Peer-to-peer networks are often large, collaborative networks where peers can join openly. The essence of a collaborative, distributed system is that every node performs tasks for other nodes. The peers often help in singular interactions and without direct reciprocity. Malicious peers can abuse and freeride the public goods. The network without countermeasures can fall into a tragedy of the commons where no one helps another and everyone takes advantage of the generosity of peers. Only when the reputation of a peer is publicly available at scale and peers trust this reputation can the network escape the problems of freeriding and attain high utility for all participants. This thesis focuses on designing and implementing the first step of a tamper proof reputation system within Tribler. Tribler is a peer-to-peer BitTorrent system developed at the Delft University of Technology. This first step, made by this thesis, is to create MultiChain, a proof-of-concept bookkeeping system. MultiChain tracks the upload and download amounts of peers to eliminate freeriding. Multi-Chain is cryptographically protected and validated. The bookkeeping system has to be scalable to be publicly available and be able to process enough transactions. The system has to work in an asynchronous network. A new design of a distributed data structure that can be used as a ledger is introduced by this thesis. This first step with MultiChain is already more resilient to tampering than previous work, like BarterCast. BarterCast has no security measures against tampering records. The design of MultiChain is to have a chain of blocks for every peer as a ledger. Peers are participants of a peer-to-peer network. A block contains a transaction between two peers. This block is shared and added to both chains. This makes both chains of the peers intertwined and entangled at a shared block. The proposed design abandons the typical global, full ledger. The protocol of creating these blocks between peers is described. The problems faced by MultiChain in an asynchronous network are explained. The thesis proposes how the design can overcome these problems by only allowing atomic operations to be performed on the chain and to introduce unfinished blocks in the chain. The implementation of the design is tested and experimented with within this thesis to validate it to work correctly. Furthermore, a number of weak points are discussed. These weak points have to be addressed in the future to create a tamper proof reputation system.
For the past five years, the bitcoin network constantly experience a growth in its size as more communities turn to accept the currency for payment exchanges. Using Flat File and a LevelDB of indices to save blocks on disk, bitcoin users require more memory to save the history of transaction. We focus on issues of memory management and access time in the bitcoin protocol using SQLite DataBase. With all the advantages of SQLite DataBase, it would be efficient if it is fitted in this architecture. The SQLite comes with many flavors one of which is its ability to support sql queries. Thus, instead of parsing indices to search a block from the database, a more powerful query can do the job.
Bitcoin is most radical innovation in monetary space for a very long time. It is an entirely private monetary system that runs itself and does not depend on trust in any central authority to honor its promises. Instead, it relies on trust in Bitcoin community or network that verifies transactions and maintains integrity of system. This system of distributed trust creates bitcoins and produces an automatic, tamper-proof bitcoin money supply process. (1) As such, it avoids dangers of discretionary monetary policy--namely, quantitative easing, manipulated interest rates, and need to rely on wise men or women to withstand political pressure or successfully forecast future. Indeed, under Bitcoin there is no monetary policy at all. There is just an automatic monetary rule dictated by Bitcoin protocol designed in 2009 by an anonymous programmer using alias Satoshi Nakamoto. Bitcoin has been widely hailed as a success and has won a substantial following. Unfortunately, underlying economics of Bitcoin mean that it is unsustainable and in all likelihood will be remembered as a failed experiment--at best a pointer to some superior successor. A first-pass intuition into Bitcoin can be obtained from a comparison with stone money in Milton Friedman's (1992) case study, Island of Stone Money. In this story, people of island of Yap in Micronesia used as money large round limestone disks transported from nearby island of Palau. These were too heavy to conveniently move around, so they were placed in prominent places. When ownership was to be transferred (e.g., as part of a