Xavier Bultel, Jannik Dreier, Pascal Lafourcade, Malika More
At the main cryptography conference, CRYPTO, in 1989, Quisquater and colleagues published a paper showing how to explain the complex notion of zero-knowledge proof in a simpler way that children can understand. In the same line of work, this article presents simple and intuitive explanations of various modern security concepts and technologies, including symmetric encryption, public key encryption, homomorphic encryption, intruder models (CPA, CCA1, CCA2), and security properties (OW, IND, NM). The explanations given in this article may also serve in demystifying such complex security notions for non-expert adults.
Bitcoin is a digital currency and payment system based on classical cryptographic technologies which works without a central administrator such as in traditional currencies. It has long been questioned what the impact of quantum computing would be on Bitcoin, and cryptocurrencies in general. Here, we analyse three primary directions that quantum computers might have an impact in: mining, security, and forks. We find that in the near-term the impact of quantum computers appear to be rather small for all three directions. The impact of quantum computers would require considerably larger number of qubits and breakthroughs in quantum algorithms to reverse existing hash functions.
Cryptocurrencies which evolved with bitcoin has a decentralized structure based on the ledger which is handled via proof of work mechanism, indeed generating a monetary supply. We all agree that decentralization save us from the cruel national political system but has a limitation of computational cost involved and problem related to scalability. The idea is to introduce a new cryptocurrency named UV Coin which is a cryptocurrency framework having control of the central banks but involves distributed set of authorities to prevent double spending. This coin will maintain enough transparency. The proof of the benefits is partial centralization such as elimination of wasteful hashing and involves a scalable system to avoid double spending attack.
Open access
Blockchain Technology Applications and Security
Advanced Steganography and Watermarking Techniques
This thesis presents new results in three fundamental areas of public-key cryptography: integrity, authentication and confidentiality. In each case we design new primitives or improve the features of existing ones. The first chapter, dealing with integrity, introduces a non-interactive proof for proper RSA public key generation and a contract co-signature protocol in which a breach in fairness provides the victim with transferable evidence against the cheater. The second chapter, focusing on authentication, shows how to use time measurements to shorten zeroknowledge commitments and how to exploit bias in zero-knowledge challenges to gain efficiency. This chapter also generalizes Fiat-Shamir into a one-to-many protocol and describes a very sophisticated smart card fraud illustrating what can happen when authentication protocols are wrongly designed. The third chapter is devoted to confidentiality. We propose public-key cryptosystems where traditional hardness assumptions are replaced by refinements of the CAPTCHA concept and explore the adaptation of honey encryption to natural language messages. Our final contributions focus on identity-based encryption (IBE) showing how to add broadcast features to hierarchical IBE and how to use IBE to reduce vulnerability exposure time of during software patch broadcast.
One of today’s main challenge related to cloud storage is to maintain the functionalities and the efficiency of customers’ and service providers’ usual environments while protecting the confidentiality of sensitive data. Deduplication is one of those functionalities: it enables cloud storage providers to save a lot of memory by storing only once a file uploaded several times. However, classical encryption schemes block deduplication. One needs to use a “message-locked encryption” scheme (MLE), which allows the detection of duplicates and the storage of only one encrypted file on the server, which can be decrypted by any owner of the file. However, in most existing scheme, a user can bypass this deduplication protocol. In this article, we provide servers verifiability for MLE schemes: the servers can verify that the ciphertexts are well-formed. This property forces a customer to prove that she complied to the deduplication protocol, thus preventing her to deviate from the prescribed functionality of MLE. Then, we provide an MLE scheme satisfying this new security property. To achieve the deduplication consistency, our construction primarily relies on zero-knowledge proofs. Unlike Abadi et al.’s MLE, we instantiate those proofs, so<br> that we obtain a more efficient scheme, secure in the random oracle model.
