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
This paper presents a concept for a new method to provide the authentication and confidentiality using zero knowledge protocol and key exchange. Zero knowledge proof protocol is a essential component of cryptography, which in recent years has increasingly popular amongst scholars. Its applications have widened and it has made inroads in several areas including mathematics and network safety and so on. This simple protocol based on zero knowledge proof by which user can prove to the authentication server that he has the password without having to send the password to the server either clear text or in encrypted format. This is a protocol in which the data learned by one party (i.e., The inspector) allow him/her to verify that a statement is true but does not reveal any additional information. In this paper we first discuss about zero-knowledge protocol proof system of knowledge and also key exchange between users and which then is modified into an authentication scheme with secret key exchange for confidentiality. The whole protocol involves mutual identification of two users, exchange of a random common secret key or session key for the verification of public keys.
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 has recently gained popularity as a peer-to-peer decentralized electronic currency system. It was designed to provide all the desirable properties of electronic cash. One of the desirable properties of Bitcoin is that it does not depend on a central authority like a bank or the government. This paper introduces Bitcoin and its characteristics together with its core components–block, block chain and transaction. Mechanisms for ensuring privacy and security in Bitcoin are also presented. Primitives like hash functions and digital signatures play important roles in providing cryptographic functionalities for Bitcoin. SHA-256 is used to prevent malicious parties from stealing and creating coins. Specific characteristics of this hash function and its suitability for bitcoin would be analyzed. The elliptic Curve digital signature Algorithm (ECDSA) is used for confirming identity of payer and validating truncation. Properties of the Elliptic curve used in ECDSA and the signature algorithm would also be technically analyzed together with their role in protecting the electronic currency for the next few decades.
One of the central problems in the development of distributed systems is the design of cryptographic protocols that meet specified functional requirements and preserve user privacy. Methods of conventional encryption algorithms typically use PKI. In this paper we presented a method based on other approach. The authors have developed a cryptographic framework for access control, ensured the confidentiality of social relations, secrecy of resources and anonymity of users in social networks. The implementation of the protocols in the framework is combination of zero-knowledge proof and signature schemes Camenisch-Lysyanskaya.
In recent years there has been a fantastic boom of increasingly sophisticated “cryptographic objects ” — identity-based encryption, fully-homomorphic encryption, functional encryption, and most recently, various forms of obfuscation. These objects often come in various flavors of security, and as these constructions have grown in number, complexity and inter-connectedness, the relationships between them have become increasingly confusing. We provide a new framework of cryptographic agents that unifies various cryptographic objects and security definitions, similar to how the Universal Composition framework unifies various multi-party computation tasks like commitment, coin-tossing and zero-knowledge proofs. Our contributions can be summarized as follows. • Our main contribution is a new model of cryptographic computation, that unifies and extends cryptographic primitives such as Obfuscation, Functional Encryption, Fully Homomorphic En-cryption, Witness encryption, Property Preserving Encryption and the like, all of which can be cleanly modeled as “schemata ” in our framework. We provide a new indistinguishability preserving (IND-PRE) definition of security that interpolates indistinguishability and simulation
Nicolas T. Courtois, Pinar Emirdag, Filippo Valsorda
Abstract. In this paper we study the question of key management and practical operational security in bitcoin digital currency storage systems. We study the security two most used bitcoin HD Wallet key management solutions (e.g. in BIP032 and in earlier systems). These systems have ex-tensive audit capabilities but this property comes at a very high price. They are excessively fragile. One small security incident in a remote corner of the system and everything collapses, all private keys can be re-covered and ALL bitcoins within the remit of the system can be stolen. Privilege escalation attacks on HD Wallet solutions are not new. In this paper we take it much further. We propose new more advanced combi-nation attacks in which the security of keys hold in cold storage can be compromised without executing any software exploit on the cold sys-tem, but through security incidents at operation such as bad random number or related random events. In our new attacks all bitcoins over whole large security domains can be
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.
In the existing threshold signature schemes,conspiracy attack is still a difficult problem to solved.To overcome the security weaknesses of Wang Bin's work and Wang Xin's work,a new scheme was proposed.In order to resist conspiracy attack,it utilized three ways: using model without a trusted part,it can eliminate authority cheating thoroughly and the security level was higher;adding random number and time stamp,it can prevent the attacker to recover the signer member's secret parameters and also to prevent the attack of disrupting protocol;using zero-knowledge proof verifiable method to confirm that the signer member had right secret parameters,so it can prevent the attacker to forge signature directly.The results show that this scheme is correct,and it can not only resist forgery attack and conspiracy attack,but also provide anonymity and traceability simultaneously.
