During this Thesis we investigated the possibilities that Identity-based Encryption offers when used out of their original purpose. We managed to generalize a whole class of different identity-based encryption schemes into Downgradable Identity-based Encryptions. We found a generic way to construct Blind Identity-based Encryptions. These two works leads both to applications that are not a priori linked with IBE: Attribute-based Encryption from Downgradable IBE and Oblivious Transfer for Blind IBE, in the case of Affine IBE we manage to reduce the communication cost from a linear to logarithmic. As application we also find a way to use Hierarchical IBE to construct a special type of signature called Identity-based Designated Verifier Signature. We continue the research out of the context of IBE's application with Oblivious Transfer. We manage to generalize the concept of Oblivious Transfer into a new protocol called Oblivious Language-based Envelope encompassing many kind of protocols. Finally, in the image of the whole Thesis we construct Oblivious Transfer with a very different primitive called Password Authenticated Key Exchange. Surprisingly, with some optimizations this last transformation leads to a very efficient Oblivious Transfer Protocol. The Identity-based Encryption is our main basis of work, thus efficient instantiations of this primitive were the key of our own efficiency, thus we used the instanciation from the paper of Blazy et als at crypto 2014 which is efficient, tight secure and affine.
During this Thesis we investigated the possibilities that Identity-based Encryption offers when used out of their original purpose. We managed to generalize a whole class of different identity-based encryption schemes into Downgradable Identity-based Encryptions. We found a generic way to construct Blind Identity-based Encryptions. These two works leads both to applications that are not a priori linked with IBE: Attribute-based Encryption from Downgradable IBE and Oblivious Transfer for Blind IBE, in the case of Affine IBE we manage to reduce the communication cost from a linear to logarithmic. As application we also find a way to use Hierarchical IBE to construct a special type of signature called Identity-based Designated Verifier Signature. We continue the research out of the context of IBE's application with Oblivious Transfer. We manage to generalize the concept of Oblivious Transfer into a new protocol called Oblivious Language-based Envelope encompassing many kind of protocols. Finally, in the image of the whole Thesis we construct Oblivious Transfer with a very different primitive called Password Authenticated Key Exchange. Surprisingly, with some optimizations this last transformation leads to a very efficient Oblivious Transfer Protocol. The Identity-based Encryption is our main basis of work, thus efficient instantiations of this primitive were the key of our own efficiency, thus we used the instanciation from the paper of Blazy et als at crypto 2014 which is efficient, tight secure and affine.
Abstract In the actual applications, an adversary can break the security of cryptography scheme through various leakage attacks (e.g. side-channel attacks, cold-boot attacks, etc.), even the continuous leakage attacks. That is, a practical cryptography scheme must maintain its claimed security in the continuous leakage setting. However, the previous constructions on the leakage-resilient identity-based encryption (IBE) scheme could tolerate a leakage that is bounded, and cannot resist the continuous leakage attacks. In order to further achieve the better security, a novel method to build the continuous leakage-resilient IBE scheme with tight security is presented in this paper, and the scheme’s security is proved, in the standard model, based on a stronger security assumption that depends on the number of queries made by the adversary. In addition, our proposal has several advantages over previous such constructions, e.g. shorter public parameters, higher communication efficiency, tight security, etc.
The contributions of this thesis fall into three areas of certificateless cryptography. The first area is encryption, where we propose new constructions for both identity-based and certificateless cryptography. We construct an n-out-of- n group encryption scheme for identity-based cryptography that does not require any special means to generate the keys of the trusted authorities that are participating. We also introduce a new security definition for chosen ciphertext secure multi-key encryption. We prove that our construction is secure as long as at least one authority is uncompromised, and show that the existing constructions for chosen ciphertext security from identity-based encryption also hold in the group encryption case. We then consider certificateless encryption as the special case of 2-out-of-2 group encryption and give constructions for highly efficient certificateless schemes in the standard model. Among these is the first construction of a lattice-based certificateless encryption scheme. Our next contribution is a highly efficient certificateless key encapsulation mechanism (KEM), that we prove secure in the standard model. We introduce a new way of proving the security of certificateless schemes based that are based on identity-based schemes. We leave the identity-based part of the proof intact, and just extend it to cover the part that is introduced by the certificateless scheme. We show that our construction is more efficient than any instanciation of generic constructions for certificateless key encapsulation in the standard model. The third area where the thesis contributes to the advancement of certificateless cryptography is key agreement. Swanson showed that many certificateless key agreement schemes are insecure if considered in a reasonable security model. We propose the first provably secure certificateless key agreement schemes in the strongest model for certificateless key agreement. We extend Swanson's definition for certificateless key agreement and give more power to the adversary. Our new schemes are secure as long as each party has at least one uncompromised secret. Our first construction is in the random oracle model and gives the adversary slightly more capabilities than our second construction in the standard model. Interestingly, our standard model construction is as efficient as the random oracle model construction.
Abstract We construct an identity-based encryption (IBE) scheme that is tightly secure in a very strong sense. Specifically, we consider a setting with many instances of the scheme and many encryptions per instance. In this setting, we reduce the security of our scheme to a variant of a simple assumption used for a similar purpose by Chen and Wee (CRYPTO 2013, Springer, 2013). The security loss of our reduction is O(k) O(k) (where k k is the security parameter). Our scheme is the first IBE scheme to achieve this strong flavor of tightness under a simple assumption. Technically, our scheme is a variation of the IBE scheme by Chen and Wee. However, in order to “lift” their results to the multi-instance, multi-ciphertext case, we need to develop new ideas. In particular, while we build on (and extend) their high-level proof strategy, we deviate significantly in the low-level proof steps.
