The use of blockchains is growing every day, and their utility has greatly expanded from sending and receiving crypto-coins to smart-contracts and decentralized autonomous organizations. Modern blockchains underpin a variety of applications: from designing a global identity to improving satellite connectivity. In our research we look at the ability of blockchains to store metadata in an increasing volume of transactions and with evolving focus of utilization. We further show that basic approaches to improving blockchain privacy also rely on embedding metadata. This paper identifies and classifies real-life blockchain transactions embedding metadata of a number of major protocols running essentially over the bitcoin blockchain. The empirical analysis here presents the evolution of metadata utilization in the recent years, and the discussion suggests steps towards preventing criminal use. Metadata are relevant to any blockchain, and our analysis considers primarily bitcoin as a case study. The paper concludes that simultaneously with both expanding legitimate utilization of embedded metadata and expanding blockchain functionality, the applied research on improving anonymity and security must also attempt to protect against blockchain abuse.

Bitcoin is an open source payment system with a market capitalization of about 15 G$. During the years several key management solutions have been proposed to enhance bitcoin. The common characteristic of these techniques is that they allow to derive public keys independently of the private keys, and that these keys match. In this paper we overview the historical development of such techniques, specify and compare all major variants proposed or used in practical systems. We show that such techniques can be designed based on 2 distinct ECC arithmetic properties and how to combine both. A major trend in blockchain systems is to use by Stealth Address (SA) techniques to make different payments made to the same payee unlikable. We review all known SA techniques and show that early variants are less secure. Finally we propose a new SA method which is more robust against leakage and against various attacks.

There are two major families in cryptanalytic attacks on symmetric ciphers: statistical attacks and algebraic attacks. In this position paper we argue that algebraic cryptanalysis has not yet been developed properly due to the weakness of the theory which has substantial difficulty to prove most basic results on the number of linearly independent equations in algebraic attacks. Consequently most authors present a restricted range of attacks which are shown experimentally to work with their computer but refrain from claiming results which would work on a larger computer but have not yet been tested. For example in recent 2015 work of Raddum we discover that (experimentally) ElimLin attack breaks up to 16 rounds of Simon block cipher however it is hard to know what happens for 17 rounds. In this paper we argue that one CAN predict and model the behavior of such attacks and evaluate complexity of the attacks which we cannot yet execute. To the best of our knowledge this has never been done before.

In this paper we study the question of centralisation in bitcoin digital currency. In theory bitcoin has been designed to be a totally decentralized distributed system. Satoshi Nakamoto has very clearly postulated that each node should be collecting recent transactions and trying to create new blocks (Satoshi08). In bitcoin transactions are aggregated in block in order to authenticate them and form an official ledger and history of bitcoin transactions. In practice as soon as expensive ASIC bitcoin miners have replaced general-purpose hardware, production of bitcoins and the validation of transactions has concentrated in the hands of a smaller group of people. Then at some moment in early 2012 an important decision was taken: the Stratum protocol was designed (Palatinus12) which took a deliberate decision to move the power of selecting which transactions are included in blocks from miners to pool managers. The growing difficulty of mining and large standard deviation in this process (Rosenfeld13; CourtoisBahack14) made that majority of miners naturally shifted to pooled mining. At this moment bitcoin ceased being a decentralized democratic system. In this paper we survey the question of a 51% attacks and show that there is a large variety of plausible attack scenarios. In particular we study one particularly subversive attack scenario which depends on non-trivial internal details of the bitcoin hashing process. How does it compare with the current mining practices? We have study the Stratum protocol in four popular real-life mining configurations. Our analysis shows that pools could very easily cheat the majority of people. However the most subversive versions of the attack are NOT facilitated and could potentially be detected.

Bitcoin is a crypto currency, a distributed peer-to-peer financial system. Well actually it is an electronic system which manages the provisional ownership of a strictly fixed supply of abstract fungible units which really works as a distributed property register or a digital notary service. This is not so different than managing the ownership of shares in traditional financial markets. Modern financial institutions increasingly just do NOT trust each other, they build co-operative robust and decentralized and increasingly transparent, electronic systems which are and able to both serve the diverse objectives of participants (e.g. traders) and uphold certain security policies. Is Bitcoin actually so brilliant to be called the Internet of money as it is sometimes claimed? Not quite. Consider just the question of speed. Super low latency transactions are a norm in the financial industry, and even ordinary people have access to super fast bank transfers and real-time credit card transactions. Bitcoin remains rather the horse carriage of money. In this paper we look at the question of fast transaction acceptance in bitcoin and other crypto currencies. We claim that bitcoin needs to change in order to be able to satisfy the most basic needs of modern users.

Recently, two families of ultra-lightweight block ciphers were proposed, SIMON and SPECK, which come in a variety of block and key sizes (Beaulieu et al., 2013). They are designed to offer excellent performance for hardware and software implementations (Beaulieu et al., 2013; Aysu et al., 2014). In this paper, we study the resistance of SIMON-64/128 with respect to algebraic attacks. Its round function has very low Multiplicative Complexity (MC) (Boyar et al., 2000; Boyar and Peralta, 2010) and very low non-linearity (Boyar et al., 2013; Courtois et al., 2011) since the only non-linear component is the bitwise multiplication operation. Such ciphers are expected to be very good candidates to be broken by algebraic attacks and combinations with truncated differentials (additional work by the same authors). We algebraically encode the cipher and then using guess-then-determine techniques, we try to solve the underlying system using either a SAT solver (Bard et al., 2007) or by ElimLin al gorithm (Courtois et al., 2012b). We consider several settings where P-C pairs that satisfy certain properties are available, such as low Hamming distance or follow a strong truncated differential property (Knudsen, 1995). We manage to break faster than brute force up to 10(/44) rounds for most cases we have tried. Surprisingly, no key guessing is required if pairs which satisfy a strong truncated differential property are available. This reflects the power of combining truncated differentials with algebraic attacks in ciphers of low non-linearity and shows that such ciphers require a large number of rounds to be secure.

Secure and reliable authentication is an essential prerequisite for many online systems, yet achieving this in a way which is acceptable to customers remains a challenge. GrIDsure, a one-time PIN scheme using random grids and personal patterns, has been proposed as a way to overcome some of these challenges. We present an analytical study which demonstrates that GrIDsure in its current form is vulnerable to interception. To strengthen the scheme, we propose a way to fortify GrIDsure against Man-in-the-Middle attacks through (i) an additional secret transmitted out-of-band and (ii) multiple patterns. Since the need to recall multiple patterns increases user workload, we evaluated user performance with multiple captures with 26 participants making 15 authentication attempts each over a 3-week period. In contrast with other research into the use of multiple graphical passwords, we find no significant difference in the usability of GrIDsure with single and with multiple patterns.