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Fuzzy extractors: How to generate strong keys from biometrics and other noisy data. Technical Report 2003/235, Cryptology ePrint archive, http://eprint.iacr.org, 2006. Previous version appeared at EUROCRYPT 2004
 34 [DRS07] [DS05] [EHMS00] [FJ01] Yevgeniy Dodis, Leonid Reyzin, and Adam
, 2004
"... We provide formal definitions and efficient secure techniques for • turning noisy information into keys usable for any cryptographic application, and, in particular, • reliably and securely authenticating biometric data. Our techniques apply not just to biometric information, but to any keying mater ..."
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Cited by 292 (35 self)
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We provide formal definitions and efficient secure techniques for • turning noisy information into keys usable for any cryptographic application, and, in particular, • reliably and securely authenticating biometric data. Our techniques apply not just to biometric information, but to any keying material that, unlike traditional cryptographic keys, is (1) not reproducible precisely and (2) not distributed uniformly. We propose two primitives: a fuzzy extractor reliably extracts nearly uniform randomness R from its input; the extraction is errortolerant in the sense that R will be the same even if the input changes, as long as it remains reasonably close to the original. Thus, R can be used as a key in a cryptographic application. A secure sketch produces public information about its input w that does not reveal w, and yet allows exact recovery of w given another value that is close to w. Thus, it can be used to reliably reproduce errorprone biometric inputs without incurring the security risk inherent in storing them. We define the primitives to be both formally secure and versatile, generalizing much prior work. In addition, we provide nearly optimal constructions of both primitives for various measures of “closeness” of input data, such as Hamming distance, edit distance, and set difference.
Detection of Algebraic Manipulation with Applications to Robust Secret Sharing and Fuzzy Extractors
, 2008
"... Abstract. Consider an abstract storage device Σ(G) that can hold a single element x from a fixed, publicly known finite group G. Storage is private in the sense that an adversary does not have read access to Σ(G) at all. However, Σ(G) is nonrobust in the sense that the adversary can modify its cont ..."
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Cited by 25 (4 self)
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Abstract. Consider an abstract storage device Σ(G) that can hold a single element x from a fixed, publicly known finite group G. Storage is private in the sense that an adversary does not have read access to Σ(G) at all. However, Σ(G) is nonrobust in the sense that the adversary can modify its contents by adding some offset ∆ ∈ G. Due to the privacy of the storage device, the value ∆ can only depend on an adversary’s a priori knowledge of x. We introduce a new primitive called an algebraic manipulation detection (AMD) code, which encodes a source s into a value x stored on Σ(G) so that any tampering by an adversary will be detected, except with a small error probability δ. We give a nearly optimal construction of AMD codes, which can flexibly accommodate arbitrary choices for the length of the source s and security level δ. We use this construction in two applications: – We show how to efficiently convert any linear secret sharing scheme into a robust secret sharing scheme, which ensures that no unqualified subset of players can modify their shares and cause the reconstruction of some value s ′ � = s. – We show how how to build nearly optimal robust fuzzy extractors for several natural metrics. Robust fuzzy extractors enable one to reliably extract and later recover random keys from noisy and nonuniform secrets, such as biometrics, by relying only on nonrobust public storage. In the past, such constructions were known only in the random oracle model, or required the entropy rate of the secret to be greater than half. Our construction relies on a randomly chosen common reference string (CRS) available to all parties. 1
On cryptography with auxiliary input
 DKL09] [DS05] [FGK+ 10] [FOR12] [GHV10
, 2009
"... We study the question of designing cryptographic schemes which are secure even if an arbitrary function f(sk) of the secret key is leaked, as long as the secret key sk is still (exponentially) hard to compute from this auxiliary input. This setting of auxiliary input is more general than the more tr ..."