Chaos-based Image/Signal Encryption
Advanced Steganography and Watermarking Techniques
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.
Abstract Password-authenticated key exchange (PAKE) protocols allow two players to agree on a shared high entropy secret key, that depends on their own passwords only. Following the Gennaro and Lindell’s approach, with a new kind of smooth-projective hash functions (SPHFs), Katz and Vaikuntanathan recently came up with the first concrete one-round PAKE protocols, where the two players just have to send simultaneous flows to each other. The first one is secure in the Bellare-Pointcheval-Rogaway (BPR) model and the second one in the Canetti’s UC framework, but at the cost of simulation-sound non-interactive zero-knowledge (SS-NIZK) proofs (one for the BPR-secure protocol and two for the UC-secure one), which make the overall constructions not really efficient. This paper follows their path with, first, a new efficient instantiation of SPHF on Cramer-Shoup ciphertexts, which allows to get rid of the SS-NIZK proof and leads to the design of the most efficient one-round PAKE known so far, in the BPR model, and in addition without pairings. In the UC framework, the security proof required the simulator to be able to extract the hashing key of the SPHF, hence the additional SS-NIZK proof. We improve the way the latter extractability is obtained by introducing the notion of trapdoor smooth projective hash functions (TSPHFs). Our concrete instantiation leads to the most efficient one-round PAKE UC-secure against static corruptions to date. We additionally show how these SPHFs and TSPHFs can be used for blind signatures and zero-knowledge proofs with straight-line extractability. 1
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.
The question of how to construct optimally efficient secure protocols is a central question in cryptography and in the computer security world at large. We focus in this work on several facets of this question, with a particular view towards the role of randomness in secure computation.
The use of random inputs is ubiquitous in cryptographic primitives. However, the ability to consistently draw from large random sources may be a barrier in practice. We examine the existence of optimally strong pseudorandom sources with particularly efficient implementations; namely, we construct exponentially hard pseudorandom generators that can be computed by circuits that have size linear in the generator output.
Conversely, we also examine efficient protocols that rely only minimally on their random inputs. In the resettable security model, parties may be forced to use the same random input across polynomially many interactions with other parties. We examine this security model in the case of zero knowledge proofs, which are a primitive frequently required in secure computation protocols when one party must prove that they have executed the protocol correctly to another party without revealing secret inputs and compromising security.
Finally, we examine the secure and efficient implementation of a specific functionality, including its various required cryptographic primitives (such as zero knowledge arguments of knowledge). In particular, we construct a protocol that securely realizes pattern matching, including single character wildcards and substring matching.
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.
In a world that relies heavily on technology, privacy is sought by many. Privacy, among other things, is especially desired when making an online payment. This motivates the use of electronic cash, a form of electronic payment system based on the paper cash system used daily. The most successful and widely used of these services is Bitcoin – a decentralized peer-to-peer electronic cash system. This paper provides a broad introduction to Bitcoin, while analyzing its construction and investigating some of its perks and flaws. It can be seen that, when compared to paper cash and electronic cash, Bitcoin is in a class of its own.
The diploma thesis deals with cryptographic building blocks and schemes that enable us to implement secure e-voting. It is desirable for an e-vote to remain secret, but at the same time we would like the possibility for a third party to verify various levels of correctness of execution. Cryptography provides us the necessary tools to simultaneously realize these two seemingly conflicting requirements. Public-key cryptosystems, secret sharing schemes, zero-knowledge proofs and mixnets are particularly important among aforementioned tools. These building blocks are presented and studied in the context of e-voting. In the thesis we also describe two schemes for cryptographic e-voting. The first one is a paper-based scheme, and the second is an open source system for internet voting, Helios.
The existing direct anonymous attestation scheme achieves anonymous attestation using group signature,identity managed,certificate system technology and so on,which has the defect being slow implementation of the efficiency and poor safety.The computing speed of the new XTR public key cryptosystem is fast,and its safety is well.Based on the XTR public key cryptosystem,using zero-knowledge proof,this article improved the original direct anonymous attestation scheme.Demonstrated by analyzing,compared with the original mechanism,the security and implementation of this new scheme have been improved greatly.