Abstract. We formalize the use of Bitcoin as a source of publicly-verifiable randomness. As a side-effect of Bitcoinās proof-of-work-based consensus system, random values are broadcast every time new blocks are mined. We can derive strong lower bounds on the computational min-entropy in each block: currently, at least 68 bits of min-entropy are produced every 10 minutes, from which one can derive over 32 near-uniform bits using standard extractor techniques. We show that any attack on this beacon would form an attack on Bitcoin itself and hence have a monetary cost that we can bound, unlike any other construc-tion for a public randomness beacon in the literature. In our simplest construction, we show that a lottery producing a single unbiased bit is manipulation-resistant against an attacker with a stake of less than 50 bitcoins in the output, or about US$12,000 today. Finally, we propose making the beacon output available to smart contracts and demonstrate that this simple tool enables a number of interesting applications. 1
Blockchain Technology Applications and Security
Advanced Steganography and Watermarking Techniques
Arthur Gervais, Hubert Ritzdorf, Ghassan Karame, SrÄjan Äapkun
Given the increasing adoption of Bitcoin, the number of transactions and the block sizes within the system are only expected to increase. To sustain its correct operation in spite of its ever-increasing use, Bitcoin implements a number of necessary optimizations and scalability measures. These measures limit the amount of information broadcast in the system to the minimum necessary. In this paper, we show that current scalability measures adopted by Bitcoin come at odds with the security of the system. More specifically, we show that an adversary can exploit these measures in order to effectively delay the propagation of transactions and blocks to specific nodes for a considerable amount of time---without causing a network partitioning in the system. Notice that this attack alters the information received by Bitcoin nodes, and modifies their views of the ledger state. Namely, we show that this allows the adversary to considerably increase its mining advantage in the network, and to double-spend transactions in spite of the current countermeasures adopted by Bitcoin. Based on our results, we propose a number of countermeasures in order to enhance the security of Bitcoin without deteriorating its scalability.
Open access
2 source records
Blockchain Technology Applications and Security
Advanced Steganography and Watermarking Techniques
Feroz Ahmad Ahmad, Prashant Kumar, Gulshan Shrivastava, Med Salim Bouhlel
ON 12 JANUARY 2009 a pseudonymous entity signed a transaction that instructed a distributed network to transfer a small amount of digital currency to Hal Finney, one ofthe key figures of the cypherpunk movement. After a few minutes, the transaction was recorded on a distributed public ledger, permanently updating the balance ofbothparties. This transactionā the first Bitcoin transactionāmarked the beginning of a new era of decentralized payment systems, ushering in a variety of financial Services that do not depend on any centralized clearinghouse or other financial middleman. Bitcoin is regarded by many as a powerful technological innovation that could disrupt many sectors, in the realm of finance and beyond. But the underlying technology on which the network operates, the Bitcoin blockchain can do much more than that. Just as the internet did in the early-1990s, blockchain technology carries with it a whole new range of promises concerning how decentralization can support and promote individual freedoms and autonomy. Blockchain proponents believe that Bitcoin and other cryptocurrency platforms will revolutionize mechanisms of value exchange in the same way that the internet transformed information sharing, by providing a platform for people to exchange digital resources, in a secure and decentralized manner without the need to rely on any intermediary or trusted authority. But this revolutionary potential also carries with it serious implications for censorship, intellectual property, and the regulated flow of information. A blockchain is a decentralized database of transactions maintained by a distributed network of computers, which all contribute to the verification and the validation of transactions. Once accepted, these transactions are recorded inside a āblockā of transactions, which incorporates a reference to previous blocks. This creates a long chain of blocksāa āblockchaināāthat stores the history of all transactions in a chronological order. Every block contains information about a particular set of transactions, a reference to the preceding block in the blockchain, and the answer to a complex mathematical puzzle that is used to validate the data associated with that block. A copy of the blockchain is stored on every computer in the network, making it virtually impossible for anyone unilaterally to modify the data stored on this decentralized database: if anyone tries to modify any transaction the fraud will be immediately detected by all other network participants.
