A Survey on PoW-based Consensus

We provide a historical overview of proof-of-work techniques and the fields in which it plunges its roots. We are interested in PoW-techniques applied to blockchain technology and therefore we survey the state-of-the-art protocols employing these methods for consensus algorithms, emphasizing the differences between the efficient hashcash systems and the promising bread pudding protocols. Afterwards, the consensus mechanisms are discussed and some interesting known attacks to these algorithms are collected and classified according to their underlying ideas

Authentication Methods and Password Cracking

Na začátku této práce porovnáváme dnes běžně používané metody autentizace a také mluvíme o historii, současnosti a budoucnosti zabezpečení hesel. Později využíváme nástroj Hashcat k experimentům s útoky hrubou silou a slovníkovými útoky, které zrychlujeme s pomocí Markovových modelů a pravidel pro manipulaci se slovy. Porovnáváme také dva hardwarové přístupy --- běžný počítač a cloud computing. Nakonec na základě našich poznatků práci uzavíráme souborem doporučení na prolamování hesel s důrazem na hardware, velikost datové sady a použitou hašovací funkci.In the beginning of this thesis, we compare authentication methods commonly used today and dive into the history, state of the art as well as the future of password security. Later on, we use the tool Hashcat to experiment with brute-force and dictionary attacks accelerated with Markov models and word mangling rules. We also compare two hardware approaches --- regular computer and cloud computing. Based on our findings, we finally conclude with a set of password-cracking recommendations with focus on hardware, dataset size and used hash function

Lyra2: Efficient Password Hashing with High Security against Time-Memory Trade-Offs

We present Lyra2, a password hashing scheme (PHS) based on cryptographic sponges. Lyra2 was designed to be strictly sequential (i.e., not easily parallelizable), providing strong security even against attackers that uses multiple processing cores (e.g., custom hardware or a powerful GPU). At the same time, it is very simple to implement in software and allows legitimate users to fine tune its memory and processing costs according to the desired level of security against brute force password-guessing. Lyra2 is an improvement of the recently proposed Lyra algorithm, providing an even higher security level against different attack venues and overcoming some limitations of this and other existing schemes

Balloon Hashing: A Memory-Hard Function Providing Provable Protection Against Sequential Attacks

We present the Balloon password-hashing algorithm. This is the first practical cryptographic hash function that: (i) has proven memory-hardness properties in the random-oracle model, (ii) uses a password-independent access pattern, and (iii) meets or exceeds the performance of the best heuristically secure password-hashing algorithms. Memory-hard functions require a large amount of working space to evaluate efficiently and when used for password hashing, they dramatically increase the cost of offline dictionary attacks. In this work, we leverage a previously unstudied property of a certain class of graphs (“random sandwich graphs”) to analyze the memory-hardness of the Balloon algorithm. The techniques we develop are general: we also use them to give a proof of security of the scrypt and Argon2i password-hashing functions in the random-oracle model. Our security analysis uses a sequential model of computation, which essentially captures attacks that run on single-core machines. Recent work shows how to use massively parallel special-purpose machines (e.g., with hundreds of cores) to attack Balloon and other memory-hard functions. We discuss these important attacks, which are outside of our adversary model, and propose practical defenses against them. To motivate the need for security proofs in the area of password hashing, we demonstrate and implement a practical attack against Argon2i that successfully evaluates the function with less space than was previously claimed possible. Finally, we use experimental results to compare the performance of the Balloon hashing algorithm to other memory-hard functions

FlexiChain 2.0: NodeChain Assisting Integrated Decentralized Vault for Effective Data Authentication and Device Integrity in Complex Cyber-Physical Systems

Distributed Ledger Technology (DLT) has been introduced using the most common consensus algorithm either for an electronic cash system or a decentralized programmable assets platform which provides general services. Most established reliable networks are unsuitable for all applications such as smart cities applications, and, in particular, Internet of Things (IoT) and Cyber Physical Systems (CPS) applications. The purpose of this paper is to provide a suitable DLT for IoT and CPS that could satisfy their requirements. The proposed work has been designed based on the requirements of Cyber Physical Systems. FlexiChain is proposed as a layer zero network that could be formed from independent blockchains. Also, NodeChain has been introduced to be a distributed (Unique ID) UID aggregation vault to secure all nodes' UIDs. Moreover, NodeChain is proposed to serve mainly FlexiChain for all node security requirements. NodeChain targets the security and integrity of each node. Also, the linked UIDs create a chain of narration that keeps track not merely for assets but also for who authenticated the assets. The security results present a higher resistance against four types of attacks. Furthermore, the strength of the network is presented from the early stages compared to blockchain and central authority. FlexiChain technology has been introduced to be a layer zero network for all CPS decentralized applications taking into accounts their requirements. FlexiChain relies on lightweight processing mechanisms and creates other methods to increase security

