## CryptoDB

### Moni Naor

#### Affiliation: Weizmann Institute of Science

#### Publications

**Year**

**Venue**

**Title**

2019

JOFC

Hardness-Preserving Reductions via Cuckoo Hashing
Abstract

The focus of this work is hardness-preserving transformations of somewhat limited pseudorandom functions families (PRFs) into ones with more versatile characteristics. Consider the problem of domain extension of pseudorandom functions: given a PRF that takes as input elements of some domain $$\mathcal {U}$$U, we would like to come up with a PRF over a larger domain. Can we do it with little work and without significantly impacting the security of the system? One approach is to first hash the larger domain into the smaller one and then apply the original PRF. Such a reduction, however, is vulnerable to a “birthday attack”: after $$\sqrt{\left| \mathcal {U}\right| }$$U queries to the resulting PRF, a collision (i.e., two distinct inputs having the same hash value) is very likely to occur. As a consequence, the resulting PRF is insecure against an attacker making this number of queries. In this work, we show how to go beyond the aforementioned birthday attack barrier by replacing the above simple hashing approach with a variant of cuckoo hashing, a hashing paradigm that resolves collisions in a table by using two hash functions and two tables, cleverly assigning each element to one of the two tables. We use this approach to obtain: (i) a domain extension method that requires just two calls to the original PRF can withstand as many queries as the original domain size, and has a distinguishing probability that is exponentially small in the amount of non-cryptographic work; and (ii) a security-preserving reduction from non-adaptive to adaptive PRFs.

2018

TCC

The Security of Lazy Users in Out-of-Band Authentication
Abstract

Faced with the threats posed by man-in-the-middle attacks, messaging platforms rely on “out-of-band” authentication, assuming that users have access to an external channel for authenticating one short value. For example, assuming that users recognizing each other’s voice can authenticate a short value, Telegram and WhatApp ask their users to compare 288-bit and 200-bit values, respectively. The existing protocols, however, do not take into account the plausible behavior of users who may be “lazy” and only compare parts of these values (rather than their entirety).Motivated by such a security-critical user behavior, we study the security of lazy users in out-of-band authentication. We start by showing that both the protocol implemented by WhatsApp and the statistically-optimal protocol of Naor, Segev and Smith (CRYPTO ’06) are completely vulnerable to man-in-the-middle attacks when the users consider only a half of the out-of-band authenticated value. In this light, we put forward a framework that captures the behavior and security of lazy users. Our notions of security consider both statistical security and computational security, and for each flavor we derive a lower bound on the tradeoff between the number of positions that are considered by the lazy users and the adversary’s forgery probability.Within our framework we then provide two authentication protocols. First, in the statistical setting, we present a transformation that converts any out-of-band authentication protocol into one that is secure even when executed by lazy users. Instantiating our transformation with a new refinement of the protocol of Naor et al. results in a protocol whose tradeoff essentially matches our lower bound in the statistical setting. Then, in the computational setting, we show that the computationally-optimal protocol of Vaudenay (CRYPTO ’05) is secure even when executed by lazy users – and its tradeoff matches our lower bound in the computational setting.

2016

CRYPTO

2016

CRYPTO

2008

EPRINT

History-Independent Cuckoo Hashing
Abstract

Cuckoo hashing is an efficient and practical dynamic dictionary. It provides expected amortized constant update time, worst case constant lookup time, and good memory utilization. Various experiments demonstrated that cuckoo hashing is highly suitable for modern computer architectures and distributed settings, and offers significant improvements compared to other schemes.
In this work we construct a practical {\em history-independent} dynamic dictionary based on cuckoo hashing. In a history-independent data structure, the memory representation at any point in time yields no information on the specific sequence of insertions and deletions that led to its current content, other than the content itself. Such a property is significant when preventing unintended leakage of information, and was also found useful in several algorithmic settings.
Our construction enjoys most of the attractive properties of cuckoo hashing. In particular, no dynamic memory allocation is required, updates are performed in expected amortized constant time, and membership queries are performed in worst case constant time. Moreover, with high probability, the lookup procedure queries only two memory entries which are independent and can be queried in parallel. The approach underlying our construction is to enforce a canonical memory representation on cuckoo hashing. That is, up to the initial randomness, each set of elements has a unique memory representation.

