a CC BY license

LIPIcs, Volume 251, ITCS 2023, Complete Volume

LIPIcs, Volume 251, ITCS 2023, Complete Volum