Open access
43 source records
Blockchain Technology Applications and Security
Advanced Steganography and Watermarking Techniques
We present a technique that uses privacy enhancing technologies and biometrics to prevent the unauthorized lending of credentials. Current credential schemes suffer the weakness that issued credentials can be transferred between users. Our technique ensures the biometric identity of the individual executing the Issue and Show protocols of an existing credential system in a manner analogous to the enrollment and verification steps in traditional biometric systems. During Issue we create Pedersen commitments on biometrically derived keys obtained from fuzzy extractors. This issue-time commitment is sealed into the issued credential. During Show a verification-time commitment is generated. Correspondence of keys is verified using a zero-knowledge proof of knowledge. The proposed approach preserves the security of the underlying credential system, protects the privacy of the biometric, and generalizes to multiple biometric modalities. We illustrate the usage of our technique by showing how it can be incorporated into digital credentials and anonymous credentials.
Biometric Identification and Security
User Authentication and Security Systems
Advanced Steganography and Watermarking Techniques
Alex Biryukov, Dmitry Khovratovich, Ivan Pustogarov
Bitcoin is a digital currency which relies on a distributed set of miners to mint coins and on a peer-to-peer network to broadcast transactions. The identities of Bitcoin users are hidden behind pseudonyms (public keys) which are recommended to be changed frequently in order to increase transaction unlinkability. We present an efficient method to deanonymize Bitcoin users, which allows to link user pseudonyms to the IP addresses where the transactions are generated. Our techniques work for the most common and the most challenging scenario when users are behind NATs or firewalls of their ISPs. They allow to link transactions of a user behind a NAT and to distinguish connections and transactions of different users behind the same NAT. We also show that a natural countermeasure of using Tor or other anonymity services can be cut-off by abusing anti-DoS countermeasures of the bitcoin network. Our attacks require only a few machines and have been experimentally verified. We propose several countermeasures to mitigate these new attacks.
Open access
4 source records
Blockchain Technology Applications and Security
Internet Traffic Analysis and Secure E-voting
Advanced Steganography and Watermarking Techniques
Abstract Mix network plays a key role in electronic voting to preserve anonymity and lots of mixnet schemes have been proposed so far. However, they requires complex and costly zero-knowledge proofs to provide their correct mixing operations. In 2010, Sebe et al. proposed an efficient and lightweight mixnet scheme based on a cryptographic secure hash function instead of zero-knowledge proofs. In this paper, we present a more efficient and faster mixnet scheme than Sebe et al.'s scheme under the same assumption. Also, our scheme is secure. Key Words : Zero-knowledge proof, e-Voting, Anonimity, Mixnet, hash function ā . Introduction Mixes are a means of untraceable communication based on a public key cryptosystem, as published by D.Chaum in 1981 [1] . A mix-network or mixnet accepts as input a collection of ciphertexts, and outputs associated plaintexts(or ciphertexts) in a randomly permuted order. A well constructed mixnet makes it infeasible for an adversary to determine which plaintext output corresponds to which ciphertext input more efficiently than by guessing at random. Proposed by D.Chaum in 1981 as a technique for anonymous e-mail and e-voting, mixnet can be categorized into decryption mix-nets and re-encryption mix-nets
Internet Traffic Analysis and Secure E-voting
Cryptography and Data Security
Advanced Steganography and Watermarking Techniques
We calculate the probability of success of block-hiding mining strategies in Bitcoin-like networks. These strategies involve building a secret branch of the block-tree and publishing it opportunistically, aiming to replace the top of the main branch and rip the reward associated with the secretly mined blocks. We identify two types of block-hiding strategies and chart the parameter space where those are more beneficial than the standard mining strategy described in Nakamoto's paper. Our analysis suggests a generalization of the notion of the relative hashing power as a measure for a miner's influence on the network. Block-hiding strategies are beneficial only when this measure of influence exceeds a certain threshold.
Open access
2 source records
cs.CR
Advanced Steganography and Watermarking Techniques
Bitcoin is a "crypto currency", a decentralized electronic payment scheme based on cryptography. Bitcoin economy grows at an incredibly fast rate and is now worth some 10 billions of dollars. Bitcoin mining is an activity which consists of creating (minting) the new coins which are later put into circulation. Miners spend electricity on solving cryptographic puzzles and they are also gatekeepers which validate bitcoin transactions of other people. Miners are expected to be honest and have some incentives to behave well. However. In this paper we look at the miner strategies with particular attention paid to subversive and dishonest strategies or those which could put bitcoin and its reputation in danger. We study in details several recent attacks in which dishonest miners obtain a higher reward than their relative contribution to the network. In particular we revisit the concept of block withholding attacks and propose a new concrete and practical block withholding attack which we show to maximize the advantage gained by rogue miners. RECENT EVENTS: it seems that the attack was recently executed, see Section XI-A.