2007

EPRINT

Deterministic History-Independent Strategies for Storing Information on Write-Once Memories
Abstract

Motivated by the challenging task of designing ``secure'' vote storage mechanisms, we deal with information storage mechanisms that operate in extremely hostile environments. In such environments, the majority of existing techniques for information storage and for security are susceptible to powerful adversarial attacks. In this setting, we propose a mechanism for storing a set of at most $K$ elements from a large universe of size $N$ on write-once memories in a manner that does not reveal the insertion order of the elements. We consider a standard model for write-once memories, in which the memory is initialized to the all $0$'s state, and the only operation allowed is flipping bits from $0$ to $1$. Whereas previously known constructions were either inefficient (required $\Theta(K^2)$ memory), randomized, or employed cryptographic techniques which are unlikely to be available in hostile environments, we eliminate each of these undesirable properties. The total amount of memory used by the mechanism is linear in the number of stored elements and poly-logarithmic in the size of the universe of elements.
In addition, we consider one of the classical distributed computing problems: conflict resolution in multiple-access channels. By establishing a tight connection with the basic building block of our mechanism, we construct the first deterministic and non-adaptive conflict resolution algorithm whose running time is optimal up to poly-logarithmic factors.

2006

CRYPTO

2006

EPRINT

The Complexity of Online Memory Checking
Abstract

Suppose you want to store a large file on a remote and unreliable server. You would like to verify that your file has not been corrupted, so you store a small private (randomized)``fingerprint'' of the file on your own computer. This is the setting for the well-studied authentication problem, and the size of the required private fingerprint is well understood. We study the problem of sub-linear authentication: suppose you would like to encode and store your file in a way that allows you to verify that it has not been corrupted, but without reading all of it. If you only want to read t bits of the file, how large does the size s of the fingerprint need to be? We define this problem formally, and show a tight lower bound on the relationship between s and t when the adversary is not computationally bounded, namely: s x t= Omega(n) where n is the file size. This is an easier case of the online memory checking problem, introduced by Blum, Evans, Gemmel, Kannan and Naor in 1991, and hence the same (tight) lower bound applies also to this problem.
It was previously shown that when the adversary is not computationally bounded, under the assumption that one-way functions exist, it is possible to construct much better online memory checkers and sub-linear authentication schemes. We show that the existence of one-way functions is also a necessary condition: even slightly breaking the s x t= Omega(n) lower bound in a computational setting implies the existence of one-way functions.

2006

EPRINT

Tight Bounds for Unconditional Authentication Protocols in the Manual Channel and Shared Key Models
Abstract

We address the message authentication problem in two seemingly
different communication models. In the first model, the sender and
receiver are connected by an insecure channel and by a
low-bandwidth auxiliary channel, that enables the sender to
``manually'' authenticate one short message to the receiver (for
example, by typing a short string or comparing two short strings).
We consider this model in a setting where no computational
assumptions are made, and prove that for any $0 < \epsilon < 1$
there exists a $\log^* n$-round protocol for authenticating
$n$-bit messages, in which only $2 \log(1 / \epsilon) + O(1)$ bits
are manually authenticated, and any adversary (even
computationally unbounded) has probability of at most $\epsilon$
to cheat the receiver into accepting a fraudulent message.
Moreover, we develop a proof technique showing that our protocol
is essentially optimal by providing a lower bound of $2 \log(1 /
\epsilon) - O(1)$ on the required length of the manually
authenticated string.
The second model we consider is the traditional message
authentication model. In this model the sender and the receiver
share a short secret key; however, they are connected only by an
insecure channel. We apply the proof technique above to obtain a
lower bound of $2 \log(1 / \epsilon) - 2$ on the required Shannon
entropy of the shared key. This settles an open question posed by
Gemmell and Naor (CRYPTO '93).
Finally, we prove that one-way functions are {\em necessary} (and
sufficient) for the existence of protocols breaking the above
lower bounds in the computational setting.

2005

TCC

2002

EPRINT

On Chosen Ciphertext Security of Multiple Encryptions
Abstract

We consider the security of multiple and possibly related
plaintexts in the context of a chosen ciphertext attack.
That is the attacker in addition and concurrently to obtaining encryptions
of multiple plaintexts under the same key,
may issue encryption and decryption queries and
partial information queries.
Loosely speaking, an encryption scheme is considered
secure under such attacks if all that the adversary can learn from
such attacks about the selected plaintexts can be obtained from the
corresponding partial information queries.
The above definition extends the definition of semantic security
under chosen ciphertext attacks (CCAs),
which is also formulated in this work.
The extension is in considering the security of multiple plaintexts
rather than the security of a single plaintext. We prove that both these
formulations are equivalent to the standard formulation of CCA,
which refers to indistinguishability of encryptions. The good news
is that any encryption scheme that is secure in the standard CCA
sense is in fact secure in the extended model.
The treatment holds both for public-key and private-key
encryption schemes.