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Cited by 19 (2 self)
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We study the question of designing cryptographic schemes which are secure even if an arbitrary function f(sk) of the secret key is leaked, as long as the secret key sk is still (exponentially) hard to compute from this auxiliary input. This setting of auxiliary input is more general than the more traditional setting, which assumes that some of information about the secret key sk may be leaked, but sk still has high minentropy left. In particular, we deal with situations where f(sk) informationtheoretically determines the entire secret key sk. As our main result, we construct CPA/CCA secure symmetric encryption schemes that remain secure with exponentially hardtoinvert auxiliary input. We give several applications of such schemes. • We construct an averagecase obfuscator for the class of point functions, which remains secure with exponentially hardtoinvert auxiliary input, and is reusable. • We construct a reusable and robust extractor that remains secure with exponentially hardtoinvert auxiliary input. Our results rely on a new cryptographic assumption, Learning SubspacewithNoise (LSN), which is related to the well known Learning ParitywithNoise (LPN) assumption.
Nonmalleable extractors and symmetric key cryptography from weak secrets
 In Proceedings of the 41stACM Symposium on the Theory of Computing
, 2009
"... We study the question of basing symmetric key cryptography on weak secrets. In this setting, Alice and Bob share an nbit secret W, which might not be uniformly random, but the adversary has at least k bits of uncertainty about it (formalized using conditional minentropy). Since standard symmetrick ..."
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Cited by 19 (9 self)
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We study the question of basing symmetric key cryptography on weak secrets. In this setting, Alice and Bob share an nbit secret W, which might not be uniformly random, but the adversary has at least k bits of uncertainty about it (formalized using conditional minentropy). Since standard symmetrickey primitives require uniformly random secret keys, we would like to construct an authenticated key agreement protocol in which Alice and Bob use W to agree on a nearly uniform key R, by communicating over a public channel controlled by an active adversary Eve. We study this question in the information theoretic setting where the attacker is computationally unbounded. We show that singleround (i.e. one message) protocols do not work when k ≤ n 2, and require poor parameters even when n 2 < k ≪ n. On the other hand, for arbitrary values of k, we design a communication efficient tworound (challengeresponse) protocol extracting nearly k random bits. This dramatically improves the previous construction of Renner and Wolf [RW03], which requires Θ(λ + log(n)) rounds where λ is the security parameter. Our solution takes a new approach by studying and constructing “nonmalleable” seeded randomness extractors — if an attacker sees a random seed X and comes up with an arbitrarily related seed X ′, then we bound the relationship between R = Ext(W; X) and R ′ = Ext(W; X ′). We also extend our tworound key agreement protocol to the “fuzzy ” setting, where Alice and Bob share “close ” (but not equal) secrets WA and WB, and to the Bounded Retrieval Model (BRM) where the size of the secret W is huge.
Informationtheoretically secret key generation for fading wireless channels
 IEEE Trans on Information Forensics and Security
, 2010
"... Abstract—The multipathrich wireless environment associated with typical wireless usage scenarios is characterized by a fading channel response that is timevarying, locationsensitive, and uniquely shared by a given transmitter–receiver pair. The complexity associated with a richly scattering envir ..."
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Cited by 17 (1 self)
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Abstract—The multipathrich wireless environment associated with typical wireless usage scenarios is characterized by a fading channel response that is timevarying, locationsensitive, and uniquely shared by a given transmitter–receiver pair. The complexity associated with a richly scattering environment implies that the shortterm fading process is inherently hard to predict and best modeled stochastically, with rapid decorrelation properties in space, time, and frequency. In this paper, we demonstrate how the channel state between a wireless transmitter and receiver can be used as the basis for building practical secret key generation protocols between two entities. We begin by presenting a scheme based on level crossings of the fading process, which is wellsuited for the Rayleigh and Rician fading models associated with a richly scattering environment. Our level crossing algorithm is simple, and incorporates a selfauthenticating mechanism to prevent adversarial manipulation of message exchanges during the protocol. Since the level crossing algorithm is best suited for fading processes that exhibit symmetry in their underlying distribution, we present a second and more powerful approach that is suited for more general channel state distributions. This second approach is motivated by observations from quantizing jointly Gaussian processes, but exploits empirical measurements to set quantization boundaries and a heuristic log likelihood ratio estimate to achieve an improved secret key generation rate. We validate both proposed protocols through experimentations using a customized 802.11a platform, and show for the typical WiFi channel that reliable secret key establishment can be accomplished at rates on the order of 10 b/s. Index Terms—Informationtheoretic security, key generation, PHY layer security. I.
