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Unconditionally Secure Cryptography: Signature Schemes, User-Private Information Retrieval, and the Generalized Russian Cards Problem

Colleen Marie Swanson

Published May 23, 2013• 1 citation
OpenAlex

Abstract

We focus on three different types of multi-party cryptographic protocols. The first is in the area of unconditionally secure signature schemes, the goal of which is to provide users the ability to electronically sign documents without the reliance on computational assumptions needed in traditional digital signatures. The second is on cooperative protocols in which users help each other maintain privacy while querying a database, called user-private information retrieval protocols. The third is concerned with the generalized Russian cards problem, in which two card players wish to communicate their hands to each other via public announcements without the third player learning the card deal. The latter two problems have close ties to the field of combinatorial designs, and properly fit within the field of combinatorial cryptography. All of these problems have a common thread, in that they are grounded in the information-theoretically secure or unconditionally secure setting.

Related papers

Combinatorial Solutions Providing Improved Security for the Generalized Russian Cards Problem

Colleen Marie Swanson, Douglas R. Stinson

We present the first formal mathematical presentation of the generalized Russian cards problem, and provide rigorous security definitions that capture both basic and extended versions of weak and perfect security notions. In the generalized Russian cards problem, three players, Alice, Bob, and Cathy, are dealt a deck of $n$ cards, each given $a$, $b$, and $c$ cards, respectively. The goal is for Alice and Bob to learn each other's hands via public communication, without Cathy learning the fate of any particular card. The basic idea is that Alice announces a set of possible hands she might hold, and Bob, using knowledge of his own hand, should be able to learn Alice's cards from this announcement, but Cathy should not. Using a combinatorial approach, we are able to give a nice characterization of informative strategies (i.e., strategies allowing Bob to learn Alice's hand), having optimal communication complexity, namely the set of possible hands Alice announces must be equivalent to a large set of $t-(n, a, 1)$-designs, where $t=a-c$. We also provide some interesting necessary conditions for certain types of deals to be simultaneously informative and secure. That is, for deals satisfying $c = a-d$ for some $d \geq 2$, where $b \geq d-1$ and the strategy is assumed to satisfy a strong version of security (namely perfect $(d-1)$-security), we show that $a = d+1$ and hence $c=1$. We also give a precise characterization of informative and perfectly $(d-1)$-secure deals of the form $(d+1, b, 1)$ satisfying $b \geq d-1$ involving $d-(n, d+1, 1)$-designs.

arXiv:1207.1336•Jul 5, 2012•0 citations

Additional Constructions to Solve the Generalized Russian Cards Problem using Combinatorial Designs

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In the generalized Russian cards problem, we have a card deck $X$ of $n$ cards and three participants, Alice, Bob, and Cathy, dealt $a$, $b$, and $c$ cards, respectively. Once the cards are dealt, Alice and Bob wish to privately communicate their hands to each other via public announcements, without the advantage of a shared secret or public key infrastructure. Cathy, for her part, should remain ignorant of all but her own cards after Alice and Bob have made their announcements. Notions for Cathy's ignorance in the literature range from Cathy not learning the fate of any individual card with certainty (weak $1$-security) to not gaining any probabilistic advantage in guessing the fate of some set of $\delta$ cards (perfect $\delta$-security). As we demonstrate in this work, the generalized Russian cards problem has close ties to the field of combinatorial designs, on which we rely heavily, particularly for perfect security notions. Our main result establishes an equivalence between perfectly $\delta$-secure strategies and $(c+\delta)$-designs on $n$ points with block size $a$, when announcements are chosen uniformly at random from the set of possible announcements. We also provide construction methods and example solutions, including a construction that yields perfect $1$-security against Cathy when $c=2$. Drawing on our equivalence results, we are able to use a known combinatorial design to construct a strategy with $a=8$, $b=13$, and $c=3$ that is perfectly $2$-secure. Finally, we consider a variant of the problem that yields solutions that are easy to construct and optimal with respect to both the number of announcements and level of security achieved. Moreover, this is the first method obtaining weak $\delta$-security that allows Alice to hold an arbitrary number of cards and Cathy to hold a set of $c = \lfloor \frac{a-\delta}{2} \rfloor$ cards. Alternatively, the construction yields solutions for arbitrary $\delta$, $c$ and any $a \geq \delta + 2c$.