Tema ovog rada su kriptovalute. Budući da većina ljudi nije pravodobno upoznata s ovom temom, ovaj rad prikazuje i opisuje kriptovalute te način na koji se upotrjebljuju u svakodnevnom životu. Kriptovalute (eng. cryptocurrency) digitalne su valute dizajnirane kao sredstvo razmjene. Poznate su po tome što su državne agencije i banke isključene iz procesa razmjene. Kriptovalute omogućuju jednostavnu, jeftinu i brzu transakciju na području cijeloga svijeta. Trenutno najisplativije kriptovalute su Bitcoin i Ethereum, a u radu je opisana njihova korisnost, prednosti i mane. Budući da se Bitcoinu predviđa uspješna budućnost i sve je prisutniji i prihvatljiviji na tržištu, u radu su navedeni primjeri iz Hrvatske koji to potvrđuju. Sve veći broj poduzetnika odlučuje se za uvođenje kriptovaluta. U primjerima je obuhvaćen širok spektar djelatnosti, od frizerskih usluga, preko raznih tvrtki koji se bave prodajom računalne opreme, ugostiteljskih usluga preko mogućnosti brzog i lakog podizana gotovine na kripto bankomatima pa sve do plaćanja komunalnih usluga, pa čak i humanitarno djelovanje. Mnogi smatraju da su kriptovalute samo sinonim za prijevare i pranje novca, no programeri tvrde da su kriptovalute samo jedna vrsta tehnologije, alat koji sam po sebi ne može biti ni dobar ni loš, ovisno o tome za što se koristi. Autor ovoga rada proveo je istraživanje o tome kako se može besplatno započeti trgovanje kriptovalutama te je anketom ispitao stavove ispitanika o implementaciji kriptovaluta u društvu.
Kriptovalute su digitalni novac utemeljen na kriptografiji i decentraliziranom sustavu. Postoje samo u elektroničkom obliku kao jedinstveni digitalni novčići ("tokeni"). Iza njih ne stoji autoritet države niti ih je moguće svojevoljno proizvesti. Rad se fokusira na značajkama, postavkama, razvoju i svim međuodnosima važnih ekonomskih faktora koji utječu na kriptovalute. U prvom poglavlju navedena su obilježja kriptovaluta. Drugo poglavlje daje primjere i govori o primjeni kriptovaluta u svakodnevnom životu. U trećem poglavlju je raspravljano o trenutnim i budućim regulacijama najmoćnijih zemalja svijeta (G20) , kao i njihovoj zajedničkoj suradnji u želji za jedinstvenim i standardiziranim pravilima, a sve u svrhu što kvalitetnijeg nadzora nad kriptovalutama kako bi se spriječile malverzacije i zaštitili potrošači. Četvrto poglavlje govori o inicijalnoj ponudi kovanica, a peto poglavlje je namijenjeno sigurnosti kriptovaluta. Cilj istraživanja je utvrditi koliko je studentska populacija upoznata i usmjerena prema novim oblicima digitalnog novca, koje značajke kriptovaluta smatraju pozitivnima, a koje negativnima i u kojoj su mjeri investirali ili su spremni investirati dio svojih ulaganja u kriptovalute i sl. Metode istraživanja korištene u radu su kompilacija na temelju proučavanja postojeće literature o temi rada, prikupljanje i analiza podataka vezanih uz kriptovalute, ponajprije podataka vezanih uz cijene i tržišnu kapitalizaciju, anketiranje studenata Ekonomskog fakulteta u Rijeci i metoda dedukcije putem koje su pokazane sve važne karakteristike i obilježja kriptovaluta. Na temelju provedene ankete u kojoj je sudjelovalo 90 studenata Ekonomskog fakulteta u Rijeci zaključak toga dijela istraživanja je da je mlada populacija dobro upoznata s kriptovalutama i njenim glavnim značajkama, ali i određenim nedostatkom informiranosti o tehnologiji (trećina studenata nije čula za pojam "blockchain") i nedovoljnoj odlučnosti oko investiranja i trgovanja u kriptovalute. Povrh toga, dokazan je i negativan utjecaj hakerskih napada i određenih kriminalnih radnji, kao i nestabilnost tržišne cijene na povjerenje studenata, ali i ukupne populacije vezane uz globalni financijski sustav u kriptovalute. Ishod istraživanja omogućio je da zaključimo kako su kriptovalute trenutno u ranoj fazi razvoja i nisu se dovoljno implementirale za široku primjenu u trgovini roba i usluga ili općenito kao sredstvo razmjene. Faktor koji je uključen u istraživanje kako bi opisao veličinu, odnosno obujam neke kriptovalute je tržišna kapitalizacija u dolarima. Temeljna ideja ovog rada je informirati čitatelja o pozitivnim i negativnim značajkama koje se se vežu uz kriptovalute. Na taj način čitatelji će biti bolje informirani i educirani o potencijalnom riziku ulaganja u kriptovalute, kao i većoj razini zaštite prilikom posjedovanja neke digitalne valute.
