Anonymous Signatures Revisited

Information

In modern applications, such as Electronic Voting, e-Health, e-Cash, there is a need that the validity of a signature should be verified by only one responsible person. This is opposite to the traditional digital signature scheme where anybody can verify a signature. There have been several solutions for this problem, the first one is we combine a signature scheme with an encryption scheme; the second one is to use the group signature; and the last one is to use the strong designated verifier signature scheme with the undeniable property. In this paper, we extend the traditional digital signature scheme to propose a new solution for the aforementioned problem. Our extension is in the sense that only a designated verifier (responsible person) can verify a signer’s signature, and if necessary (in case the signer refuses to admit his/her signature) the designated verifier without revealing his/her secret key is able to prove to anybody that the signer has actually generated the signatu...

2008

In this paper, we propose a new signature scheme that is existentially unforgeable under a chosen message attack without random oracle. The security of our scheme depends on a new complexity assumption called the k+1 square roots assumption. We also discuss the relationship between the k+1 square roots assumption and some related problems and provide some conjectures. Moreover, the k+1 square roots assumption can be used to construct shorter signatures under the random oracle model. As some applications, a new chameleon hash signature scheme and a on-line/off-line signature scheme and a new efficient anonymous credential scheme based on the proposed signature scheme are presented.

2006

Universal designated verifier signatures (UDVS) were introduced in 2003 by Steinfeld et al. to allow signature holders to monitor the verification of a given signature in the sense that any plain signature can be publicly turned into a signature which is only verifiable by some specific designated verifier. Privacy issues, like non-dissemination of digital certificates, are the main motivations to study such primitives. In this paper, we propose two fairly efficient UDVS schemes which are secure (in terms of unforgeability and anonymity) in the standard model (i.e. without random oracles). Their security relies on algorithmic assumptions which are much more classical than assumptions involved in the two only known UDVS schemes in standard model to date. The latter schemes, put forth by Zhang et al. in 2005 and Vergnaud in 2006, rely on the Strong Diffie-Hellman assumption and the strange-looking knowledge of exponent assumption (KEA). Our schemes are obtained from Waters's signature and they do not need the KEA assumption. They are also the first random oracle-free constructions with the anonymity property.

Proceedings of the 8th ACM SIGSAC symposium on Information, computer and communications security - ASIA CCS '13, 2013

We develop a three-level hierarchy of privacy notions for (unforgeable) digital signature schemes. We first prove mutual independence of existing notions of anonymity and confidentiality, and then show that these are implied by higher privacy goals. The top notion in our hierarchy is pseudorandomness: signatures with this property hide the entire information about the signing process and cannot be recognized as signatures when transmitted over a public network. This implies very strong unlinkability guarantees across different signers and even different signing algorithms, and gives rise to new forms of private public-key authentication. We show that one way towards pseudorandom signatures leads over our mid-level notion, called indistinguishability: such signatures can be simulated using only the public parameters of the scheme. As we reveal, indistinguishable signatures exist in different cryptographic settings (e.g. based on RSA, discrete logarithms, pairings) and can be efficiently lifted to pseudorandomness deploying general transformations using appropriate encoding techniques. We also examine a more direct way for obtaining pseudorandomness for any unforgeable signature scheme. All our transformations work in the standard model. We keep public verifiability of signatures in the setting of system-wide known public keys. Some results even hold if signing keys are disclosed to the adversary -given that signed messages have high entropy.

Journal of Cryptology, 2009

Ring signatures, first introduced by Rivest, Shamir, and Tauman, enable a user to sign a message so that a ring of possible signers (of which the user is a member) is identified, without revealing exactly which member of that ring actually generated the signature. In contrast to group signatures, ring signatures are completely "ad-hoc" and do not require any central authority or coordination among the various users (indeed, users do not even need to be aware of each other); furthermore, ring signature schemes grant users fine-grained control over the level of anonymity associated with any particular signature. This paper has two main areas of focus. First, we examine previous definitions of security for ring signature schemes and suggest that most of these prior definitions are too weak, in the sense that they do not take into account certain realistic attacks. We propose new definitions of anonymity and unforgeability which address these threats, and give separation results proving that our new notions are strictly stronger than previous ones. Second, we show the first constructions of ring signature schemes in the standard model. One scheme is based on generic assumptions and satisfies our strongest definitions of security. Two additional schemes are more efficient, but achieve weaker security guarantees and more limited functionality.

