Universally Convertible Directed Signatures

Many variants of Chaum and van Antwerpen's undeniable signatures have been proposed to achieve specific properties desired in real-world applications of cryptography. Among them, directed signatures were introduced by Lim and Lee in 1993. Directed signatures differ from the well-known confirmer signatures in that the signer has the simultaneous abilities to confirm, deny and individually convert a signature. The universal conversion of these signatures has remained an open problem since their introduction in 1993. This paper provides a positive answer to this quest by showing a very efficient design for universally convertible directed signatures (UCDS) both in terms of computational complexity and signature size. Our construction relies on the so-called xyz-trick applicable to bilinear map groups. We define proper security notions for UCDS schemes and show that our construction is secure, in the random oracle model, under computational assumptions close to the CDH and DDH assumptions. Finally, we introduce and realize traceable universally convertible directed signatures where a master tracing key allows to link signatures to their direction

Design and Analysis of Opaque Signatures

Digital signatures were introduced to guarantee the authenticity and integrity of the underlying messages. A digital signature scheme comprises the key generation, the signature, and the verification algorithms. The key generation algorithm creates the signing and the verifying keys, called also the signer’s private and public keys respectively. The signature algorithm, which is run by the signer, produces a signature on the input message. Finally, the verification algorithm, run by anyone who knows the signer’s public key, checks whether a purported signature on some message is valid or not. The last property, namely the universal verification of digital signatures is undesirable in situations where the signed data is commercially or personally sensitive. Therefore, mechanisms which share most properties with digital signatures except for the universal verification were invented to respond to the aforementioned need; we call such mechanisms “opaque signatures”. In this thesis, we study the signatures where the verification cannot be achieved without the cooperation of a specific entity, namely the signer in case of undeniable signatures, or the confirmer in case of confirmer signatures; we make three main contributions. We first study the relationship between two security properties important for public key encryption, namely data privacy and key privacy. Our study is motivated by the fact that opaque signatures involve always an encryption layer that ensures their opacity. The properties required for this encryption vary according to whether we want to protect the identity (i.e. the key) of the signer or hide the validity of the signature. Therefore, it would be convenient to use existing work about the encryption scheme in order to derive one notion from the other. Next, we delve into the generic constructions of confirmer signatures from basic cryptographic primitives, e.g. digital signatures, encryption, or commitment schemes. In fact, generic constructions give easy-to-understand and easy-to-prove schemes, however, this convenience is often achieved at the expense of efficiency. In this contribution, which constitutes the core of this thesis, we first analyze the already existing constructions; our study concludes that the popular generic constructions of confirmer signatures necessitate strong security assumptions on the building blocks, which impacts negatively the efficiency of the resulting signatures. Next, we show that a small change in these constructionsmakes these assumptions drop drastically, allowing as a result constructions with instantiations that compete with the dedicated realizations of these signatures. Finally, we revisit two early undeniable signatures which were proposed with a conjectural security. We disprove the claimed security of the first scheme, and we provide a fix to it in order to achieve strong security properties. Next, we upgrade the second scheme so that it supports a iii desirable feature, and we provide a formal security treatment of the new scheme: we prove that it is secure assuming new reasonable assumptions on the underlying constituents

New Convertible Authenticated Encryption Scheme with Message Linkages

The digital signature provides the signing message with functions like authentication, integration and non-repudiation. However, in some of the applications, the signature has to be verified only by specific recipients of the message and it should be hidden from the public. For achieving this, authenticated encryption systems are used. Authenticated Encryption schemes are highly helpful to send a confidential message over an insecure network path. In order to protect the recipients benefit and for ensuring non-repudiation, we help the receiver to change the signature from encrypted one to an ordinary one. With this we avoid any sort of later disputes. Few years back, Araki et al. has proposed a convertible authenticated scheme for giving a solution to the problem. His scheme enables the recipient to convert the senders signature into an ordinary one. However, the conversion requires the cooperation of the signer. In this thesis, we present a convertible authenticated encryption scheme that can produce the ordinary signature without the cooperation of the signer with a greater ease. Here, we display a validated encryption plan using message linkages used to convey a message. For the collector's advantage, the beneficiary can surely change the encrypted signature into an ordinary signature that which anyone can check. A few attainable assaults shall be examined, and the security investigation will demonstrate that none of the them can effectively break the proposed plan

