TumbleBit: an untrusted Bitcoin-compatible anonymous payment hub

This paper presents TumbleBit, a new unidirectional unlinkable payment hub that is fully compatible with today s Bitcoin protocol. TumbleBit allows parties to make fast, anonymous, off-blockchain payments through an untrusted intermediary called the Tumbler. TumbleBits anonymity properties are similar to classic Chaumian eCash: no one, not even the Tumbler, can link a payment from its payer to its payee. Every payment made via TumbleBit is backed by bitcoins, and comes with a guarantee that Tumbler can neither violate anonymity, nor steal bitcoins, nor print money by issuing payments to itself. We prove the security of TumbleBit using the real/ideal world paradigm and the random oracle model. Security follows from the standard RSA assumption and ECDSA unforgeability. We implement TumbleBit, mix payments from 800 users and show that TumbleBits offblockchain payments can complete in seconds.https://eprint.iacr.org/2016/575.pdfPublished versio

Secure computation under network and physical attacks

2011 - 2012This thesis proposes several protocols for achieving secure com- putation under concurrent and physical attacks. Secure computation allows many parties to compute a joint function of their inputs, while keeping the privacy of their input preserved. It is required that the pri- vacy one party's input is preserved even if other parties participating in the protocol collude or deviate from the protocol. In this thesis we focus on concurrent and physical attacks, where adversarial parties try to break the privacy of honest parties by ex- ploiting the network connection or physical weaknesses of the honest parties' machine. In the rst part of the thesis we discuss how to construct proto- cols that are Universally Composable (UC for short) based on physical setup assumptions. We explore the use of Physically Uncloneable Func- tions (PUFs) as setup assumption for achieving UC-secure computa- tions. PUF are physical noisy source of randomness. The use of PUFs in the UC-framework has been proposed already in [14]. However, this work assumes that all PUFs in the system are trusted. This means that, each party has to trust the PUFs generated by the other parties. In this thesis we focus on reducing the trust involved in the use of such PUFs and we introduce the Malicious PUFs model in which only PUFs generated by honest parties are assumed to be trusted. Thus the secu- rity of each party relies on its own PUF only and holds regardless of the goodness of the PUFs generated/used by the adversary. We are able to show that, under this more realistic assumption, one can achieve UC- secure computation, under computational assumptions. Moreover, we show how to achieve unconditional UC-secure commitments with (ma- licious) PUFs and with stateless tamper-proof hardware tokens. We discuss our contribution on this matter in Part I. These results are contained in papers [80] and [28]. In the second part of the thesis we focus on the concurrent setting, and we investigate on protocols achieving round optimality and black- box access to a cryptographic primitive. We study two fundamental functionalities: commitment scheme and zero knowledge, and we focus on some of the round-optimal constructions and lower bounds con- cerning both functionalities. We nd that such constructions present subtle issues. Hence, we provide new protocols that actually achieve the security guarantee promised by previous results. Concerning physical attacks, we consider adversaries able to re- set the machine of the honest party. In a reset attack a machine is forced to run a protocol several times using the same randomness. In this thesis we provide the rst construction of a witness indistinguish- able argument system that is simultaneous resettable and argument of knowledge. We discuss about this contribution in Part III, which is the content of the paper. [edited by author]XI n.s

PriFHEte: Achieving Full-Privacy in Account-based Cryptocurrencies is Possible

In cryptocurrencies, all transactions are public. For their adoption, it is important that these transactions, while publicly verifiable, do not leak information about the identity and the balances of the transactors. For UTXO-based cryptocurrencies, there are well-established approaches (e.g., ZCash) that guarantee full privacy to the transactors. Full privacy in UTXO means that each transaction is anonymous within the set of all private transactions ever posted on the blockchain. In contrast, for account-based cryptocurrencies (e.g., Ethereum) full privacy, that is, privacy within the set of all accounts, seems to be impossible to achieve within the constraints of blockchain transactions (e.g., they have to fit in a block). Indeed, every approach proposed in the literature achieves only a much weaker privacy guarantee called $k-$anonymity where a transactor is private within a set of $k$ account holders. $k-$anonymity is achieved by adding $k$ accounts to the transaction, which concretely limits the anonymity guarantee to a very small constant (e.g., $~$64 for QuisQuis and $~$256 for anonymous Zether), compared to the set of all possible accounts. In this paper, we propose a completely new approach that does not achieve anonymity by including more accounts in the transaction, but instead makes the transaction itself ``smarter\u27\u27. Our key contribution is to provide a mechanism whereby a compact transaction can be used to correctly update all accounts. Intuitively, this guarantees that all accounts are equally likely to be the recipients/sender of such a transaction. We, therefore, provide the first protocol that guarantees full privacy in account-based cryptocurrencies PriFHEte The contribution of this paper is theoretical. Our main objective is to demonstrate that achieving full privacy in account-based cryptocurrency is actually possible. We see our work as opening the door to new possibilities for anonymous account-based cryptocurrencies. Nonetheless, in this paper, we also discuss PriFHEte\u27s potential to be developed in practice by leveraging the power of off-chain scalability solutions such as zk rollups