Open access
2 source records
Blockchain Technology Applications and Security
Cryptography and Data Security
Advanced Steganography and Watermarking Techniques
In the digital world, so many copyrighted works are made in an illegal way because it is easy to keep and copy. Digital Rights Management has proposed to prevent this theft. Contents providers often bring in one server who has charge of managing the normal user, but there are some problems that it flocks to the server. Against this problem, P2P based DRM system has considered. All users can transfer the encrypted content to other users, so the content server does not have to load so much traffic from users. As a problem with this method, it is hard to figure out usage situation of contents because P2P based system is divided into many pieces of users. In this paper, we propose a new P2P based DRM system using Bitcoin protocol, which is one of the electric commerce. Bitcoin protocol, timestamp server saves all transactions to prevent double spending. We can bring out all usage situations to apply this system.
Open access
Digital Rights Management and Security
Advanced Steganography and Watermarking Techniques
A widespread security claim of the Bitcoin system, presented in the original Bitcoin white-paper, states that the security of the system is guaranteed as long as there is no attacker in possession of half or more of the total computational power used to maintain the system. This claim, however, is proved based on theoretically flawed assumptions. In the paper we analyze two kinds of attacks based on two theoretical flaws: the Block Discarding Attack and the Difficulty Raising Attack. We argue that the current theoretical limit of attacker's fraction of total computational power essential for the security of the system is in a sense not $\frac{1}{2}$ but a bit less than $\frac{1}{4}$, and outline proposals for protocol change that can raise this limit to be as close to $\frac{1}{2}$ as we want. The basic idea of the Block Discarding Attack has been noted as early as 2010, and lately was independently though-of and analyzed by both author of this paper and authors of a most recently pre-print published paper. We thus focus on the major differences of our analysis, and try to explain the unfortunate surprising coincidence. To the best of our knowledge, the second attack is presented here for the first time.
Open access
2 source records
cs.CR
Blockchain Technology Applications and Security
Advanced Steganography and Watermarking Techniques
Thesis (M.S.C.S.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
Cryptography and Residue Arithmetic
Cryptography and Data Security
Advanced Steganography and Watermarking Techniques
There is the advantage of zero-knowledge proofs of identity without letting out any users' secret when prover and verifier are communicating.But most of existing zero-knowledge proofs are iterative in nature,and require multiple communication rounds.To solve this problem,a new protocol was proposed based on challenge-response mode and elliptic curve algorithm.The interactive process of the new scheme was analyzed in detail.The analysis shows that the scheme ensures high security and also is resettable with only one-round communication,and low computing cost.
Cryptography and Data Security
Cryptography and Residue Arithmetic
Advanced Steganography and Watermarking Techniques
The zero knowledge proof protocol is a powerful cryptographic system.The security of most of the zero knowledge proof identification protocols is based on complex mathematical algorithms and requires heavy interactive computations for both parties,involving the prover and the verifier.However,visual cryptography is special since it does not require one to know the cryptography and the corresponding complex mathematical computations.In this paper,a new zero knowledge proof identification method based on the visual cryptography is proposed,it reduces current zero knowledge proof identification protocolās dependence on computing devices and the complex computation.
Advanced Steganography and Watermarking Techniques
In this paper, a novel biometric authentication scheme is proposed, which combines zero-knowledge proof, - protocols and bit commitment scheme. The remote server compares biometric template using committed values, this can keep privacy of user's biometrics. To the best knowledge of us, this is the first scheme which uses - protocols as a basic tool to implement biometric authentication. Compared with the previous schemes, this scheme has advantages as higher security, lower computation complexity and privacy keeping of biometric template.
Biometric Identification and Security
User Authentication and Security Systems
Advanced Steganography and Watermarking Techniques
A public watermark detection scheme based on secure multiparty computation is proposed. It removes the expensive zero-knowledge interactive proofs and replaces the traditional trusted third party with a group of proxies. Then, the proposed scheme is robust against "single point failure", and efficient in terms of computation cost, the number of rounds and bandwidth required in the communications. Because of using integer comparison technique, the scheme achieves the provable security against sensitivity attacks without leaking any information about the secret watermark.
Cryptography and Data Security
Advanced Steganography and Watermarking Techniques