76 research outputs found
Cryptographic Schemes based on Elliptic Curve Pairings
This thesis introduces the concept of certificateless public key
cryptography (CLPKC). Elliptic curve pairings are then used to
make concrete CL-PKC schemes and are also used to make other
efficient key agreement protocols.
CL-PKC can be viewed as a model for the use of public key cryptography
that is intermediate between traditional certificated PKC and ID-PKC.
This is because, in contrast to traditional public key cryptographic
systems, CL-PKC does not require the use of certificates to guarantee
the authenticity of public keys. It does rely on the use of a trusted
authority (TA) who is in possession of a master key. In this
respect, CL-PKC is similar to identity-based public key
cryptography (ID-PKC). On the other hand, CL-PKC does not suffer
from the key escrow property that is inherent in ID-PKC.
Applications for the new infrastructure are discussed.
We exemplify how CL-PKC schemes can be constructed by constructing
several certificateless public key encryption schemes and
modifying other existing ID based schemes. The lack of
certificates and the desire to prove the schemes secure in the
presence of an adversary who has access to the master key or has
the ability to replace public keys, requires the careful
development of new security models. We prove that some of our
schemes are secure, provided that the Bilinear Diffie-Hellman
Problem is hard.
We then examine Joux’s protocol, which is a one round, tripartite
key agreement protocol that is more bandwidth-efficient than any
previous three-party key agreement protocol, however, Joux’s protocol
is insecure, suffering from a simple man-in-the-middle attack. We
show how to make Joux’s protocol secure, presenting several tripartite,
authenticated key agreement protocols that still require only one round
of communication. The security properties of the new protocols are
studied. Applications for the protocols are also discussed
Pairing-based cryptosystems and key agreement protocols.
For a long time, pairings on elliptic curves have been considered to be destructive in elliptic curve cryptography. Only recently after some pioneering works, particularly the well-known Boneh-Franklin identity-based encryption (IBE), pairings have quickly become an important
tool to construct novel cryptographic schemes.
In this thesis, several new cryptographic schemes with pairings are proposed, which are both efficient and secure with respect to a properly defined security model, and some
relevant previous schemes are revisited.
IBE provides a public key encryption mechanism where a public key can be an arbitrary string such as an entity identifier and unwieldy certificates are unnecessary. Based on the Sakai-Kasahara key construction, an IBE scheme which is secure in the Boneh-Franklin IBE model is constructed, and two identity-based key encapsulation mechanisms are proposed. These schemes achieve the best efficiency among the existing schemes to date. Recently Al-Riyami and Paterson introduced the certificateless public key encryption (CL-PKE) paradigm, which eliminates the need of certificates and at the same time retains the desirable properties of IBE without the key escrow problem. The security formulation of CL-PKE is revisited and a strong security model for this type of mechanism is defined.
Following a heuristic approach, three efficient CL-PKE schemes which are secure in the defined strong security model are proposed. Identity-based two-party key agreement protocols from pairings are also investigated.
The Bellare-Rogaway key agreement model is enhanced and within the model several previously unproven protocols in the literature are formally analysed. In considering that the user identity may be sensitive information in many environments, an identity-based key agreement protocol with unilateral identity privacy is proposed
Efficient Certificateless Public Key Encryption
In [3] Al-Riyami and Paterson introduced the notion of Certificateless Public Key Cryptography and presented an instantiation. In this paper, we revisit the formulation of certificateless public key encryption and construct a more efficient scheme and then extend it to an authenticated
encryption
The Double Ratchet: Security Notions, Proofs, and Modularization for the Signal Protocol
Signal is a famous secure messaging protocol used by billions of people, by virtue of many secure text messaging applications including Signal itself, WhatsApp, Facebook Messenger, Skype, and Google Allo. At its core it uses the concept of double ratcheting, where every message is encrypted and authenticated using a fresh symmetric key; it has many attractive properties, such as forward security, post-compromise security, and immediate (no-delay) decryption, which had never been achieved in combination by prior messaging protocols.
While the formal analysis of the Signal protocol, and ratcheting in general, has attracted a lot of recent attention, we argue that none of the existing analyses is fully satisfactory. To address this problem, we give a clean and general definition of secure messaging, which clearly indicates the types of security we expect, including forward security, post-compromise security, and immediate decryption. We are the first to explicitly formalize and model the immediate decryption property, which implies (among other things) that parties seamlessly recover if a given message is permanently lost---a property not achieved by any of the recent provable alternatives to Signal. We build a modular generalized Signal protocol from the following components: (a) continuous key agreement (CKA), a clean primitive we introduce and which can be easily and generically built from public-key encryption (not just Diffie-Hellman as is done in the current Signal protocol) and roughly models public-key ratchets; (b) forward-secure authenticated encryption with associated data (FS-AEAD), which roughly captures symmetric-key ratchets; and (c) a two-input hash function that is a pseudorandom function (resp. generator with input) in its first (resp. second) input, which we term PRF-PRNG. As a result, in addition to instantiating our framework in a way resulting in the existing, widely-used Diffie-Hellman based Signal protocol, we can easily get post-quantum security and not rely on random oracles in the analysis.
