We propose a tool for automated truncation of differential trails in ciphers using modular addition, bitwise rotation, and XOR (ARX). The tool takes as input a differential trail and produces as output a set of truncated differential trails. The set represents all possible truncations of the input trail according to certain predefined rules. A linear-time algorithm for the exact computation of the differential probability of a truncated trail that follows the truncation rules is proposed. We further describe a method to merge the set of truncated trails into a compact set of non-overlapping truncated trails with associated probability and we demonstrate the application of the tool on block cipher Speck64. We have also investigated the effect of clustering of differential trails around a fixed input trail. The best cluster that we have found for 15 rounds has probability $$2^{-55.03}$$ (consisting of 389 unique output differences) which allows us to build a distinguisher using 128 times less data than the one based on just the single best trail, which has probability $$2^{-62}$$ . Moreover, we show examples for Speck64 where a cluster of trails around a suboptimal (in terms of probability) input trail results in higher overall probability compared to a cluster obtained around the best differential trail.

In this paper we describe how to couple reputation systems with distributed consensus protocols to provide a scalable permissionless consensus protocol with a low barrier of entry, while still providing strong resistance against Sybil attacks for large peer-to-peer networks of untrusted validators. We introduce reputation module ReCon, which can be laid on top of various consensus protocols such as PBFT or HoneyBadger. The protocol takes external reputation ranking as input and then ranks nodes based on the outcomes of consensus rounds run by a small committee, and adaptively selects the committee based on the current reputation. ReCon can tolerate larger threshold of malicious nodes (up to slightly above 1/2) compared to the 1/3 limit of BFT consensus algorithms.

Mining is one of the core elements of the proof-of-work based cryptocurrency economy. In this paper we investigate the generic landscape and hierarchy of miners on the example of Ethereum and Zcash, two blockchains that are among the top 5 in terms of USD value of created coins. Both chains used ASIC resistant proofs-of-work which favors GPU mining in order to keep mining decentralized. This however has changed with recent introduction of ASIC miners for these chains. This transition allows us to develop methods that might detect hidden ASIC mining in a chain (if it exists), and to study how the introduction of ASICs effects the decentralization of mining power. Finally, we describe how an attacker might use public blockchain information to invalidate the privacy of miners, deducing the mining hardware of individual miners and their mining rewards.

With the growth in popularity for cryptocurrencies the need for privacy preserving blockchains is growing as well. Zcash is such a blockchain, providing transaction privacy through zero-knowledge proofs. In this paper we analyze transaction linkability in Zcash based on the currency minting transactions (mining). Using predictable usage patterns and clustering heuristics on mining transactions an attacker can link to publicly visible addresses over 84% of the volume of the transactions that use a ZK-proof. Since majority of Zcash transactions are not yet using ZK-proofs, we show that overall 95.5% of the total number of Zcash transactions are potentially linkable to public addresses by just observing the mining activity.

In this paper we analyze two privacy and security issues for the privacy-oriented cryptocurrency Zcash. First we study shielded transactions and show ways to fingerprint user transactions, including active attacks. We introduce two new attacks which we call Danaan-gift attack and Dust attack. Following the recent Sapling update of Zcash protocol we study the interaction between the new and the old zk-SNARK protocols and the effects of their interaction on transaction privacy. In the second part of the paper we check for the presence of subliminal channels in the zk-SNARK protocol and in Pedersen Commitments. We show presence of efficient 70-bit channels which could be used for tagging of shielded transactions which would allow the attacker (malicious transaction verifier) to link transactions issued by a maliciously modified zk-SNARK prover, while would be indistinguishable from regular transactions for the honest verifier/user. We discuss countermeasures against both of these privacy issues.

On May 3 the ASIC manufacturer Bitmain announced an ASIC miner for Equihash (and another company, ASICminer as well, on May 10). Bitmain is one of the biggest ASIC provider companies in the world, while also having large shares in mining power in Bitcoin, and other large cryptocurrencies. These announcements raise the question when did these companies develop an ASIC, did they launch and test it on Equihash-based cryptocurrencies before the announcement. Are ASIC miners present in the current Zcash mining ecosystem and if yes, to what extent. By italics we marked items for which we would appreciate feedback from the community.