Round-Optimal Blind Signatures in the Plain Model from Classical and Quantum Standard Assumptions

Blind signatures, introduced by Chaum (Crypto’82), allows a user to obtain a signature on a message without revealing the message itself to the signer. Thus far, all existing constructions of round-optimal blind signatures are known to require one of the following: a trusted setup, an interactive assumption, or complexity leveraging. This state-of-the-affair is somewhat justified by the few known impossibility results on constructions of round-optimal blind signatures in the plain model (i.e., without trusted setup) from standard assumptions. However, since all of these impossibility results only hold under some conditions, fully (dis)proving the existence of such round-optimal blind signatures has remained open. In this work, we provide an affirmative answer to this problem and construct the first round-optimal blind signature scheme in the plain model from standard polynomial-time assumptions. Our construction is based on various standard cryptographic primitives and also on new primitives that we introduce in this work, all of which are instantiable from classical and post-quantum standard polynomial-time assumptions. The main building block of our scheme is a new primitive called a blind-signature-conforming zero-knowledge (ZK) argument system. The distinguishing feature is that the ZK property holds by using a quantum polynomial-time simulator against non-uniform classical polynomial-time adversaries. Syntactically one can view this as a delayed-input three-move ZK argument with a reusable first message, and we believe it would be of independent interest

Efficient Round-Optimal Blind Signatures in the Standard Model

Blind signatures are at the core of e-cash systems and have numerous other applications. In this work we construct efficient blind and partially blind signature schemes over bilinear groups in the standard model. Our schemes yield short signatures consisting of only a couple of elements from the shorter source group and have very short communication overhead consisting of $1$ group element on the user side and $3$ group elements on the signer side. At $80$-bit security, our schemes yield signatures consisting of only $40$ bytes which is $67\%$ shorter than the most efficient existing scheme with the same security in the standard model. Verification in our schemes requires only a couple of pairings. Our schemes compare favorably in every efficiency measure to all existing counterparts offering the same security in the standard model. In fact, the efficiency of our signing protocol as well as the signature size compare favorably even to many existing schemes in the random oracle model. For instance, our signatures are shorter than those of Brands\u27 scheme which is at the heart of the U-Prove anonymous credential system used in practice. The unforgeability of our schemes is based on new intractability assumptions of a ``one-more\u27\u27 type which we show are intractable in the generic group model, whereas their blindness holds w.r.t.~malicious signing keys in the information-theoretic sense. We also give variants of our schemes for a vector of messages

Practical Round-Optimal Blind Signatures in the ROM from Standard Assumptions

Blind signatures serve as a foundational tool for privacy-preserving applications and have recently seen renewed interest due to new applications in blockchains and privacy-authentication tokens. With this, constructing practical round-optimal (i.e., signing consists of the minimum two rounds) blind signatures in the random oracle model (ROM) has been an active area of research, where several impossibility results indicate that either the ROM or a trusted setup is inherent. In this work, we present two round-optimal blind signatures under standard assumptions in the ROM with different approaches: one achieves the smallest sum of the signature and communication sizes, while the other achieves the smallest signature size. Both of our instantiations are based on standard assumptions over asymmetric pairing groups, i.e., CDH, DDH, and/or SXDH. Our first construction is a highly optimized variant of the generic blind signature construction by Fischlin (CRYPTO\u2706) and has signature and communication sizes 447 B and 303 B, respectively. We progressively weaken the building blocks required by Fischlin and we result in the first blind signature where the sum of the signature and communication sizes fit below 1 KB based on standard assumptions. Our second construction is a semi-generic construction from a specific class of randomizable signature schemes that admits an all-but-one reduction. The signature size is only 96 B while the communication size is 2.2 KB. This matches the previously known smallest signature size while improving the communication size by several orders of magnitude. Finally, both of our constructions rely on a (non-black box) fine-grained analysis of the forking lemma that may be of independent interest