<p>The password which is a more secure and valuable data should be highly protected from eavesdropper. This paper presents how password required for authentication of members of group communication is securely delivered by the source or initiator of the group. The password delivery uses zero knowledge proof and sent to the group member in an encrypted format using cipher block mode encryption. The password delivered is a One Time Password which can be used for certain amount of time in order to ensure a highly secure communication environment among the group.</p>
Trusted timestamping is a process for proving that certain information existed at a given point in time. This paper presents a trusted timestamping concept and its implementation in form of a web-based service that uses the decentralized Bitcoin block chain to store anonymous, tamper-proof timestamps for digital content. The service allows users to hash files, such as text, photos or videos, and store the created hashes in the Bitcoin block chain. Users can then retrieve and verify the timestamps that have been committed to the block chain. The non-commercial service enables anyone, e.g., researchers, authors, journalists, students, or artists, to prove that they were in possession of certain information at a given point in time. Common use cases include proving that a contract has been signed, a photo taken, a video recorded, or a task completed prior to a certain date. All procedures maintain complete privacy of the user's data.
Open access
2 source records
Advanced Steganography and Watermarking Techniques
The protocols for member expansion in secret sharing schemes are very useful for key management in dynamic topology networks. In order to reduce the computation complexity of the existed protocols for member expansion in secret sharing schemes, a new protocol is proposed based on the problem of elliptic curve discrete logarithm. This paper examines fifteen most recent patens that were awarded in the area of secret sharing. Unlike traditional detailed patent reviews that are focused on applying the simple secret sharing method, the proposed protocol has the following merits: 1) there is no trust center ; 2) only requesting broadcast 2 1 t + times to generate the sub-secret for the new participant and the new participant can verify the truth of the sub-secret; 3) the old participants can verify the new sub-secret by the noninteractive zero-knowledge proof protocol; 4) In the sub-secret generation stage, not only sub-secrets of old participants but also the sub-secret of new participant is secure. Compared to the existed protocols, the proposed protocol has lower computational complexity and less communications. Therefore, the proposed protocol has higher performance and is suitable for resource-constrained terminals of dynamic networks.
ECDSA has become a popular choice as lightweight alternative to RSA and classic DL based signature algorithms in recent years. As standardized, the signature produced by ECDSA for a pair of a message and a key is not deterministic. This work shows how this non-deterministic choice can be exploited by an attacker to leak private information through the signature without any side channels, an attack first discovered by Young and Yung for classic DL-based cryptosystems in 1997, and how this attack affects the application of ECDSA in the Bitcoin protocol.
Abstract. LowMC is a collection of block cipher families introduced at Eurocrypt 2015 by Albrecht et al. Its design is optimized for instanti-ations of multi-party computation, fully homomorphic encryption, and zero-knowledge proofs. A unique feature of LowMC is that its internal affine layers are chosen at random, and thus each block cipher family contains a huge number of instances. The Eurocrypt paper proposed two specific block cipher families of LowMC, having 80-bit and 128-bit keys. In this paper, we mount interpolation attacks (algebraic attacks intro-duced by Jakobsen and Knudsen) on LowMC, and show that a practically significant fraction of 2−38 of its 80-bit key instances could be broken 223 times faster than exhaustive search. Moreover, essentially all instances that are claimed to provide 128-bit security could be broken about 1000 times faster. In order to obtain these results, we had to develop novel techniques and optimize the original interpolation attack in new ways. While some of our new techniques exploit specific internal properties of LowMC, others are more generic and could be applied, in principle, to any block cipher.