2001

EPRINT

Pseudo-Random Functions and Factoring
Abstract

Factoring integers is the most established problem on which
cryptographic primitives are based. This work presents an efficient
construction of {\em pseudorandom functions} whose security is based
on the intractability of factoring. In particular, we are able to
construct efficient length-preserving pseudorandom functions where
each evaluation requires only a {\em constant} number of modular
multiplications per output bit. This is substantially more efficient
than any previous construction of pseudorandom functions based on
factoring, and matches (up to a constant factor) the efficiency of
the best known factoring-based {\em pseudorandom bit generators}.

2001

EPRINT

Anti-persistence: History Independent Data Structures
Abstract

Many data structures give away much more information than they
were intended to. Whenever privacy is important, we need to be
concerned that it might be possible to infer information from the
memory representation of a data structure that is not available
through its ``legitimate'' interface. Word processors that
quietly maintain old versions of a document are merely the most
egregious example of a general problem.
We deal with data structures whose current memory representation does
not reveal their history. We focus on dictionaries, where this means
revealing nothing
about the order of insertions or deletions. Our first algorithm is
a hash table based on open addressing,
allowing $O(1)$ insertion and search. We also present
a history independent dynamic perfect hash table that uses space
linear in the number of elements inserted and has expected amortized
insertion
and deletion time $O(1)$. To solve the dynamic perfect hashing
problem we devise a general scheme for history independent
memory allocation.
For fixed-size records this is quite efficient, with insertion and
deletion both linear in the size of the record. Our variable-size
record
scheme is efficient enough for dynamic perfect hashing but not for
general use. The main open problem we leave is whether it is possible
to implement a variable-size record scheme with low overhead.

2001

EPRINT

Revocation and Tracing Schemes for Stateless Receivers
Abstract

We deal with the problem of a center sending a message to a group
of users such that some subset of the users is considered revoked
and should not be able to obtain the content of the message. We
concentrate on the stateless receiver case, where the users
do not (necessarily) update their state from session to session.
We present a framework called the Subset-Cover framework,
which abstracts a variety of revocation schemes including some
previously known ones. We provide sufficient conditions that
guarantee the security of a revocation algorithm in this class.
We describe two explicit Subset-Cover revocation algorithms; these
algorithms are very flexible and work for any number of revoked
users. The schemes require storage at the receiver of $\log N$ and
$\frac{1}{2} \log^2 N$ keys respectively ($N$ is the total number
of users), and in order to revoke $r$ users the required message
lengths are of $r \log N$ and $2r$ keys respectively. We also
provide a general traitor tracing mechanism that can be
integrated with any Subset-Cover revocation scheme that satisfies
a ``bifurcation property''. This mechanism does not need an a
priori bound on the number of traitors and does not expand the
message length by much compared to the revocation of the same set
of traitors.
The main improvements of these methods over previously suggested
methods, when adapted to the stateless scenario, are: (1) reducing
the message length to $O(r)$ regardless of the coalition
size while maintaining a single decryption at the user's end (2)
provide a seamless integration between the revocation and
tracing so that the tracing mechanisms does not require any change
to the revocation algorithm.

2001

EPRINT

Communication Complexity and Secure Function Evaluation
Abstract

A secure function evaluation protocol allows two parties to jointly compute a function $f(x,y)$ of their inputs in a manner not leaking more information than necessary. A major result in this field is: ``any function $f$ that can be computed using polynomial resources can be computed securely using polynomial resources'' (where `resources' refers to communication and computation). This result follows by a general transformation from any circuit for $f$ to a secure protocol that evaluates $f$.
Although the resources used by protocols resulting from this transformation are polynomial in the circuit size, they are much higher (in general) than those required for an insecure computation of $f$.
For the design of efficient secure protocols we suggest two new methodologies, that differ with respect to their underlying computational models. In one methodology we utilize the communication complexity tree (or branching program) representation of $f$. We start with an efficient (insecure) protocol for $f$ and transform it into
a secure protocol. In other words, ``any function $f$ that can be computed using communication complexity $c$ can be can be computed securely using communication complexity that is polynomial in $c$ and a security parameter''. The second methodology uses the circuit computing $f$, enhanced with look-up tables as its underlying computational model.
It is possible to simulate any RAM machine in this model with polylogarithmic blowup. Hence it is possible to start with a computation of $f$ on a RAM machine and transform it into a secure protocol.
We show many applications of these new methodologies resulting in protocols efficient either in communication or in computation. In particular, we exemplify a protocol for the ``millionaires problem'', where two participants want to compare their values but reveal no other information. Our protocol is more efficient than previously known ones in either communication or computation.