We further show that our design can be elegantly extended to include other forms of fine-grained state compromise recently studied at CRYPTO\u2718, but without sacrificing the immediate decryption property. However, we argue that the additional security offered by these modifications is unlikely to justify the efficiency hit of using much heavier public-key cryptography in place of symmetric-key cryptography
Two-message Key Exchange with Strong Security from Ideal Lattices
In this paper, we first revisit the generic two-message key exchange (TMKE) scheme (which will be referred to as KF) introduced by Kurosawa and Furukawa (CT-RSA 2014). This protocol is mainly based on key encapsulation mechanism (KEM) which is assumed to be secure against chosen plaintext attacks (IND-CPA). However, we find out that the security of the KF protocol cannot be reduced to IND-CPA KEM. The concrete KF protocol instantiated from ElGamal KEM is even subject to key compromise impersonation (KCI) attacks. In order to overcome the flaws of the KF scheme, we introduce a new generic TMKE scheme from KEM. Instead, we require that the KEM should be secure against one-time adaptive chosen ciphertext attacks (OT-IND-CCA2). We call this class of KEM as OTKEM.
In particular, we propose a new instantiation of OTKEM from Ring Learning with Errors (Ring-LWE) problem in the standard model. This yields a concrete post-quantum TMKE protocol with strong security. The security of our TMKE scheme is shown in the extended Canetti-Krawczyk model with perfect forward secrecy (eCK-PFS)
Evolving Bitcoin Custody
The broad topic of this thesis is the design and analysis of Bitcoin custody
systems. Both the technology and threat landscape are evolving constantly.
Therefore, custody systems, defence strategies, and risk models should be
adaptive too.
We introduce Bitcoin custody by describing the different types, design
principles, phases and functions of custody systems. We review the technology
stack of these systems and focus on the fundamentals; key-management and
privacy. We present a perspective we call the systems view. It is an attempt to
capture the full complexity of a custody system, including technology, people,
and processes. We review existing custody systems and standards.
We explore Bitcoin covenants. This is a mechanism to enforce constraints on
transaction sequences. Although previous work has proposed how to construct and
apply Bitcoin covenants, these require modifying the consensus rules of
Bitcoin, a notoriously difficult task. We introduce the first detailed
exposition and security analysis of a deleted-key covenant protocol, which is
compatible with current consensus rules. We demonstrate a range of security
models for deleted-key covenants which seem practical, in particular, when
applied in autonomous (user-controlled) custody systems. We conclude with a
comparative analysis with previous proposals.
Covenants are often proclaimed to be an important primitive for custody
systems, but no complete design has been proposed to validate that claim. To
address this, we propose an autonomous custody system called Ajolote which uses
deleted-key covenants to enforce a vault sequence. We evaluate Ajolote with; a
model of its state dynamics, a privacy analysis, and a risk model. We propose a
threat model for custody systems which captures a realistic attacker for a
system with offline devices and user-verification. We perform ceremony analysis
to construct the risk model.Comment: PhD thesi
Pairing-based cryptosystems and key agreement protocols
For a long time, pairings on elliptic curves have been considered to be destructive in elliptic curve cryptography. Only recently after some pioneering works, particularly the well-known Boneh-Franklin identity-based encryption (IBE), pairings have quickly become an important tool to construct novel cryptographic schemes. In this thesis, several new cryptographic schemes with pairings are proposed, which are both efficient and secure with respect to a properly defined security model, and some relevant previous schemes are revisited. IBE provides a public key encryption mechanism where a public key can be an arbitrary string such as an entity identifier and unwieldy certificates are unnecessary. Based on the Sakai-Kasahara key construction, an IBE scheme which is secure in the Boneh-Franklin IBE model is constructed, and two identity-based key encapsulation mechanisms are proposed. These schemes achieve the best efficiency among the existing schemes to date. Recently Al-Riyami and Paterson introduced the certificateless public key encryption (CL-PKE) paradigm, which eliminates the need of certificates and at the same time retains the desirable properties of IBE without the key escrow problem. The security formulation of CL-PKE is revisited and a strong security model for this type of mechanism is defined. Following a heuristic approach, three efficient CL-PKE schemes which are secure in the defined strong security model are proposed. Identity-based two-party key agreement protocols from pairings are also investigated. The Bellare-Rogaway key agreement model is enhanced and within the model several previously unproven protocols in the literature are formally analysed. In considering that the user identity may be sensitive information in many environments, an identity-based key agreement protocol with unilateral identity privacy is proposed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
- …