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 transaction— the first Bitcoin transaction—marked 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 “block” of transactions, which incorporates a reference to previous blocks. This creates a long chain of blocks—a “blockchain”—that 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
Recent technological advances have given rise to the popularity and success of cloud. This new paradigm is gaining an expanding interest, since it provides cost efficient architectures that support the transmission, storage, and intensive computing of data. However, these promising storage services bring many challenging design issues, considerably due to the loss of data control. These challenges, namely data confidentiality and data integrity, have significant influence on the security and performances of the cloud system. This thesis aims at overcoming this trade-off, while considering two data security concerns. On one hand, we focus on data confidentiality preservation which becomes more complex with flexible data sharing among a dynamic group of users. It requires the secrecy of outsourced data and an efficient sharing of decrypting keys between different authorized users. For this purpose, we, first, proposed a new method relying on the use of ID-Based Cryptography (IBC), where each client acts as a Private Key Generator (PKG). That is, he generates his own public elements and derives his corresponding private key using a secret. Thanks to IBC properties, this contribution is shown to support data privacy and confidentiality, and to be resistant to unauthorized access to data during the sharing process, while considering two realistic threat models, namely an honest but curious server and a malicious user adversary. Second, we define CloudaSec, a public key based solution, which proposes the separation of subscription-based key management and confidentiality-oriented asymmetric encryption policies. That is, CloudaSec enables flexible and scalable deployment of the solution as well as strong security guarantees for outsourced data in cloud servers. Experimental results, under OpenStack Swift, have proven the efficiency of CloudaSec in scalable data sharing, while considering the impact of the cryptographic operations at the client side. On the other hand, we address the Proof of Data Possession (PDP) concern. In fact, the cloud customer should have an efficient way to perform periodical remote integrity verifications, without keeping the data locally, following three substantial aspects : security level, public verifiability, and performance. This concern is magnified by the client’s constrained storage and computation capabilities and the large size of outsourced data. In order to fulfill this security requirement, we first define a new zero-knowledge PDP proto- col that provides deterministic integrity verification guarantees, relying on the uniqueness of the Euclidean Division. These guarantees are considered as interesting, compared to several proposed schemes, presenting probabilistic approaches. Then, we propose SHoPS, a Set-Homomorphic Proof of Data Possession scheme, supporting the 3 levels of data verification. SHoPS enables the cloud client not only to obtain a proof of possession from the remote server, but also to verify that a given data file is distributed across multiple storage devices to achieve a certain desired level of fault tolerance. Indeed, we present the set homomorphism property, which extends malleability to set operations properties, such as union, intersection and inclusion. SHoPS presents high security level and low processing complexity. For instance, SHoPS saves energy within the cloud provider by distributing the computation over multiple nodes. Each node provides proofs of local data block sets. This is to make applicable, a resulting proof over sets of data blocks, satisfying several needs, such as, proofs aggregation
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
Quantum zero-knowledge proofs and quantum proofs of knowledge are inherently difficult to analyze because their security analysis uses rewinding. Certain cases of quantum rewinding are handled by the results by Watrous (SIAM J Comput, 2009) and Unruh (Eurocrypt 2012), yet in general the problem remains elusive. We show that this is not only due to a lack of proof techniques: relative to an oracle, we show that classically secure proofs and proofs of knowledge are insecure in the quantum setting. More specifically, sigma-protocols, the Fiat-Shamir construction, and Fischlin's proof system are quantum insecure under assumptions that are sufficient for classical security. Additionally, we show that for similar reasons, computationally binding commitments provide almost no security guarantees in a quantum setting. To show these results, we develop the "pick-one trick", a general technique that allows an adversary to find one value satisfying a given predicate, but not two.
Bitcoin is a "crypto currency", a decentralized electronic payment scheme based on cryptography which has recently gained excessive popularity. Scientific research on bitcoin is less abundant. A paper at Financial Cryptography 2012 conference explains that it is a system which "uses no fancy cryptography", and is "by no means perfect". It depends on a well-known cryptographic standard SHA-256. In this paper we revisit the cryptographic process which allows one to make money by producing bitcoins. We reformulate this problem as a Constrained Input Small Output (CISO) hashing problem and reduce the problem to a pure block cipher problem. We estimate the speed of this process and we show that the cost of this process is less than it seems and it depends on a certain cryptographic constant which we estimated to be at most 1.86. These optimizations enable bitcoin miners to save tens of millions of dollars per year in electricity bills. Miners who set up mining operations face many economic incertitudes such as high volatility. In this paper we point out that there are fundamental incertitudes which depend very strongly on the bitcoin specification. The energy efficiency of bitcoin miners have already been improved by a factor of about 10,000, and we claim that further improvements are inevitable. Better technology is bound to be invented, would it be quantum miners. More importantly, the specification is likely to change. A major change have been proposed in May 2013 at Bitcoin conference in San Diego by Dan Kaminsky. However, any sort of change could be flatly rejected by the community which have heavily invested in mining with the current technology. Another question is the reward halving scheme in bitcoin. The current bitcoin specification mandates a strong 4-year cyclic property. We find this property totally unreasonable and harmful and explain why and how it needs to be changed.