Vulnerability of blockchain technologies to quantum attacks

Quantum computation represents a threat to many cryptographic protocols in operation today. It has been esti- mated that by 2035, there will exist a quantum computer capable of breaking the vital cryptographic scheme RSA2048. Blockchain technologies rely on cryptographic protocols for many of their essential sub-routines. Some of these protocols, but not all, are open to quantum attacks. Here we analyze the major blockchain-based cryp- tocurrencies deployed today—including Bitcoin, Ethereum, Litecoin and ZCash, and determine their risk exposure to quantum attacks. We finish with a comparative analysis of the studied cryptocurrencies and their underlying blockchain technologies and their relative levels of vulnerability to quantum attacks

Analysis Design & Applications of Cryptographic Building Blocks

This thesis deals with the basic design and rigorous analysis of cryptographic schemes and primitives, especially of authenticated encryption schemes, hash functions, and password-hashing schemes. In the last decade, security issues such as the PS3 jailbreak demonstrate that common security notions are rather restrictive, and it seems that they do not model the real world adequately. As a result, in the first part of this work, we introduce a less restrictive security model that is closer to reality. In this model it turned out that existing (on-line) authenticated encryption schemes cannot longer beconsidered secure, i.e. they can guarantee neither data privacy nor data integrity. Therefore, we present two novel authenticated encryption scheme, namely COFFE and McOE, which are not only secure in the standard model but also reasonably secure in our generalized security model, i.e. both preserve full data inegrity. In addition, McOE preserves a resonable level of data privacy. The second part of this thesis starts with proposing the hash function Twister-Pi, a revised version of the accepted SHA-3 candidate Twister. We not only fixed all known security issues of Twister, but also increased the overall soundness of our hash-function design. Furthermore, we present some fundamental groundwork in the area of password-hashing schemes. This research was mainly inspired by the medial omnipresence of password-leakage incidences. We show that the password-hashing scheme scrypt is vulnerable against cache-timing attacks due to the existence of a password-dependent memory-access pattern. Finally, we introduce Catena the first password-hashing scheme that is both memory-consuming and resistant against cache-timing attacks

HashCore: Proof-of-Work Functions for General Purpose Processors

Over the past five years, the rewards associated with mining Proof-of-Work blockchains have increased substantially. As a result, miners are heavily incentivized to design and utilize Application Specific Integrated Circuits (ASICs) that can compute hashes far more efficiently than existing general purpose hardware. Currently, it is difficult for most users to purchase and operate ASICs due to pricing and availability constraints, resulting in a relatively small number of miners with respect to total user base for most popular cryptocurrencies. In this work, we aim to invert the problem of ASIC development by constructing a Proof-of-Work function for which an existing general purpose processor (GPP, such as an x86 IC) is already an optimized ASIC. In doing so, we will ensure that any would-be miner either already owns an ASIC for the Proof-of-Work system they wish to participate in or can attain one at a competitive price with relative ease. In order to achieve this, we present HashCore, a Proof-of-Work function composed of "widgets" generated pseudo-randomly at runtime that each execute a sequence of general purpose processor instructions designed to stress the computational resources of such a GPP. The widgets will be modeled after workloads that GPPs have been optimized for, for example, the SPEC CPU 2017 benchmark suite for x86 ICs, in a technique we refer to as inverted benchmarking. We provide a proof that HashCore is collision-resistant regardless of how the widgets are implemented. We observe that GPP designers/developers essentially create an ASIC for benchmarks such as SPEC CPU 2017. By modeling HashCore after such benchmarks, we create a Proof-of-Work function that can be run most efficiently on a GPP, resulting in a more accessible, competitive, and balanced mining market