2000

EPRINT

Constructing Pseudo-Random Permutations with a Prescribed Structure
Abstract

We show how to construct pseudo-random permutations that satisfy a
certain cycle restriction, for example that the permutation be
cyclic (consisting of one cycle containing all the elements) or an
involution (a self-inverse permutation) with no fixed points. The
construction can be based on any (unrestricted) pseudo-random
permutation. The resulting permutations
are defined succinctly and their
evaluation at a given point is efficient. Furthermore, they enjoy
a {\em fast forward} property, i.e. it is possible to iterate
them at a very small cost.

1999

EPRINT

Concurrent Zero-Knowledge
Abstract

One of the toughest challenges in designing
cryptographic protocols is to design them so that they will remain
secure even when composed. For example, concurrent executions of a
zero-knowledge protocol by a single prover (with one or more
verifiers) may leak information and may not be zero-knowledge in
toto. In this work we:
(1) Suggest time as a mechanism to design concurrent cryptographic
protocols and in particular maintaining zero-knowledge under
concurrent execution.
(2) Introduce the notion of of Deniable Authentication
and connect it to the problem of concurrent zero-knowledge.
We do not assume global synchronization, however we assume an
(alpha,beta) timing constraint: for any two processors $P_1$
and $P_2$, if $P_1$ measures alpha elapsed time on its local
clock and $P_2$ measures beta elapsed time on its local clock, and
$P_2$ starts after $P_1$ does, then $P_2$ will finish after
$P_1$ does. We show that for an adversary controlling all the
processors clocks (as well as their communication channels) but
which is constrained by an (alpha,beta) constraint
there exist four-round almost concurrent zero-knowledge interactive proofs
and perfect concurrent zero-knowledge arguments for every language in NP.
We also address the more specific problem of Deniable Authentication,
for which we propose several particularly efficient solutions.
Deniable Authentication is of independent interest, even in the
sequential case; our concurrent solutions yield sequential
solutions, without recourse to timing, i.e., in the standard model.

1998

EPRINT

Private Information Retrieval by Keywords
Abstract

Private information retrieval (PIR) schemes enable a user to
access one or more servers that hold copies of
a database and {\em privately} retrieve parts of the $n$
bits of data stored in the database. This means that the queries
give each individual
database no partial information (in the information theoretic or computational
sense) on the identity of the item retrieved by the user.
All known PIR schemes assume that the user knows the {\em physical address}
of the sought item. This is usually not the case when accessing a public
database that is not managed by the user. Such databases are typically
presented with keywords, which are then internally translated (at the
database end) to physical addresses, using an appropriate search
structure (for example, a hash table or a binary tree). In this note we
describe a simple, modular way to privately access data by keywords.
It combines {\em any} conventional search structure with {\em any}
underlying PIR scheme (including single server schemes). The transformation
requires no modification in the way that the search structure is maintained.
Therefore the same database will support both private and regular (non
private) searches.

1997

EPRINT

Visual Authentication and Identification
Abstract

The problems of authentication and identification have
received wide interest in cryptographic research.
However, there has been no satisfactory solution for the problem of
authentication by a human recipient
who does not use any trusted computational device.
The problem of authentication arises for example in
the context of smartcard--human interaction, in particular in
the context of electronic wallets. The problem of identification is ubiquitous
in communication over insecure networks.
This paper introduces visual authentication and visual
identification methods, which are
authentication and identification methods for human users
based on visual cryptography. These methods are
very natural and easy to use, and can be implemented using very
common ``low tech'' technology. The methods we suggest are efficient
in the sense that a single transparency can be used
for several authentications or for several identifications. The visual
authentication methods we suggest are not limited to authenticating
textual messages, and can be used to authenticate any image.
An important contribution of this paper is the introduction of a
framework for proving the security of protocols in which humans take an
active part. We rigorously prove the security of our schemes using this
framework.

1996

EPRINT

Deniable Encryption
Abstract

Consider a situation in which the transmission of encrypted
messages is intercepted by an adversary who can later
ask the sender to reveal
the random choices (and also the secret key, if one exists)
used in generating
the ciphertext, thereby exposing the cleartext.
An encryption scheme is <B>deniable</B> if the sender can generate
`fake random choices' that will make the ciphertext `look like'
an encryption of a different cleartext, thus keeping the
real cleartext private.
Analogous requirements can be formulated with respect to
attacking the receiver and with respect to attacking both parties.
In this paper we introduce deniable encryption and propose
constructions of schemes with polynomial deniability. In addition to
being interesting by itself, and having several applications, deniable
encryption provides a simplified and elegant construction of
<B>adaptively secure</B> multiparty computation.

