International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Shafi Goldwasser

Affiliation: MIT

Publications

Year
Venue
Title
2017
TCC
2017
JOFC
2016
TCC
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
TCC
2015
TCC
2014
CRYPTO
2014
EUROCRYPT
2014
PKC
2014
EPRINT
2014
EPRINT
2014
EPRINT
2014
JOFC
2013
TCC
2013
CRYPTO
2012
TCC
2011
TCC
2011
ASIACRYPT
2010
TCC
2010
CRYPTO
2010
CRYPTO
2010
EPRINT
Circular and Leakage Resilient Public-Key Encryption Under Subgroup Indistinguishability (or: Quadratic Residuosity Strikes Back)
Zvika Brakerski Shafi Goldwasser
The main results of this work are new public-key encryption schemes that, under the quadratic residuosity (QR) assumption (or Paillier's decisional composite residuosity (DCR) assumption), achieve key-dependent message security as well as high resilience to secret key leakage and high resilience to the presence of auxiliary input information. In particular, under what we call the {\it subgroup indistinguishability assumption}, of which the QR and DCR are special cases, we can construct a scheme that has: * Key-dependant message (circular) security. Achieves security even when encrypting affine functions of its own secret-key (in fact, w.r.t. affine ``key-cycles'' of predefined length). Our scheme also meets the requirements for extending key-dependant message security to broader classes of functions beyond affine functions using the techniques of [BGK, ePrint09] or [BHHI, ePrint09]. * Leakage resiliency. Remains secure even if any adversarial low-entropy (efficiently computable) function of the secret-key is given to the adversary. A proper selection of parameters allows for a ``leakage rate'' of $(1-o(1))$ of the length of the secret-key. * Auxiliary-input security. Remains secure even if any sufficiently \emph{hard to invert} (efficiently computable) function of the secret-key is given to the adversary. Our scheme is the first to achieve key-dependant security and auxiliary-input security based on the DCR and QR assumptions. Previous schemes that achieved these properties relied either on the DDH or LWE assumptions. The proposed scheme is also the first to achieve leakage resiliency for leakage rate $(1-o(1))$ of the secret-key length, under the QR assumption. We note that leakage resilient schemes under the DCR and the QR assumptions, for the restricted case of composite modulus product of safe primes, were implied by the work of [NS, Crypto09], using hash proof systems. However, under the QR assumption, known constructions of hash proof systems only yield a leakage rate of $o(1)$ of the secret-key length.
2009
TCC
2009
TCC
2009
EUROCRYPT
2008
CRYPTO
2008
EPRINT
How to Protect Yourself without Perfect Shredding
Erasing old data and keys is an important tool in cryptographic protocol design. It is useful in many settings, including proactive security, adaptive security, forward security, and intrusion resilience. Protocols for all these settings typically assume the ability to perfectly erase information. Unfortunately, as amply demonstrated in the systems literature, perfect erasures are hard to implement in practice. We propose a model of partial erasures where erasure instructions leave almost all the data erased intact, thus giving the honest players only a limited capability for disposing of old data. Nonetheless, we provide a general compiler that transforms any secure protocol using perfect erasures into one that maintains the same security properties when only partial erasures are available. The key idea is a new redundant representation of secret data which can still be computed on, and yet is rendered useless when partially erased. We prove that any such a compiler must incur a cost in additional storage, and that our compiler is near optimal in terms of its storage overhead.
2007
EUROCRYPT
2007
TCC
2005
TCC
2005
JOFC
2004
TCC
2003
EPRINT
On the (In)security of the Fiat-Shamir Paradigm
Shafi Goldwasser Yael Tauman
In 1986, Fiat and Shamir suggested a general method for transforming secure 3-round public-coin identification schemes into digital signature schemes. The significant contribution of this method is a means for designing efficient digital signatures, while hopefully achieving security against chosen message attacks. All other known constructions which achieve such security are substantially more inefficient and complicated in design. In 1996, Pointcheval and Stern proved that the signature schemes obtained by the Fiat-Shamir transformation are secure in the so called `Random Oracle Model'. The question is: does the proof of the security of the Fiat-Shamir transformation in the Random Oracle Model, imply that the transformation yields secure signature schemes in the ``real-world"? In this paper we answer this question negatively. We show that there exist secure 3-round public-coin identification schemes for which the Fiat-Shamir methodology produces {\bf insecure} digital signature schemes for {\bf any} implementation of the `Random Oracle Model' in the `real-world' by a function ensemble.