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A Distributed Computing Perspective of Unconditionally Secure Information Transmission in Russian Cards Problems

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The problem of $A$ privately transmitting information to $B$ by a public announcement overheard by an eavesdropper $C$ is considered. To do so by a deterministic protocol, their inputs must be correlated. Dependent inputs are represented using a deck of cards. There is a publicly known signature $(a,b,c)$, where $n = a + b + c + r$, and $A$ gets $a$ cards, $B$ gets $b$ cards, and $C$ gets $c$ cards, out of the deck of $n$ cards. Using a deterministic protocol, $A$ decides its announcement based on her hand. Using techniques from coding theory, Johnson graphs, and additive number theory, a novel perspective inspired by distributed computing theory is provided, to analyze the amount of information that $A$ needs to send, while preventing $C$ from learning a single card of her hand. In one extreme, the generalized Russian cards problem, $B$ wants to learn all of $A$'s cards, and in the other, $B$ wishes to learn something about $A$'s hand.

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Ueli M. Maurer

Two types of generalizations in secure multi-party computation are described. First, we report on a new general adversary model: while previous protocols were designed to tolerate misbehavior of one of two types (active or passive adversaries) the new protocols tolerate mixed types of adversaries, i.e. an adversary that can corrupt players actively, passively, or can let them crash (i.e. stop execution). We derive the necessary and sufficient conditions under which such a general adversary can be tolerated. Second, we present a generic protocol framework that unifies previous approaches achieving unconditional (or information-theoretic) security and cryptographic security (i.e. security based on a cryptographic intractability assumption). This is achieved by introducing secret sharing schemes with an additional property called the multiplication property and a new type of homomorphic commitment scheme, and by generalizing the concept of commitments.