1996

EPRINT

Visual Cryptography II: Improving the Contrast Via the Cover Base
Abstract

In Eurocrypt'94 we proposed a a new type of cryptographic scheme,
which can decode concealed images without any cryptographic computations,
by placing two transparencies on top of each other and using the decoder's
(human) visual systems.
One of the drawback of that proposal was a loss in contrast: a black pixel
is translated in the reconstruction into a black region, but a white
pixel is translated into a grey region (half black and half white).
In this paper we propose am alternative model for reconstruction with a
different set of operation (which we call the ``Cover" semi-group) is proposed.
In this model we are able to obtain a better contrast than
is possible in the previous one.
We prove tight bounds on the contrast
as a function of the complexity of the scheme. We also show
that for constructing k-out-n secret sharing schemes when
n and k are greater than 2 the new method is not applicable.

1996

EPRINT

On the Construction of Pseudo-Random Permutations: Luby-Rackoff Revisited
Abstract

Luby and Rackoff showed a method for constructing a pseudo-random permutation
from a pseudo-random function. The method is based on composing four
(or three for weakened security) so called Feistel permutations
each of which requires the evaluation of a pseudo-random function.
We reduce somewhat the complexity of the construction
and simplify its proof of security by showing
that two Feistel permutations are sufficient together with initial
and final pair-wise independent permutations.
The revised construction and proof provide a framework in which
similar constructions may be brought up and their security can
be easily proved.
We demonstrate this by presenting some additional adjustments
of the construction that achieve the following:
1. Reduce the success probability of the adversary.
2. Provide a construction of pseudo-random permutations with large input
size using pseudo-random functions with small input size.
3. Provide a construction of a pseudo-random permutation
using a single pseudo-random function.

1992

CRYPTO

#### Program Committees

- TCC 2015
- TCC 2008
- Eurocrypt 2007 (Program chair)
- Crypto 2006
- Crypto 2005
- TCC 2004 (Program chair)
- Eurocrypt 2003
- PKC 2003
- Asiacrypt 2000
- Eurocrypt 2000
- Crypto 1997
- Crypto 1995
- Crypto 1991

#### Coauthors

- Joël Alwen (1)
- Prabhanjan Ananth (1)
- Mihir Bellare (1)
- Itay Berman (2)
- Matt Blaze (1)
- Dan Boneh (1)
- Elette Boyle (1)
- Zvika Brakerski (1)
- Ran Canetti (2)
- David Chaum (1)
- Benny Chor (2)
- Yevgeniy Dodis (1)
- Cynthia Dwork (14)
- Uriel Feige (1)
- Joan Feigenbaum (1)
- Amos Fiat (3)
- Ben Fisch (2)
- Ben A. Fisch (1)
- Daniel Freund (2)
- Daniel Freund (1)
- Peter Gemmell (1)
- Niv Gilboa (1)
- Sharon Goldberg (1)
- Andrew Goldberg (1)
- Oded Goldreich (1)
- Iftach Haitner (2)
- Danny Harnik (2)
- Russell Impagliazzo (1)
- Aayush Jain (1)
- Krishnaram Kenthapadi (1)
- Joe Kilian (2)
- Gillat Kol (1)
- Ilan Komargodski (8)
- Jeffery Lotspiech (2)
- Yoad Lustig (1)
- Frank McSherry (1)
- Ilya Mironov (1)
- Tal Moran (6)
- Dalit Naor (2)
- Kobbi Nissim (1)
- Asaf Nussboim (1)
- Rafail Ostrovsky (4)
- Dimitrios Papadopoulos (1)
- Rafael Pass (1)
- Benny Pinkas (8)
- Omer Reingold (11)
- Leonid Reyzin (1)
- Thomas Ristenpart (1)
- Alon Rosen (4)
- Lior Rotem (1)
- Guy N. Rothblum (4)
- Shmuel Safra (1)
- Amit Sahai (2)
- Gil Segev (10)
- Hovav Shacham (1)
- Adi Shamir (2)
- Adam Smith (2)
- Vanessa Teague (1)
- Eran Tromer (1)
- Vinod Vaikuntanathan (1)
- Sachin Vasant (1)
- Ramarathnam Venkatesan (2)
- Shabsi Walfish (1)
- Hoeteck Wee (1)
- Daniel Wichs (1)
- Udi Wieder (1)
- Scott Yilek (1)
- Eylon Yogev (9)
- Moti Yung (2)
- Asaf Ziv (2)