2002
EPRINT
Secure Computation Without Agreement
Shafi Goldwasser Yehuda Lindell
It has recently been shown that authenticated Byzantine agreement, in which more than a third of the parties are corrupted, cannot be securely realized under concurrent or parallel (stateless) composition. This result puts into question any usage of authenticated Byzantine agreement in a setting where many executions take place. In particular, this is true for the whole body of work of secure multi-party protocols in the case that a third or more of the parties are corrupted. This is because these protocols strongly rely on the extensive use of a broadcast channel, which is in turn realized using authenticated Byzantine agreement. We remark that it was accepted folklore that the use of a broadcast channel (or authenticated Byzantine agreement) is actually essential for achieving meaningful secure multi-party computation whenever a third or more of the parties are corrupted. In this paper we show that this folklore is false. We present a mild relaxation of the definition of secure computation allowing abort. Our new definition captures all the central security issues of secure computation, including privacy, correctness and independence of inputs. However, the novelty of the definition is in {\em decoupling} the issue of agreement from these issues. We then show that this relaxation suffices for achieving secure computation in a point-to-point network. That is, we show that secure multi-party computation for this definition can be achieved for {\em any} number of corrupted parties and {\em without} a broadcast channel (or trusted preprocessing phase as required for running authenticated Byzantine agreement). Furthermore, this is achieved by just replacing the broadcast channel in known protocols with a very simple and efficient echo-broadcast protocol. An important corollary of our result is the ability to obtain multi-party protocols that remain secure under composition, without assuming a broadcast channel.
2001
EUROCRYPT
2001
EPRINT
Resettably-Sound Zero-Knowledge and its Applications
Resettably-sound proofs and arguments remain sound even when the prover can reset the verifier, and so force it to use the same random coins in repeated executions of the protocol. We show that resettably-sound zero-knowledge {\em arguments} for NP exist if collision-resistant hash functions exist. In contrast, resettably-sound zero-knowledge {\em proofs} are possible only for languages in P/poly. We present two applications of resettably-sound zero-knowledge arguments. First, we construct resettable zero-knowledge arguments of knowledge for NP, using a natural relaxation of the definition of arguments (and proofs) of knowledge. We note that, under the standard definition of proofs of knowledge, it is impossible to obtain resettable zero-knowledge arguments of knowledge for languages outside BPP. Second, we construct a constant-round resettable zero-knowledge argument for NP in the public-key model, under the assumption that collision-resistant hash functions exist. This improves upon the sub-exponential hardness assumption required by previous constructions. We emphasize that our results use non-black-box zero-knowledge simulations. Indeed, we show that some of the results are {\em impossible} to achieve using black-box simulations. In particular, only languages in BPP have resettably-sound arguments that are zero-knowledge with respect to black-box simulation.
2000
EPRINT
Identification Protocols Secure Against Reset Attacks
We provide identification protocols that are secure even when the adversary can reset the internal state and/or randomization source of the user identifying itself, and when executed in an asynchronous environment like the Internet that gives the adversary concurrent access to instances of the user. These protocols are suitable for use by devices (like smartcards) which when under adversary control may not be able to reliably maintain their internal state between invocations.
1999
EUROCRYPT
1999
EPRINT
Resettable Zero-Knowledge