IEEE Information Theory Workshop (ITW)•Nov 27, 2002•0 citations
We focus on three different types of multi-party cryptographic protocols. The first is in the area of unconditionally secure signature schemes, the goal of which is to provide users the ability to electronically sign documents without the reliance on computational assumptions needed in traditional digital signatures. The second is on cooperative protocols in which users help each other maintain privacy while querying a database, called user-private information retrieval protocols. The third is concerned with the generalized Russian cards problem, in which two card players wish to communicate their hands to each other via public announcements without the third player learning the card deal. The latter two problems have close ties to the field of combinatorial designs, and properly fit within the field of combinatorial cryptography. All of these problems have a common thread, in that they are grounded in the information-theoretically secure or unconditionally secure setting.
We present the first formal mathematical presentation of the generalized Russian cards problem, and provide rigorous security definitions that capture both basic and extended versions of weak and perfect security notions. In the generalized Russian cards problem, three players, Alice, Bob, and Cathy, are dealt a deck of nnn cards, each given aaa, bbb, and ccc cards, respectively. The goal is for Alice and Bob to learn each other's hands via public communication, without Cathy learning the fate of any particular card. The basic idea is that Alice announces a set of possible hands she might hold, and Bob, using knowledge of his own hand, should be able to learn Alice's cards from this announcement, but Cathy should not. Using a combinatorial approach, we are able to give a nice characterization of informative strategies (i.e., strategies allowing Bob to learn Alice's hand), having optimal communication complexity, namely the set of possible hands Alice announces must be equivalent to a large set of t−(n,a,1)t-(n, a, 1)t−(n,a,1)-designs, where t=a−ct=a-ct=a−c. We also provide some interesting necessary conditions for certain types of deals to be simultaneously informative and secure. That is, for deals satisfying c=a−dc = a-dc=a−d for some d≥2d \geq 2d≥2, where b≥d−1b \geq d-1b≥d−1 and the strategy is assumed to satisfy a strong version of security (namely perfect (d−1)(d-1)(d−1)-security), we show that a=d+1a = d+1a=d+1 and hence c=1c=1c=1. We also give a precise characterization of informative and perfectly (d−1)(d-1)(d−1)-secure deals of the form (d+1,b,1)(d+1, b, 1)(d+1,b,1) satisfying b≥d−1b \geq d-1b≥d−1 involving d−(n,d+1,1)d-(n, d+1, 1)d−(n,d+1,1)-designs.
In the generalized Russian cards problem, we have a card deck XXX of nnn cards and three participants, Alice, Bob, and Cathy, dealt aaa, bbb, and ccc cards, respectively. Once the cards are dealt, Alice and Bob wish to privately communicate their hands to each other via public announcements, without the advantage of a shared secret or public key infrastructure. Cathy, for her part, should remain ignorant of all but her own cards after Alice and Bob have made their announcements. Notions for Cathy's ignorance in the literature range from Cathy not learning the fate of any individual card with certainty (weak 111-security) to not gaining any probabilistic advantage in guessing the fate of some set of δ\deltaδ cards (perfect δ\deltaδ-security). As we demonstrate in this work, the generalized Russian cards problem has close ties to the field of combinatorial designs, on which we rely heavily, particularly for perfect security notions. Our main result establishes an equivalence between perfectly δ\deltaδ-secure strategies and (c+δ)(c+\delta)(c+δ)-designs on nnn points with block size aaa, when announcements are chosen uniformly at random from the set of possible announcements. We also provide construction methods and example solutions, including a construction that yields perfect 111-security against Cathy when c=2c=2c=2. Drawing on our equivalence results, we are able to use a known combinatorial design to construct a strategy with a=8a=8a=8, b=13b=13b=13, and c=3c=3c=3 that is perfectly 222-secure. Finally, we consider a variant of the problem that yields solutions that are easy to construct and optimal with respect to both the number of announcements and level of security achieved. Moreover, this is the first method obtaining weak δ\deltaδ-security that allows Alice to hold an arbitrary number of cards and Cathy to hold a set of c=⌊a−δ2⌋c = \lfloor \frac{a-\delta}{2} \rfloorc=⌊2a−δ​⌋ cards. Alternatively, the construction yields solutions for arbitrary δ\deltaδ, ccc and any a≥δ+2ca \geq \delta + 2ca≥δ+2c.
Security and privacy are of paramount importance in the modern information age. Secure multi-party computation and private information retrieval are canonical and representative problems in cryptography that capture the key challenges in understanding the fundamentals of security and privacy. In this dissertation, we use information theoretic tools to tackle these two classical cryptographic primitives. In the first part, we consider the secure multi-party computation problem, where multiple users, each holding an independent message, wish to compute a function on the messages without revealing any additional information. We present an efficient protocol in terms of randomness cost to securely compute a vector linear function. In the second part, we discuss the symmetric private information retrieval problem, where a user wishes to retrieve one message from a number of replicated databases while keeping the desired message index a secret from each individual database. Further, the user learns nothing about the other messages. We present an optimal protocol that achieves the minimum upload cost for symmetric private information retrieval, i.e., the queries sent from the user to the databases have the minimum number of bits.
The design of secure protocols that implement arbitrarily desired functionalities is a major part of modern cryptography. Taking the opposite perspective, the design of any cryptographic scheme may be viewed as the design of a secure protocol for implementing a suitable functionality. Still, we believe that it makes sense to differentiate between basic cryptographic primitives (which involve little interaction) like encryption and signature schemes, on the one hand, and general cryptographic protocols, on the other hand. In this chapter we consider general results concerning secure multi –party computations, where the two –party case is an important special case. In a nutshell, these results assert that one can construct protocols for securely computing any desirable multi–party functionality (see the following terminology). Indeed, what is striking about these results is their generality, and we believe that the wonder is not diminished by the (various alternative) conditions under which these results hold. Our focus on the general study of secure multi–party computation (rather than on protocols for solving specific problems) is natural in the context of the theoretical treatment of the subject matter. We wish to highlight the importance of this general study to practice. Firstly, this study clarifies fundamental issues regarding security in a multi–party environment. Secondly, it draws the lines between what is possible in principle and what is not. Thirdly, it develops general techniques for designing secure protocols. And last, sometimes it may even yield schemes (or modules) that may be incorporated in practical systems.
The problem of AAA privately transmitting information to BBB by a public announcement overheard by an eavesdropper CCC is considered. To do so by a deterministic protocol, their inputs must be correlated. Dependent inputs are represented using a deck of cards. There is a publicly known signature (a,b,c)(a,b,c)(a,b,c), where n=a+b+c+rn = a + b + c + rn=a+b+c+r, and AAA gets aaa cards, BBB gets bbb cards, and CCC gets ccc cards, out of the deck of nnn cards. Using a deterministic protocol, AAA decides its announcement based on her hand. Using techniques from coding theory, Johnson graphs, and additive number theory, a novel perspective inspired by distributed computing theory is provided, to analyze the amount of information that AAA needs to send, while preventing CCC from learning a single card of her hand. In one extreme, the generalized Russian cards problem, BBB wants to learn all of AAA's cards, and in the other, BBB wishes to learn something about AAA's hand.
Two types of generalizations in secure multi-party computation are described. First, we report on a new general adversary model: while previous protocols were designed to tolerate misbehavior of one of two types (active or passive adversaries) the new protocols tolerate mixed types of adversaries, i.e. an adversary that can corrupt players actively, passively, or can let them crash (i.e. stop execution). We derive the necessary and sufficient conditions under which such a general adversary can be tolerated. Second, we present a generic protocol framework that unifies previous approaches achieving unconditional (or information-theoretic) security and cryptographic security (i.e. security based on a cryptographic intractability assumption). This is achieved by introducing secret sharing schemes with an additional property called the multiplication property and a new type of homomorphic commitment scheme, and by generalizing the concept of commitments.