dowry, inheritance, or ransom payment), current owner would publicly announce change in ownership but stone would typically remain where it was and islanders would maintain a collective memory of ownership history of stones. This collective memory ensured that there was no dispute over who owned which stones. Similarly, in Bitcoin, record of all transactions, blockchain, is also public knowledge and is regarded as die definitive record of who owns which bitcoins. Both stone money and Bitcoin share a critical feature that is highly unusual for a monetary system: both systems operate via a decentralized collective memory. On February 11, 2009, Nakamoto gave an explanation of thinking behind Bitcoin in an e-mail announcing its launch: root problem with conventional currency is all trust that is required to make it work. The central bank must be trusted not to debase currency, but history of fiat currencies is full of breaches of that trust.... With e-currency based on cryptographic proof, without die need to trust a third-party middleman, money can be secure and transactions complete. Cryptocurrencies, however, face problem of As Nakamoto notes, owner could try to re-spend an already spent coin by [digitally] signing it again to another owner. The usual solution is for a trusted company with a central database to check for double-spending, but that just gets back to trust model.... Bitcoin's solution is to use a peer-to-peer network to check for double-spending. Consequently, the result is a distributed system with no single point of failure. (2) Kevin Dowd is Professor of Finance and Economics at Durham University in United Kingdom and a partner at Cobden Partners. Martin Hutchinson is a journalist and author of Bear's Lair column (www.tbwns.com/category/the-bears-lair). The authors thank Ferdinando Ametrano, Gavin Andresen, Raadhiyah Anees, Steve Baker MP, Roger Brown, Dave Campbell, Akin Fernandez, Dominic Frisby, Jim Harper, Doug Jackson, Gordon Kerr, Jim Rapp, Eric Samieski, Lawrence H. White and Basil Zafiriou for much helpful feedback. We note that several of our readers have expressed serious reservations about our analysis and conclusions. Any remaining mistakes are authors' own. …
Bitcoin is widely represented in the popular press, but far less so in serious academic inquiry. Researchers have analyzed Bitcoin from various discipline-specific perspectives using their own sets of theories and jargon. Yet cross-disciplinary research has been muddled by the inaccurate interpretation of terminology across fields of research. This results in polarized assessments. In an effort to examine the Bitcoin phenomenon in a more holistic and multidisciplinary manner, this paper compares Bitcoin with another innovative technology – the World Wide Web – as first envisioned by Tim Berners-Lee. By exploring the early development of the World Wide Web, we seek to compare and contrast its development with that of Bitcoin and blockchain technology. The goal of this study is to show similarities and differences in their historic development, in order to identify key success factors related to the adoption of these technologies. Through identification of these factors we seek to guide both academics and practitioners towards fruitful avenues of research and development.
Kourosh Davarpanah, Dan Kaufman, Ophelie Pubellier
NeuCoin is a decentralized peer-to-peer cryptocurrency derived from Sunny\nKing's Peercoin, which itself was derived from Satoshi Nakamoto's Bitcoin. As\nwith Peercoin, proof-of-stake replaces proof-of-work as NeuCoin's security\nmodel, effectively replacing the operating costs of Bitcoin miners\n(electricity, computers) with the capital costs of holding the currency.\nProof-of-stake also avoids proof-of-work's inherent tendency towards\ncentralization resulting from competition for coinbase rewards among miners\nbased on lowest cost electricity and hash power.\n NeuCoin increases security relative to Peercoin and other existing\nproof-of-stake currencies in numerous ways, including: (1) incentivizing nodes\nto continuously stake coins over time through substantially higher mining\nrewards and lower minimum stake age; (2) abandoning the use of coin age in the\nmining formula; (3) causing the stake modifier parameter to change over time\nfor each stake; and (4) utilizing a client that punishes nodes that attempt to\nmine on multiple branches with duplicate stakes.\n This paper demonstrates how NeuCoin's proof-of-stake implementation addresses\nall commonly raised "nothing at stake" objections to generic proof-of-stake\nsystems. It also reviews many of the flaws of proof-of-work designs to\nhighlight the potential for an alternate cryptocurrency that solves these\nflaws.\n
Saramago, Rodrigo Q., Meling, Hein, Jehl, Leander N.