Designs, Codes and Cryptography, 2007

Certificateless cryptography involves a Key Generation Center (KGC) which issues a partial key to a user and the user also independently generates an additional public/secret key pair in such a way that the KGC who knows only the partial key but not the additional secret key is not able to do any cryptographic operation on behalf of the user; and a third party who replaces the public/secret key pair but does not know the partial key cannot do any cryptographic operation as the user either. We call this attack launched by the third party as the key replacement attack. In ACISP 2004, Yum and Lee proposed a generic construction of digital signature schemes under the framework of certificateless cryptography. In this paper, we show that their generic construction is insecure against key replacement attack. In particular, we give some concrete examples to show that the security requirements of some building blocks they specified are insufficient to support some of their security claims. We then propose a modification of their scheme and show its security in a new and simplified security model. We show that our simplified definition and adversarial model not only capture all the distinct features of certificateless signature but are also more versatile when compared with all the comparable ones. We believe that the model itself is * A preliminary version of the extended abstract of partial results appeared in ACISP 2006 [9]. Girault's Level 2 security. For achieving Level 3 security, that a conventional signature scheme in Public Key Infrastructure does, we propose an extension to our definition of certificateless signature scheme and introduce an additional security model for this extension. We show that our generic construction satisfies Level 3 security after some appropriate and simple modification.

Lecture Notes in Computer Science, 2009

Traceable signature scheme extends a group signature scheme with an enhanced anonymity management mechanism. The group manager can compute a tracing trapdoor which enables anyone to test if a signature is signed by a given misbehaving user, while the only way to do so for group signatures requires revealing the signer of all signatures. Nevertheless, it is not tracing in a strict sense. For all existing schemes, T tracing agents need to recollect all N signatures ever produced and perform RN "checks" for R revoked users. This involves a high volume of transfer and computations. Increasing T increases the degree of parallelism for tracing but also the probability of "missing" some signatures in case some of the agents are dishonest. We propose a new and efficient way of tracing-the tracing trapdoor allows the reconstruction of tags such that each of them can uniquely identify a signature of a misbehaving user. Identifying N signatures out of the total of N signatures (N << N) just requires the agent to construct N small tags and send them to the signatures holder. N here gives a trade-off between the number of unlinkable signatures a member can produce and the efforts for the agents to trace the signatures. We present schemes with simple design borrowed from anonymous credential systems. Our schemes are proven secure respectively in the random oracle model and in the common reference string model (or in the standard model if there exists a trusted party for system parameters initialization).

2006

It has been demonstrated by Bellare, Neven, and Namprempre (Eurocrypt 2004) that identity-based signature schemes can be constructed from any PKI-based signature scheme. In this paper we consider the following natural extension: is there a generic construction of “identity-based signature schemes with additional properties” (such as identity-based blind signatures, verifiably encrypted signatures, ...) from PKI-based signature schemes with the same properties? Our results show that this is possible for great number of properties including proxy signatures; (partially) blind signatures; verifiably encrypted signatures; undeniable signatures; forward-secure signatures; (strongly) key insulated signatures; online/offline signatures; threshold signatures; and (with some limitations) aggregate signatures. Using well-known results for PKI-based schemes, we conclude that such identity-based signature schemes with additional properties can be constructed, enjoying some better properties than specific schemes proposed until know. In particular, our work implies the existence of identity-based signatures with additional properties that are provably secure in the standard model, do not need bilinear pairings, or can be based on general assumptions.

2013

We study the Sign then Encrypt, Commit then Encrypt and Sign, and Encrypt then Sign paradigms in the context of three cryptographic primitives, namely designated confirmer signatures, signcryption, and verifiably encrypted signatures. Our study identifies weaknesses in those paradigms which impose the use of expensive encryption (as a building block) in order to meet a reasonable security level. Next, we propose some optimizations which annihilate the found weaknesses and allow consequently cheap encryption without compromising the overall security. Our optimizations further enjoy verifiability, a property profoundly needed in many real-life applications of the studied primitives.

… Key Cryptography–PKC …, 2009

Sanitizable signature schemes, as defined by Ateniese et al. (ESORICS 2005), allow a signer to partly delegate signing rights to another party, called the sanitizer. That is, the sanitizer is able to modify a predetermined part of the original message such that the integrity and authenticity of the unchanged part is still verifiable. Ateniese et al. identify five security requirements for such schemes (unforgeability, immutability, privacy, transparency and accountability) but do not provide formal specifications for these properties. They also present a scheme that is supposed to satisfy these requirements.