We introduce the notion of Resettable Zero-Knowledge (rZK), a new security measure for cryptographic protocols which strengthens the classical notion of zero-knowledge. In essence, an rZK protocol is one that remains zero knowledge even if an adeversary can interact with the prover many times, each time resetting the prover to its initial state and forcing him to use the same random tape. Under general complexity asumptions, which hold for example if the Discrete Logarithm Problem is hard, we construct (1) rZK proof-systems for NP: (2) constant-round resettable witness-indistinguishable proof-systems for NP; and (3) constant-round rZK arguments for NP in the public key model where verifiers have fixed, public keys associated with them. In addition to shedding new light on what makes zero knowledge possible (by constructing ZK protocols that use randomness in a dramatically weaker way than before), rZK has great relevance to applications. Firstly, we show that rZK protocols are closed under parallel and concurrent execution and thus are guaranteed to be secure when implemented in fully asynchronous networks, even if an adversary schedules the arrival of every message sent. Secondly, rZK protocols enlarge the range of physical ways in which provers of a ZK protocols can be securely implemented, including devices which cannot reliably toss coins on line, nor keep state betweeen invocations. (For instance, because ordinary smart cards with secure hardware are resattable, they could not be used to implement securely the provers of classical ZK protocols, but can now be used to implement securely the provers of rZK protocols.)
1998
EPRINT
On the possibility of basing Cryptography on the assumption that $P \neq NP$
Oded Goldreich Shafi Goldwasser
Recent works by Ajtai and by Ajtai and Dwork bring to light the old (general) question of whether it is at all possible to base the security of cryptosystems on the assumption that $\P\neq\NP$. We discuss this question and in particular review and extend a two-decade old result of Brassard regarding this question. Our conclusion is that the question remains open.
1998
EPRINT
A Random Server Model for Private Information Retrieval (or How to Achieve Information Theoretic PIR Avoiding Data Replication)
Yael Gertner Shafi Goldwasser Tal Malkin
Private information retrieval (PIR) schemes enable users to obtain information from databases while keeping their queries secret from the database managers. We propose a new model for PIR, utilizing auxiliary random servers to provide privacy services for database access. In this model, prior to any on-line communication where users request queries, the database engages in an initial preprocessing setup stage with the random servers. Using this model we achieve the first PIR information theoretic solution in which the database does not need to give away its data to be replicated, and with minimal on-line computation cost for the database. This solves privacy and efficiency problems inherent to all previous solutions. In particular, all previous information theoretic PIR schemes required multiple replications of the database into separate entities which are not allowed to communicate with each other; and in all previous schemes (including ones which do not achieve information theoretic security), the amount of computation performed by the database on-line for every query is at least linear in the size of the database. In contrast, in our solutions the database does not give away its contents to any other entity; and after the initial setup stage, which costs at most O(n log n) in computation, the database needs to perform only O(1) amount of computation to answer questions of users on-line. All the extra on-line computation is done by the auxiliary random servers.
1997
CRYPTO
1997
CRYPTO
1997
CRYPTO
1996
EPRINT
Collision-Free Hashing from Lattice Problems
Oded Goldreich Shafi Goldwasser Shai Halevi
Recently Ajtai described a construction of one-way functions whose security is equivalent to the difficulty of some well known approximation problems in lattices. We show that essentially the same construction can also be used to obtain collision-free hashing.
1996
EPRINT
Verifiable Partial Key Escrow
Mihir Bellare Shafi Goldwasser
One of the main objections to existing proposals for key escrow is that the individual's privacy relies on too high a level of trust in the law enforcement agencies. In particular, even if the government is trustworthy today, it may be replaced by an un-trustworthy government tomorrow which could immediately and suddenly recover the secret keys of all users.
1996
EPRINT
Public-Key Cryptosystems from Lattice Reduction Problems
Oded Goldreich Shafi Goldwasser Shai Halevi
We present a new proposal for a trapdoor one-way function, from which we derive public-key encryption and digital signatures. The security of the new construction is based on the conjectured computational difficulty of lattice-reduction problems, providing a possible alternative to existing public-key encryption algorithms and digital signatures such as RSA and DSS.
1994
CRYPTO
1992
CRYPTO
1990
CRYPTO
1989
CRYPTO
1989
CRYPTO
1989
CRYPTO
1989
CRYPTO
1988
CRYPTO
1985
CRYPTO
1984
CRYPTO
1984
CRYPTO
1984
CRYPTO
1982
CRYPTO

Program Committees

Crypto 2009
Eurocrypt 2005
TCC 2005 (Program chair)
TCC 2004 (Program chair)
Eurocrypt 1997
Asiacrypt 1991
Crypto 1988 (Program chair)
Crypto 1986