530 research outputs found
Contributions to Lattice–based Cryptography
Post–quantum cryptography (PQC) is a new and fast–growing part of Cryptography. It focuses on developing cryptographic algorithms and protocols that resist quantum adversaries (i.e., the adversaries who have access to quantum computers). To construct a new PQC primitive, a designer must use a mathematical problem intractable for the quantum adversary. Many intractability assumptions are being used in PQC. There seems to be a consensus in the research community that the most promising are intractable/hard problems in lattices. However, lattice–based cryptography still needs more research to make it more efficient and practical. The thesis contributes toward achieving either the novelty or the practicality of lattice– based cryptographic systems
Constant Size Ciphertext HIBE in the Augmented Selective-ID Model and its Extensions
At Eurocrypt 2005, Boneh, Boyen and Goh presented a constant size ciphertext hierarchical identity based encryption (HIBE) protocol. Our main contribution is to present a variant of the BBG-HIBE. The new HIBE is proved to be secure (without any degradation) in an extension of the sID model (denoted the s-ID model) and the components of the identities are from \bbbz_p, where is a suitable large prime. The BBG-HIBE is proved to be secure in the selective-ID (sID) security model and the components of the identities are from
\bbbz_p^*. In the s-ID model the adversary is allowed to vary the length of the challenge identity whereas this is not allowed in the sID model. The new HIBE shares all the good features of the BBG-HIBE. The drawback is that the public parameters and the private key are longer than that of the BBG-HIBE. We also provide two more extensions of the basic constant size ciphertext HIBE. The first is a constant size ciphertext HIBE secure in the generalised selective-ID model \clsM_2. The second one is a product construction composed of two HIBEs and a trade-off is possible between the private key size and the ciphertext size
Beyond the Hype: On Using Blockchains in Trust Management for Authentication
Trust Management (TM) systems for authentication are vital to the security of
online interactions, which are ubiquitous in our everyday lives. Various
systems, like the Web PKI (X.509) and PGP's Web of Trust are used to manage
trust in this setting. In recent years, blockchain technology has been
introduced as a panacea to our security problems, including that of
authentication, without sufficient reasoning, as to its merits.In this work, we
investigate the merits of using open distributed ledgers (ODLs), such as the
one implemented by blockchain technology, for securing TM systems for
authentication. We formally model such systems, and explore how blockchain can
help mitigate attacks against them. After formal argumentation, we conclude
that in the context of Trust Management for authentication, blockchain
technology, and ODLs in general, can offer considerable advantages compared to
previous approaches. Our analysis is, to the best of our knowledge, the first
to formally model and argue about the security of TM systems for
authentication, based on blockchain technology. To achieve this result, we
first provide an abstract model for TM systems for authentication. Then, we
show how this model can be conceptually encoded in a blockchain, by expressing
it as a series of state transitions. As a next step, we examine five prevalent
attacks on TM systems, and provide evidence that blockchain-based solutions can
be beneficial to the security of such systems, by mitigating, or completely
negating such attacks.Comment: A version of this paper was published in IEEE Trustcom.
http://ieeexplore.ieee.org/document/8029486
Puncturable Encryption: A Generic Construction from Delegatable Fully Key-Homomorphic Encryption
Puncturable encryption (PE), proposed by Green and Miers at IEEE S&P 2015, is
a kind of public key encryption that allows recipients to revoke individual
messages by repeatedly updating decryption keys without communicating with
senders. PE is an essential tool for constructing many interesting
applications, such as asynchronous messaging systems, forward-secret zero
round-trip time protocols, public-key watermarking schemes and forward-secret
proxy re-encryptions. This paper revisits PEs from the observation that the
puncturing property can be implemented as efficiently computable functions.
From this view, we propose a generic PE construction from the fully
key-homomorphic encryption, augmented with a key delegation mechanism (DFKHE)
from Boneh et al. at Eurocrypt 2014. We show that our PE construction enjoys
the selective security under chosen plaintext attacks (that can be converted
into the adaptive security with some efficiency loss) from that of DFKHE in the
standard model. Basing on the framework, we obtain the first post-quantum
secure PE instantiation that is based on the learning with errors problem,
selective secure under chosen plaintext attacks (CPA) in the standard model. We
also discuss about the ability of modification our framework to support the
unbounded number of ciphertext tags inspired from the work of Brakerski and
Vaikuntanathan at CRYPTO 2016
A Provably Secure Conditional Proxy Re-Encryption Scheme without Pairing
Blaze, Bleumer and Strauss introduced the notion of proxy re-encryption (PRE), which enables a semi-trusted proxy to transform ciphertexts under Alice\u27s public key into ciphertexts under Bob\u27s public key. The important property to note here is, the proxy should not learn anything about the plaintext encrypted. In 2009, Weng et al. introduced the concept of conditional proxy re-encryption (CPRE), which permits the proxy to re-encrypt only ciphertexts satisfying a condition specified by Alice into a ciphertext for Bob. CPRE enables fine-grained delegation of decryption rights useful in many practical scenarios, such as blockchain-enabled distributed cloud storage and encrypted email forwarding. Several CPRE schemes exist in the literature based on costly bilinear pairing operation in the random oracle model. We propose the first construction of an efficient CPRE scheme without pairing, satisfying chosen ciphertext security under the computational Diffie Hellman (CDH) assumption and its variant in the random oracle model
Account Management in Proof of Stake Ledgers
Blockchain protocols based on Proof-of-Stake (PoS) depend — by nature — on the active participation of stakeholders. If users are offline and abstain from the PoS consensus mechanism, the system’s security is at risk, so it is imperative to explore ways to both maximize the level of participation and minimize the effects of non-participation. One such option is stake representation, such that users can delegate their participation rights and, in the process, form stake pools . The core idea is that stake pool operators always participate on behalf of regular users, while the users retain the ownership of their assets. Our work provides a formal PoS wallet construction that enables delegation and stake pool formation. While investigating the construction of addresses in this setting, we distil and explore address malleability, a security property that captures the ability of an attacker to manipulate the delegation information associated with an address. Our analysis consists of identifying multiple levels of malleability, which are taken into account in our paper’s core result. We then introduce the first ideal functionality of a PoS wallet’s core which captures the PoS wallet’s capabilities and is realized as a secure protocol based on standard cryptographic primitives. Finally, we cover how to use the wallet core in conjunction with a PoS ledger, as well as investigate how delegation and stake pools affect a PoS system’s security
Society-oriented cryptographic techniques for information protection
Groups play an important role in our modern world. They are more reliable and more trustworthy than individuals. This is the reason why, in an organisation, crucial decisions are left to a group of people rather than to an individual. Cryptography supports group activity by offering a wide range of cryptographic operations which can only be successfully executed if a well-defined group of people agrees to co-operate. This thesis looks at two fundamental cryptographic tools that are useful for the management of secret information. The first part looks in detail at secret sharing schemes. The second part focuses on society-oriented cryptographic systems, which are the application of secret sharing schemes in cryptography. The outline of thesis is as follows
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