Abstract Non-malleable codes were introduced by Dziembowski et al. (in: Yao (ed) ICS2010, Tsinghua University Press, 2010), and its main application is the protection of cryptographic devices against tampering attacks on memory. In this work, we initiate a comprehensive study on non-malleable codes for the class of partial functions, that read/write on an arbitrary subset of codeword bits with specific cardinality. We present two constructions: the first one is in the CRS model and allows the adversary to selectively choose the subset of codeword bits, while the latter is in the standard model and adaptively secure. Our constructions are efficient in terms of information rate, while allowing the attacker to access asymptotically almost the entire codeword. In addition, they satisfy a notion which is stronger than non-malleability, that we call non-malleability with manipulation detection, guaranteeing that any modified codeword decodes to either the original message or to $$\bot $$ ⊥ . We show that our primitive implies All-Or-Nothing Transforms (AONTs), and as a result our constructions yield efficient AONTs under standard assumptions (only one-way functions), which, to the best of our knowledge, was an open question until now. Furthermore, we construct a notion of continuous non-malleable codes (CNMC), namely CNMC with light updates, that avoids the full re-encoding process and only uses shuffling and refreshing operations. Finally, we present a number of additional applications of our primitive in tamper resilience.

The Messaging Layer Security (MLS) project is an IETF effort aiming to establish an industry-wide standard for secure group messaging (SGM). Its development is supported by several major secure-messaging providers (with a combined user base in the billions) and a growing body of academic research. MLS has evolved over many iterations to become a complex, non-trivial, yet relatively ad-hoc cryptographic protocol. In an effort to tame its complexity and build confidence in its security, past analyses of MLS have restricted themselves to sub-protocols of MLS---most prominently a type of sub-protocol embodying so-called continuous group key agreement (CGKA). However, to date the task of proving or even defining the security of the full MLS protocol has been left open. In this work, we fill in this missing piece. First, we formally capture the security of SGM protocols by defining a corresponding security game, which is parametrized by a safety predicate that characterizes the exact level of security achieved by a construction. Then, we cast MLS as an SGM protocol, showing how to modularly build it from the following three main components (and some additional standard cryptographic primitives) in a black-box fashion: (a) CGKA, (b) forward-secure group AEAD (FS-GAEAD), which is a new primitive and roughly corresponds to an "epoch'' of group messaging, and (c) a so-called PRF-PRNG, which is a two-input hash function that is a pseudorandom function (resp.\ generator with input) in its first (resp.\ second) input. Crucially, the security predicate for the SGM security of MLS can be expressed purely as a function of the security predicates of the underlying primitives, which allows to swap out any of the components and immediately obtain a security statement for the resulting SGM construction. Furthermore, we provide instantiations of all component primitives, in particular of CGKA with MLS's TreeKEM sub-protocol (which we prove adaptively secure) and of FS-GAEAD with a novel construction (which has already been adopted by MLS). Along the way we introduce a collection of new techniques, primitives, and results with applications to other SGM protocols and beyond. For example, we extend the Generalized Selective Decryption proof technique (which is central in CGKA literature) and prove adaptive security for another (practical) more secure CGKA protocol called RTreeKEM (Alwen et al.,\ CRYPTO '20). The modularity of our approach immediately yields a corollary characterizing the security of an SGM construction using RTreeKEM.

In this work, we significantly improve the efficiency of non-malleable codes in the split state model, by constructing a code with codeword length (roughly), where |s| is the length of the message, and k is the security parameter. This is a substantial improvement over previous constructions, both asymptotically and concretely.

We study the strategic considerations of miners participating in the bitcoin's protocol. We formulate and study the stochastic game that underlies these strategic considerations. The miners collectively build a tree of blocks, and they are paid when they create a node (mine a block) which will end up in the path of the tree that is adopted by all. Since the miners can hide newly mined nodes, they play a game with incomplete information. Here we consider two simplified forms of this game in which the miners have complete information. In the simplest game the miners release every mined block immediately, but are strategic on which blocks to mine. In the second more complicated game, when a block is mined it is announced immediately, but it may not be released so that other miners cannot continue mining from it. A miner not only decides which blocks to mine, but also when to release blocks to other miners. In both games, we show that when the computational power of each miner is relatively small, their best response matches the expected behavior of the bitcoin designer. However, when the computational power of a miner is large, he deviates from the expected behavior, and other Nash equilibria arise.

We study flow optimization in cognitive Wireless Mesh Networks (WMN) that lease bandwidth from a collocated cellular network (CN). We develop a distributed mechanism that such WMNs can use to optimize their traffic given their own utilities and costs and the costs that the CN is charging for the use of its channels. We evaluate the mechanism in a synthetic scenario using the topology of a deployed WMN and time-varying traffic requirements induced from real-life vehicle mobility traces. Simulations show that the mechanism can track well the changes in the problem parameters, continuously keeping close to the optimum operation point.