On Security Proofs of Existing Equivalence Class Signature Schemes

Equivalence class signatures (EQS), introduced by Hanser and Slamanig (AC\u2714), sign vectors of elements from a bilinear group. Signatures can be ``adapted\u27\u27, meaning that anyone can transform a signature on a vector to a (random) signature on any multiple of that vector. (Signatures thus authenticate equivalence classes.) A transformed signature/message pair is then indistinguishable from a random signature on a random message. EQS have been used to efficiently instantiate (delegatable) anonymous credentials, (round-optimal) blind signatures, ring and group signatures and anonymous tokens. The original EQS construction (J.Crypto\u2719) is only proven in the generic group model, while the first construction from standard assumptions (PKC\u2718) only yields security guarantees insufficient for most applications. Two works (AC\u2719, PKC\u2722) propose applicable schemes which assume the existence of a common reference string for the anonymity notion. Their unforgeability is argued via a security proof from standard (or non-interactive) assumptions. In this work we show that their security proof is flawed and explain the subtle issue

Pairing-Free Blind Signatures from Standard Assumptions in the ROM

Blind Signatures are a useful primitive for privacy preserving applications such as electronic payments, e-voting, anonymous credentials, and more. However, existing practical blind signature schemes based on standard assumptions require either pairings or lattices. We present the first construction of a round-optimal blind signature in the random oracle model based on standard assumptions without resorting to pairings or lattices. In particular, our construction is secure under the strong RSA assumption and DDH (in pairing-free groups). For our construction, we provide a NIZK-friendly signature based on strong RSA, and efficiently instantiate Fischlin\u27s generic framework (CRYPTO\u2706). Our Blind Signature scheme has signatures of size 4.28 KB and communication cost 62.19 KB. On the way, we develop techniques that might be of independent interest. In particular, we provide efficient relaxed range-proofs with subversion zero-knowledge and compact commitments to elements of arbitrary groups

On the (Im)possibility of Projecting Property in Prime-Order Setting

Projecting bilinear pairings have frequently been used for designing cryptosystems since they were first derived from composite order bilinear groups. There have been only a few studies on the (im)possibility of projecting bilinear pairings. Groth and Sahai (EUROCRYPT 2008) showed that projecting bilinear pairings can be achieved in a prime-order group setting. They constructed both projecting asymmetric bilinear pairings and projecting symmetric bilinear pairings, where a bilinear pairing $e$ is symmetric if it satisfies $e(g,h)=e(h,g)$ for any group elements $g$ and $h$; otherwise, it is asymmetric. Subsequently, Freeman (EUROCRYPT 2010) generalized Groth-Sahai\u27s projecting asymmetric bilinear pairings. In this paper, we provide impossibility results on projecting bilinear pairings in a prime-order group setting. More precisely, we specify the lower bounds of 1. the image size of a projecting asymmetric bilinear pairing 2. the image size of a projecting symmetric bilinear pairing 3. the computational cost for a projecting asymmetric bilinear pairing 4. the computational cost for a projecting symmetric bilinear pairing in a prime-order group setting naturally induced from the $k$-linear assumption, where the computational cost means the number of generic operations. Our lower bounds regarding a projecting asymmetric bilinear pairing are tight, i.e., it is impossible to construct a more efficient projecting asymmetric bilinear pairing than the constructions of Groth-Sahai and Freeman. However, our lower bounds regarding a projecting symmetric bilinear pairing differ from Groth and Sahai\u27s results regarding a symmetric bilinear pairing; We fill these gaps by constructing projecting symmetric bilinear pairings. In addition, on the basis of the proposed symmetric bilinear pairings, we construct more efficient instantiations of cryptosystems that essentially use the projecting symmetric bilinear pairings in a modular fashion. Example applications include new instantiations of the Boneh-Goh-Nissim cryptosystem, the Groth-Sahai non-interactive proof system, and Seo-Cheon round optimal blind signatures proven secure under the DLIN assumption. These new instantiations are more efficient than the previous ones, which are also provably secure under the DLIN assumption. These applications are of independent interest

Hecate: abuse reporting in secure messengers with sealed sender

End-to-end encryption provides strong privacy protections to billions of people, but it also complicates efforts to moderate content that can seriously harm people. To address this concern, Tyagi et al. [CRYPTO 2019] introduced the concept of asymmetric message franking (AMF), which allows people to report abusive content to a moderator, while otherwise retaining end-to-end privacy by default and even compatibility with anonymous communication systems like Signal’s sealed sender. In this work, we provide a new construction for asymmetric message franking called Hecate that is faster, more secure, and introduces additional functionality compared to Tyagi et al. First, our construction uses fewer invocations of standardized crypto primitives and operates in the plain model. Second, on top of AMF’s accountability and deniability requirements, we also add forward and backward secrecy. Third, we combine AMF with source tracing, another approach to content moderation that has previously been considered only in the setting of non-anonymous networks. Source tracing allows for messages to be forwarded, and a report only identifies the original source who created a message. To provide anonymity for senders and forwarders, we introduce a model of "AMF with preprocessing" whereby every client authenticates with the moderator out-of-band to receive a token that they later consume when sending a message anonymously.CNS-1718135 - National Science Foundation; CNS-1801564 - National Science Foundation; OAC-1739000 - National Science Foundation; CNS-1931714 - National Science Foundation; CNS-1915763 - National Science Foundation; HR00112020021 - Department of Defense/DARPA; 000000000000000000000000000000000000000000000000000000037211 - SRI Internationalhttps://www.usenix.org/system/files/sec22-issa.pdfPublished versio