A certification system is responsible for issuing digital credentials, which attest claims about a subject, e.g., an academic diploma. Such credentials are valuable for individuals and society, and widespread adoption requires a trusted certification system. Trust can be gained by being transparent when issuing and verifying digital credentials. However, there is a fundamental tradeoff between privacy and transparency. For instance, admitting a student to an academic program must preserve the student’s privacy, i.e., the student’s grades must not be revealed to unauthorized parties. At the same time, other applicants may demand transparency to ensure fairness in the admission process. Thus, building a certification system with the right balance between privacy and transparency is challenging. This paper proposes a novel design for a certification system that provides sufficient transparency and preserves privacy through selective disclosure of claims such that authorized parties can verify them. Moreover, unauthorized parties can also verify the correctness of the certification process without compromising privacy. We achieve this using an incremental Merkle tree of cryptographic commitments to users' credentials. The commitments are added to the tree based on verifying zero-knowledge issuance proofs. Users store credentials off-chain and can prove the ownership and authenticity of credentials without revealing their commitments. Further, our approach enables users to prove statements about the credential’s claims in zero-knowledge. Our design offers a cost-efficient solution, reducing the amount of linkable on-chain data by up to 79% per credential compared to prior work, while maintaining transparency.
In this paper we revisit some major orthodoxies which lie at the heart of the bitcoin crypto currency and its numerous clones. In particular we look at The Longest Chain Rule, the monetary supply policies and the exact mechanisms which implement them. We claim that these built-in properties are not as brilliant as they are sometimes claimed. A closer examination reveals that they are closer to being... engineering mistakes which other crypto currencies have copied rather blindly. More precisely we show that the capacity of current crypto currencies to resist double spending attacks is poor and most current crypto currencies are highly vulnerable. Satoshi did not implement a timestamp for bitcoin transactions and the bitcoin software does not attempt to monitor double spending events. As a result major attacks involving hundreds of millions of dollars can occur and would not even be recorded. Hundreds of millions have been invested to pay for ASIC hashing infrastructure yet insufficient attention was paid to network neutrality and to insure that the protection layer it promises is effective and cannot be abused. In this paper we develop a theory of Programmed Self-Destruction of crypto currencies. We observe that most crypto currencies have mandated abrupt and sudden transitions. These affect their hash rate and therefore their protection against double spending attacks which we do not limit the to the notion of 51% attacks which is highly misleading. In addition we show that smaller bitcoin competitors are substantially more vulnerable. In addition to small hash rate, many bitcoin competitors mandate incredibly important adjustments in miner reward. We exhibit examples of 'alt-coins' which validate our theory and for which the process of programmed decline and rapid self-destruction has clearly already started.
Andrew Miller, Ari Juels, Elaine Shi, Bryan Parno · 5 authors
Bit coin is widely regarded as the first broadly successful e-cash system. An oft-cited concern, though, is that mining Bit coins wastes computational resources. Indeed, Bit coin's underlying mining mechanism, which we call a scratch-off puzzle (SOP), involves continuously attempting to solve computational puzzles that have no intrinsic utility. We propose a modification to Bit coin that repurposes its mining resources to achieve a more broadly useful goal: distributed storage of archival data. We call our new scheme Perm coin. Unlike Bit coin and its proposed alternatives, Perm coin requires clients to invest not just computational resources, but also storage. Our scheme involves an alternative scratch-off puzzle for Bit coin based on Proofs-of-Retrievability (PORs). Successfully minting money with this SOP requires local, random access to a copy of a file. Given the competition among mining clients in Bit coin, this modified SOP gives rise to highly decentralized file storage, thus reducing the overall waste of Bit coin. Using a model of rational economic agents we show that our modified SOP preserves the essential properties of the original Bit coin puzzle. We also provide parameterizations and calculations based on realistic hardware constraints to demonstrate the practicality of Perm coin as a whole.
Contracts enable mutually suspicious parties to cooperate safely through the exchange of rights. Smart contracts are programs whose behavior enforces the terms of the contract. This paper shows how such contracts can be specified elegantly and executed safely, given an appropriate distributed, secure, persistent, and ubiquitous computational fabric. JavaScript provides the ubiquity but must be significantly extended to deal with the other aspects. The first part of this paper is a progress report on our efforts to turn JavaScript into this fabric. To demonstrate the suitability of this design, we describe an escrow exchange contract implemented in 42 lines of JavaScript code.