Henry Corrigan-Gibbs, Wendy Mu, Dan Boneh, Bryan Ford
The security of any cryptosystem relies on the secrecy of the system's secret keys. Yet, recent experimental work demonstrates that tens of thousands of devices on the Internet use RSA and DSA secrets drawn from a small pool of candidate values. As a result, an adversary can derive the device's secret keys without breaking the underlying cryptosystem. We introduce a new threat model, under which there is a systemic solution to such randomness flaws. In our model, when a device generates a cryptographic key, it incorporates some random values from an "entropy authority" into its cryptographic secrets and then proves to the authority, using zero-knowledge-proof techniques, that it performed this operation correctly. By presenting an entropy-authority-signed public key certificate to a third party (like a certificate authority or SSH client), the device can demonstrate that its public key incorporates randomness from the authority and is therefore drawn from a large pool of candidate values. Where possible, our protocol protects against eavesdroppers, entropy authority misbehavior, and devices attempting to discredit the entropy authority. To demonstrate the practicality of our protocol, we have implemented and evaluated its performance on a commodity wireless home router. When running on a home router, our protocol incurs a $1.7\times$ slowdown over conventional RSA key generation and it incurs a $3.6\times$ slowdown over conventional EC-DSA key generation.
CertiCrypt is a framework that enables the machine-checked construction and verification of cryptographic proofs in the Coq proof assistant. CertiCrypt instruments the code-based game-based approach to cryptographic proofs, and builds upon many areas, including probability and complexity theory, algebra, semantics of programming languages, and program optimizations. In this thesis, we illustrate the application of CertiCrypt on two examples: the Hashed ElGamal encryption scheme and zero-knowledge protocols. Like previous case studies in CertiCrypt, these examples demonstrate the feasibility of formalizing complex cryptographic proofs. However, using CertiCrypt requires a high level of expertise in Coq, and is time consuming. In order to ease the adoption of formal proofs by the cryptographic community, we develop a semi-automated tool, called EasyCrypt, for elaborating security proofs of cryptographic systems from proof sketches. Proof sketches are checked automatically using SMT solvers and automated theorem provers, and then compiled into verifiable proofs in the CertiCrypt framework. We illustrate the application of EasyCrypt with two examples: the Hashed ElGamal encryption system, and the Cramer-Shoup encryption system. Finally, we extend the language of CertiCrypt with a formalization of polytime functions.
When initializing cryptographic systems or running cryptographic protocols, the randomness of critical parameters, like keys or key components, is one of the most crucial aspects. But, randomly chosen parameters come with the intrinsic chance of duplicates, which finally may cause cryptographic systems including RSA, ElGamal and Zero-Knowledge proofs to become insecure. When concerning digital identifiers, we need uniqueness in order to correctly identify a specific action or object. Unfortunately we also need randomness here. Without randomness, actions become linkable to each other or to their initiator’s digital identity. So ideally the employed (cryptographic) parameters should fulfill two potentially conflicting requirements simultaneously: randomness and uniqueness. This article proposes an efficient mechanism to provide both attributes at the same time without highly constraining the first one and never violating the second one. After defining five requirements on random number generators and discussing related work, we will describe the core concept of the generation mechanism. Subsequently we will prove the postulated properties (security, randomness, uniqueness, efficiency and privacy protection) and present some application scenarios including system-wide unique parameters, cryptographic keys and components, identifiers and digital pseudonyms.
Commitment schemes, secret sharing schemes, interactive zero-knowledge proofs which underlie the offered interactive secrete sharing schemes are considered. Such schemes are one of the main computing primitives in the theory of secure computations. Base elements of the theory of secure computations and schemes of construction of synchronous verifiable secrete sharing as schemes of secure function evaluation are resulted.