Structure-Preserving Signatures on Equivalence Classes From Standard Assumptions

Structure-preserving signatures on equivalence classes (SPS-EQ) introduced at ASIACRYPT 2014 are a variant of SPS where a message is considered as a projective equivalence class, and a new representative of the same class can be obtained by multiplying a vector by a scalar. Given a message and corresponding signature, anyone can produce an updated and randomized signature on an arbitrary representative from the same equivalence class. SPS-EQ have proven to be a very versatile building block for many cryptographic applications. In this paper, we present the first EUF-CMA secure SPS-EQ scheme under standard assumptions. So far only constructions in the generic group model are known. One recent candidate under standard assumptions are the weakly secure equivalence class signatures by Fuchsbauer and Gay (PKC\u2718), a variant of SPS-EQ satisfying only a weaker unforgeability and adaption notion. Fuchsbauer and Gay show that this weaker unforgeability notion is sufficient for many known applications of SPS-EQ. Unfortunately, the weaker adaption notion is only proper for a semi-honest (passive) model and as we show in this paper, makes their scheme unusable in the current models for almost all of their advertised applications of SPS-EQ from the literature. We then present a new EUF-CMA secure SPS-EQ scheme with a tight security reduction under the SXDH assumption providing the notion of perfect adaption (under malicious keys). To achieve the strongest notion of perfect adaption under malicious keys, we require a common reference string (CRS), which seems inherent for constructions under standard assumptions. However, for most known applications of SPS-EQ we do not require a trusted CRS (as the CRS can be generated by the signer during key generation). Technically, our construction is inspired by a recent work of Gay et al. (EUROCRYPT\u2718), who construct a tightly secure message authentication code and translate it to an SPS scheme adapting techniques due to Bellare and Goldwasser (CRYPTO\u2789)

Lattice-Based Blind Signatures, Revisited

We observe that all previously known lattice-based blind signature schemes contain subtle flaws in their security proofs (e.g., Rückert, ASIACRYPT \u2708) or can be attacked (e.g., BLAZE by Alkadri et al., FC \u2720). Motivated by this, we revisit the problem of constructing blind signatures from standard lattice assumptions. We propose a new three-round lattice-based blind signature scheme whose security can be proved, in the random oracle model, from the standard SIS assumption. Our starting point is a modified version of the (insecure) BLAZE scheme, which itself is based Lyubashevsky\u27s three-round identification scheme combined with a new aborting technique to reduce the correctness error. Our proof builds upon and extends the recent modular framework for blind signatures of Hauck, Kiltz, and Loss (EUROCRYPT \u2719). It also introduces several new techniques to overcome the additional challenges posed by the correctness error which is inherent to all lattice-based constructions. While our construction is mostly of theoretical interest, we believe it to be an important stepping stone for future works in this area

A SECURE MECHANISM FOR PROVIDING A HACK FREE MESH IN OPEN NETS

The thought of deniability arises from undeniable fact that coercers cannot show the forecasted evidence is wrong and so haven't any motive to refuse the needed evidence. This process tries to obstruct coercion efforts as coercers observe that their attempts are ineffective. We utilize this idea to make sure that providers of cloud storage can offer audit-free storage services. A lot of the techniques of deniable file encryption consist of the problems of understanding error including techniques of designed understanding. Inside our work we provide a powerful file encryption plan of cloud storage that allows the providers of cloud storage to produce convincing false user strategies for defend user privacy. We employ top features of attribute basis file encryption for obtaining of knowledge that's stored in the method of proper-grained access control additionally to deniable file encryption to postpone outdoors auditing. Our recommended plan will grant clients to get capable of offer fake secrets that appear genuine to exterior coercers