Threshold Ring Signatures: New Definitions and Post-Quantum Security

A $t$-out-of-$N$ threshold ring signature allows $t$ parties to jointly and anonymously compute a signature on behalf on $N$ public keys, selected in an arbitrary manner among the set of all public keys registered in the system. Existing definitions for $t$-out-of-$N$ threshold ring signatures guarantee security only when the public keys are honestly generated, and many even restrict the ability of the adversary to actively participate in the computation of the signatures. Such definitions do not capture the open settings envisioned for threshold ring signatures, where parties can independently add themselves to the system, and join other parties for the computation of the signature. Furthermore, known constructions of threshold ring signatures are not provably secure in the post-quantum setting, either because they are based on non-post quantum secure problems (e.g. Discrete Log, RSA), or because they rely on transformations such as Fiat-Shamir, that are not always secure in the quantum random oracle model (QROM). In this paper, we provide the first definition of $t$-out-of-$N$ threshold ring signatures against {\em active} adversaries who can participate in the system and arbitrarily deviate from the prescribed procedures. Second, we present a post-quantum secure realization based on {\em any} (post-quantum secure) trapdoor commitment, which we prove secure in the QROM. Our construction is black-box and it can be instantiated with any trapdoor commitment, thus allowing the use of a variety of hardness assumptions

One-time Traceable Ring Signatures

A ring signature allows a party to sign messages anonymously on behalf of a group, which is called ring. Traceable ring signatures are a variant of ring signatures that limits the anonymity guarantees, enforcing that a member can sign anonymously at most one message per tag. Namely, if a party signs two different messages for the same tag, it will be de-anomymized. This property is very useful in decentralized platforms to allow members to anonymously endorse statements in a controlled manner. In this work we introduce one-time traceable ring signatures, where a member can sign anonymously only one message. This natural variant suffices in many applications for which traceable ring signatures are useful, and enables us to design a scheme that only requires a few hash evaluations and outperforms existing (non one-time) schemes. Our one-time traceable ring signature scheme presents many advantages: it is fast, with a signing time of less than 1 second for a ring of $2^{10}$ signers (and much less for smaller rings); it is {\em post-quantum resistant}, as it only requires hash evaluations; it is extremely simple, as it requires only a black-box access to a generic hash function (modeled as a random oracle) and no other cryptographic operation is involved. From a theoretical standpoint our scheme is also the first anonymous signature scheme based on a black-box access to a symmetric-key primitive. All existing anonymous signatures are either based on specific hardness assumptions (e.g., LWE, SIS, etc.) or use the underlying symmetric-key primitive in a non-black-box way, i.e., they leverage the circuit representation of the primitive

Unconditionally Secure and Universally Composable Commitments from Physical Assumptions

We present a constant-round unconditional black-box compiler that transforms any ideal (i.e., statistically-hiding and statistically-binding) straight-line extractable commitment scheme, into an extractable and equivocal commitment scheme, therefore yielding to UC-security [9]. We exemplify the usefulness of our compiler by providing two (constant-round) instantiations of ideal straight-line extractable commitment based on (malicious) PUFs [36] and stateless tamper-proof hardware tokens [26], therefore achieving the first unconditionally UC-secure commitment with malicious PUFs and stateless tokens, respectively. Our constructions are secure for adversaries creating arbitrarily malicious stateful PUFs/tokens. Previous results with malicious PUFs used either computational assumptions to achieve UC- secure commitments or were unconditionally secure but only in the indistinguishability sense [36]. Similarly, with stateless tokens, UC-secure commitments are known only under computational assumptions [13, 24, 15], while the (not UC) unconditional commitment scheme of [23] is secure only in a weaker model in which the adversary is not allowed to create stateful tokens. Besides allowing us to prove feasibility of unconditional UC-security with (malicious) PUFs and stateless tokens, our compiler can be instantiated with any ideal straight-line extractable commitment scheme, thus allowing the use of various setup assumptions which may better fit the application or the technology available