Identity-based encryption (IBE) is one of the most important primitives in cryptography, and various security notions of IBE (e. g., IND-ID-CCA2, NM-ID-CCA2, IND-sID-CPA etc.) have been introduced. The relations among them have been clarified recently. This paper, for the first time, investigates the security of IBE in the universally composable (UC) framework. This paper first defines the UC-security of IBE, i. e., we define the ideal functionality of IBE, FIBE. We then show that UC-secure IBE is equivalent to conventionally-secure (IND-ID-CCA2-secure) IBE.
Secure Multi-Party Computation (MPC) is a widely acknowledged framework enabling the design of multi-party protocols that preserve the privacy of parties' inputs while ensuring the correct evaluation of the desired functionality. Crucially, these security guarantees should hold even in the presence of external entities who are empowered with some adversarial capabilities, such as controlling the communication channels used throughout the protocol run or forcing a subset of the parties to behave arbitrarily (so-called ``malicious" or ``byzantine" corruptions). Concretely, a user can instantiate secure MPC protocols on a device to carry out computations involving sensitive information with other untrusted parties. Despite capturing very general classes of real-world threats, one limitation of ``traditional" MPC lies in assuming at least one ``honest" party who, throughout the protocol run, behaves exactly as per the theoretical specification of the protocol itself. For several practical settings this may be unrealistic, as the devices used to run the protocol are themselves exposed to a plethora of threats, such as attacks on software or hardware components. Moreover, the security guarantees provided by secure MPC could be voided if a protocol is found to be faulty, be it from cryptographic assumptions falling short or from an incorrect formalization of the protocol itself. In this composition, we explore more expressive frameworks that enable the design of secure MPC protocols and cryptographic primitives even in the presence of untrusted devices. We first consider cryptographic reverse firewalls: lightweight devices that sanitize a party's traffic while preserving the correctness of the protocol. These objects were originally introduced by Mironov and Stephens-Davidowitz (EUROCRYPT'15) and later embedded in the framework of subversion-resilient Universal Composability (srUC) due to Chakraborty et al. (EUROCRYPT'22). Under the srUC framework, it is possible to design protocols that provide meaningful security guarantees even if the devices of honest parties have been tampered with in an undetectable manner with the goal of exfiltrating information (so-called ``specious subversion attacks"). In particular, we focus on the design of protocols for Password-Authenticated Key Exchange (PAKE): a cryptographic primitive that enables two parties to mutually authenticate by establishing a shared high-entropy key leveraging exclusively some (possibly low-entropy) pre-shared password. (1) Our first contribution focuses on sanitizing the PAKE protocol from Oblivious Transfer (OT) due to Canetti et al. (PKC'12). For that, we design and instantiate novel cryptographic primitives with sanitation-friendly properties that may be of independent interest, including sanitizable variants of oblivious transfer, dual-mode cryptosystems, and signature schemes. As an additional contribution, we formalize the unauthenticated setting in the srUC framework by extending the framework of split-authentication due to Barak et al. (CRYPTO'05, JoC'07). This is the first PAKE protocol ever designed in the srUC framework. (2) Our second contribution consists of sanitizing the PAKE protocol from trapdoor smooth-projective hashing due to Benhamouda and Pointcheval (CRYPTO'13). The sanitation requires non-trivial modifications to the original protocol, whose security relies on a CCA-secure encryption scheme - an inherently non-malleable primitive. Along the way, we bring advances to the field of malleable smooth-projective hash functions, originally introduced by Chen et al. (ASIACRYPT'16), and coin the notion of malleable trapdoor smooth-projective hashing. Our resulting PAKE protocol has better communication and round complexity compared to the aforementioned PAKE-from-OT. We then shift our attention to t-out-of-n robust combiners: constructions that take as input n candidate instantiations of some cryptographic primitive to securely realize the same primitive, as long as at least t of the candidates are secure. These objects were first formalized by Harnik et al. (EUROCRYPT'05), where robustness is characterized by explicitly forbidding combiners from re-implementing the desired primitive from scratch. Here, we focus on Non-Interactive Zero-Knowledge (NIZK): a cryptographic primitive that allows a prover to convince a verifier of the veracity of some NP-statement by using a single message (commonly referred to as a ``proof"). (3) Our third contribution provides a comprehensive characterization of robust combiners for NIZK. We show the first formal definition of these objects, and prove that no robust NIZK combiner exists for t ≤ n/2 unless the polynomial hierarchy collapses. To complement our negative results, we provide three incomparable constructions: (i) A black-box combiner for {\em homomorphic} NP languages, where n,t are polynomial and t > n/2; (ii) A non-black-box combiner for any NP language, where n,t are constant and t > n/2; (iii) A non-black-box combiner for any NP language, where n,t are polynomial and t > 2n/3.
In 1998, Blaze, Bleumer, and Strauss proposed a kind of cryptographic primitive called proxy re-encryption. In proxy re-encryption, a proxy can transform a ciphertext computed under Alice’s public key into one that can be opened under Bob’s decryption key. In 2007, Matsuo proposed the concept of four types of proxy re-encryption schemes: CBE (Certificate Based Public Key Encryption) to IBE (Identity Based Encryption) (type 1), IBE to IBE (type 2), IBE to CBE (type 3), CBE to CBE (type 4). In this paper, we find that if we allow the PKG to use its masterkey in the process of generating re-encryption key for proxy re-encryption in identity based setting, many open problems can be solved. We give the new security models for proxy reencryption in identity based setting, especially considering PKG’s involving in the re-encryption key generation process and PKG’s master-key’s security. We construct the new IND-ID-CPA and the first IND-ID-CCA2 secure proxy reencryption schemes based on BB1 IBE. We also prove their security by introducing some new techniques which maybe have independent interest. At last, we compare our new schemes with existing ones, the results show that our scheme can achieve high security levels and are very efficient for re-encryption and, which are very important for practical applications.