New Constructions of Convertible Undeniable Signature Schemes without Random Oracles

In Undeniable Signature, a signature\u27s validity can only be confirmed or disavowed with the help of an alleged signer via a confirmation or disavowal protocol. A Convertible undeniable signature further allows the signer to release some additional information which can make an undeniable signature become publicly verifiable. In this work we introduce a new kind of attacks, called \emph{claimability attacks}, in which a dishonest/malicious signer both disavows a signature via the disavowal protocol and confirms it via selective conversion. Conventional security requirement does not capture the claimability attacks. We show that some convertible undeniable signature schemes are vulnerable to this kind of attacks. We then propose a new efficient construction of fully functional convertible undeniable signature, which supports both selective conversion and universal conversion, and is immune to the claimability attacks. To the best of our knowledge, it is the most efficient convertible undeniable signature scheme with provable security in the standard model. A signature is comprised of three elements of a bilinear group. Both the selective converter of a signature and the universal converter consist of one group element only. Besides, the confirmation and disavowal protocols are also very simple and efficient. Furthermore, the scheme can be extended to support additional features which include the delegation of conversion and confirmation/disavowal, threshold conversion and etc. We also propose an alternative generic construction of convertible undeniable signature schemes. Unlike the conventional sign-then-encrypt paradigm, the signer encrypts its (standard) signature with an identity-based encryption instead of a public key encryption. It enjoys the advantage of short selective converter, which is simply an identity-based user private key, and security against claimability attacks

Identity-Based Directed Signature Scheme from Bilinear Pairings

In a directed signature scheme, a verifier can exclusively verify the signatures designated to himself, and shares with the signer the ability to prove correctness of the signature to a third party when necessary. Directed signature schemes are suitable for applications such as bill of tax and bill of health. This paper studies directed signatures in the identity-based setting. We first present the syntax and security notion that includes unforgeability and invisibility, then propose a concrete identity-based directed signature scheme from bilinear pairings. We then prove our scheme existentially unforgeable under the computational Diffie-Hellman assumption, and invisible under the decisional Bilinear Diffie-Hellman assumption, both in the random oracle model

Security Proofs for Identity-Based Identification and Signature Schemes

This paper provides either security proofs or attacks for a large number of identity-based identification and signature schemes defined either explicitly or implicitly in existing literature. Underlying these is a framework that on the one hand helps explain how these schemes are derived, and on the other hand enables modular security analyses, thereby helping to understand, simplify and unify previous work. We also analyze a generic folklore construction that in particular yields identity-based identification and signature schemes without random oracles

An efficient ID- based directed signature scheme from bilinear pairings

A directed signature scheme allows a designated verifier to directly verify a signature issued to him, and a third party to check the signature validity with the help of the signer or the designated verifier as well. Directed signatures are applicable where the signed message is sensitive to the signature receiver. Due to its merits, directed signature schemes are suitable for applications such as bill of tax and bill of health. In this paper, we proposed an efficient identity based directed signature scheme from bilinear pairings. Our scheme is efficient than the existing directed signature schemes. In the random oracle model, our scheme is unforgeable under the Computational Diffie-Hellman (CDH) assumption, and invisible under the Decisional Bilinear Diffie-Hellman (DBDH)

Efficient Deniable Authentication for Signatures, Application to Machine-Readable Travel Document

Releasing a classical digital signature faces to privacy issues. Indeed, there are cases where the prover needs to authenticate some data without making it possible for any malicious verifier to transfer the proof to anyone else. It is for instance the case for e-passports where the signature from the national authority authenticates personal data. To solve this problem, we can prove knowledge of a valid signature without revealing it. This proof should be non-transferable. We first study deniability for signature verification. Deniability is essentially a weaker form of non-transferability. It holds as soon as the protocol is finished (it is often called offline non-transferability). We introduce Offline Non-Transferable Authentication Protocol (ON-TAP) and we show that it can be built by using a classical signature scheme and a deniable zero-knowledge proof of knowledge. For that reason, we use a generic transform for Σ-protocols. Finally, we give examples to upgrade signature standards based on RSA or ElGamal into an ONTAP. Our examples are well-suited for implementation in e-passports