Simon Barber, Xavier Boyen, Elaine Shi, Ersin Uzun
Abstract. Bitcoin is a distributed digital currency which has attracted a substan-tial number of users. We perform an in-depth investigation to understand what made Bitcoin so successful, while decades of research on cryptographic e-cash has not lead to a large-scale deployment. We ask also how Bitcoin could become a good candidate for a long-lived stable currency. In doing so, we identify several issues and attacks of Bitcoin, and propose suitable techniques to address them. 1
Computational grids enable the sharing, aggregation, and selection of (geographically distributed) computational resources and can be used for solving large scale and data intensive computing applications. Computational grids are an appealing target application for market-based resource allocation especially given the attention in recent years to “virtual organizations ” and policy requirements. In this paper, we present a framework for truthful, decentralized, dynamic auctions in computational grids. Rather than a fullyspecified auction, we propose an open, extensible framework that is sufficient to promote simple, truthful bidding by endusers while supporting distributed and autonomous control by resource owners. Our auction framework incorporates resource prediction in enabling an expressive language for end-users, and highlights the role of infrastructure in enforcing rules that balance the goal of simplicity for end users with autonomy for resource owners. The technical analysis leverages simplifying assumptions of “uniform failure” and “threshold-reliability” beliefs.
Serge Abiteboul, Bogdan Cautis, Amos Fiat, T. Milo
The common assumption about digital signatures is that they disallow any kind of modification on signed data. However, a more flexible approach is often needed and has been advocated lately, one in which some restricted modifications may still occur, without invalidating the data. This is made possible by offering signatures which are homomorphic with respect to some operation on the message domain. Starting from the signature(s) of some data instance(s), computed by the data owner, anybody else can derive the signature corresponding to a new data instance, if obtained only via some accepted operation from the previous one(s). More, updated signatures should be indistinguishable from the ones computed by the data owner and this updating step should be applicable as many times as needed. This paper deals with the signing of insert-only collections, in which element insertions are accepted but no removals should occur. Newly inserted elements do not have to be signed or known by the initial signer. We propose two techniques: one which transposes the insert-only problem into a delete-only one (which is already solved), and another technique based on zero-knowledge proofs. We also give performance measures and discuss applications.
Wanzong Peng, Tongliang Lu, Wenju Peng, Zhongpan Wang
File sharing, being the foundation of the Internet, has traditionally relied on a centralized service architecture resulting in significant maintenance costs. Moreover, due to the lack of an effective file management system, instances of sensitive information going out of control and loss of confidentiality in file sharing have occurred frequently. In order to address the difficulty of tamper detection and the lack of supervision in the entire process of file transfer in the current Internet environment, this paper designs a blockchain-based system architecture for secure sharing of electronic documents. An efficient blockchain model is used in our framework, and with the help of distributed storage system and asymmetric encryption technology, file sharing can be controlled, reliable and traceable in the transfer process. Referring to existing consensus mechanisms, e.g., Delegated Proof of Stake (DPoS) and Practical Byzantine Fault Tolerance (PBFT), we propose a new consensus for efficient and secure file sharing. Our experimental results show that our framework can maintain a higher throughput than existing schemes.
We present a protocol for controlling a shared ATM multicast tree supporting many-to-many communication. The protocol supports one or several ATM virtual channel connections (VCCs) of the many-to-many type. The number of VCCs is independent of the number of endpoints. The protocol guarantees that there is no interleaving on any VCC of the tree. The protocol also guarantees that the traffic contract associated with the VCCs is respected, thus making it possible to use ordinary VCCs of the constant bit rate (CBR), variable bit rate (VBR), or unspecified bit rate (UBR) class. No resequencing server or cell buffering inside the network is required, and all cell forwarding is performed at the ATM layer. We describe the protocol both informally and formally.