Improved OR-Composition of Sigma-Protocols

In [CDS94] Cramer, Damg̊ard and Schoenmakers (CDS) devise an OR-composition technique for Σ-protocols that allows to construct highly-efficient proofs for compound statements. Since then, such technique has found countless applications as building block for designing efficient protocols. Unfortunately, the CDS OR-composition technique works only if both statements are fixed before the proof starts. This limitation restricts its usability in those protocols where the theorems to be proved are defined at different stages of the protocol, but, in order to save rounds of communication, the proof must start even if not all theorems are available. Many round-optimal protocols ([KO04, DPV04, YZ07, SV12]) crucially need such property to achieve round-optimality, and, due to the inapplicability of CDS’s technique, are currently implemented using proof systems that requires expensive NP reductions, but that allow the proof to start even if no statement is defined (a.k.a., LS proofs from Lapidot-Shamir [LS90]). In this paper we show an improved OR-composition technique for Σ-protocols, that requires only one statement to be fixed when the proof starts, while the other statement can be define

Sublinear Zero-Knowledge Arguments for RAM Programs

We describe a new succinct zero-knowledge argument protocol with the following properties. The prover commits to a large data-set $M$, and can thereafter prove many statements of the form $\exists w : \mathcal{R}_i(M,w)=1$, where $\mathcal{R}_i$ is a public function. The protocol is {\em succinct} in the sense that the cost for the verifier (in computation \& communication) does not depend on $|M|$, not even in any initialization phase. In each proof, the computation/communication cost for {\em both} the prover and the verifier is proportional only to the running time of an oblivious RAM program implementing $\mathcal{R}_i$ (in particular, this can be sublinear in $|M|$). The only costs that scale with $|M|$ are the computational costs of the prover in a one-time initial commitment to $M$. Known sublinear zero-knowledge proofs either require an initialization phase where the work of the verifier is proportional to $|M|$ and are therefore sublinear only in an amortized sense, or require that the computational cost for the prover is proportional to $|M|$ upon {\em each proof}. Our protocol uses efficient crypto primitives in a black-box way and is UC-secure in the {\em global}, non-programmable random oracle, hence it does not rely on any trusted setup assumption

How to Recover a Cryptographic Secret From the Cloud

Clouds have replaced local backup systems due to their stronger reliability and availability guarantees compared to local machines, which are prone to hardware/software failure or can be stolen or lost, especially in the case of portable devices In recent years, some digital assets are managed solely through the knowledge of cryptographic secrets (e.g., cryptocurrency, encrypted datasets), whose loss results in the permanent loss of the digital asset. Since the security of such systems relies on the assumption that the cryptographic key remains secret, a secret owner Alice cannot simply store a backup copy of such secret on the cloud, since this corresponds to giving away her ownership over the digital assets. Thus Alice must rely on her personal machines to maintain these secrets. Is it possible to obtain the best of the two worlds, where Alice benefits from the convenience of storing a backup copy of her cryptographic secrets on the cloud such that she can recover them even when she loses her devices and forgets all credentials, while at the same time retaining full ownership of her secrets? In this paper, we show that this is indeed possible, by revisiting and expanding the concept of Break-glass Encryption pioneered by Scafuro [PKC19]. We provide a secret-recovery mechanism where confidentiality is always guaranteed when Alice has not lost her credentials, even in the presence of a malicious cloud and users ([PKC19] only guarantees that a violation of confidentiality will be {\em detected}, not prevented). Recoverability is achieved in most circumstances. We design and prove security of a credential-less authentication mechanism, that enables Alice to access her secret, without remembering any credentials. This tool was assumed in [PKC19] but not implemented. We redesign the storage mechanism on the cloud side so that the cloud needs to perform no operations during the storage phase. This is in contrast with [PKC19] where the cloud must re-encrypt the stored file continuously with the help of a secure enclave (regardless of whether a recovery procedure will happen). Our protocols are proved secure in the Universal Composition framework