Secure identification and QKD in the boundedquantumstorage model
 In Advances in Cryptology— CRYPTO ’07
, 2007
"... Abstract. We consider the problem of secure identification: user U proves to server S that he knows an agreed (possibly lowentropy) password w, while giving away as little information on w as possible, namely the adversary can exclude at most one possible password for each execution of the scheme. ..."
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Cited by 11 (6 self)
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Abstract. We consider the problem of secure identification: user U proves to server S that he knows an agreed (possibly lowentropy) password w, while giving away as little information on w as possible, namely the adversary can exclude at most one possible password for each execution of the scheme. We propose a solution in the boundedquantumstorage model, where U and S may exchange qubits, and a dishonest party is assumed to have limited quantum memory. No other restriction is posed upon the adversary. An improved version of the proposed identification scheme is also secure against a maninthemiddle attack, but requires U and S to additionally share a highentropy key k. However, security is still guaranteed if one party loses k to the attacker but notices the loss. In both versions of the scheme, the honest participants need no quantum memory, and noise and imperfect quantum sources can be tolerated. The schemes compose sequentially, and w and k can securely be reused. A small modification to the identification scheme results in a quantumkeydistribution (QKD) scheme, secure in the boundedquantumstorage model, with the same reusability properties of the keys, and without assuming authenticated channels. This is in sharp contrast to known QKD schemes (with unbounded adversary) without authenticated channels, where authentication keys must be updated, and unsuccessful executions can cause the parties to run out of keys. 1
Privacy Amplification with Asymptotically Optimal Entropy Loss
, 2010
"... We study the problem of “privacy amplification”: key agreement between two parties who both know a weak secret w, such as a password. (Such a setting is ubiquitous on the internet, where passwords are the most commonly used security device.) We assume that the key agreement protocol is taking place ..."
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Cited by 10 (4 self)
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We study the problem of “privacy amplification”: key agreement between two parties who both know a weak secret w, such as a password. (Such a setting is ubiquitous on the internet, where passwords are the most commonly used security device.) We assume that the key agreement protocol is taking place in the presence of an active computationally unbounded adversary Eve. The adversary may have partial knowledge about w, so we assume only that w has some entropy from Eve’s point of view. Thus, the goal of the protocol is to convert this nonuniform secret w into a uniformly distributed string R that is fully secret from Eve. R may then be used as a key for running symmetric cryptographic protocols (such as encryption, authentication, etc.). Because we make no computational assumptions, the entropy in R can come only from w. Thus such a protocol must minimize the entropy loss during its execution, so that R is as long as possible. The best previous results have entropy loss of Θ(κ 2), where κ is the security parameter, thus requiring the password to be very long even for small values of κ. In this work, we present the first protocol for informationtheoretic key agreement that has entropy loss linear in the security parameter. The result is optimal up
Key agreement from close secrets over unsecured channels
, 2009
"... We consider informationtheoretic key agreement between two parties sharing somewhat different versions of a secret w that has relatively little entropy. Such key agreement, also known as information reconciliation and privacy amplification over unsecured channels, was shown to be theoretically feas ..."
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Cited by 10 (3 self)
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We consider informationtheoretic key agreement between two parties sharing somewhat different versions of a secret w that has relatively little entropy. Such key agreement, also known as information reconciliation and privacy amplification over unsecured channels, was shown to be theoretically feasible by Renner and Wolf (Eurocrypt 2004), although no protocol that runs in polynomial time was described. We propose a protocol that is not only polynomialtime, but actually practical, requiring only a few seconds on consumergrade computers. Our protocol can be seen as an interactive version of robust fuzzy extractors (Boyen et al., Eurocrypt 2005, Dodis et al., Crypto 2006). While robust fuzzy extractors, due to their noninteractive nature, require w to have entropy at least half its length, we have no such constraint. In fact, unlike in prior solutions, in our solution the entropy loss is essentially unrelated to the length or the entropy of w, and depends only on the security parameter.
Intrusionresilient key exchange in the bounded retrieval model
 TCC 2007: 4th Theory of Cryptography Conference, volume 4392 of Lecture
"... Abstract. We construct an intrusionresilient symmetrickey authenticated key exchange (AKE) protocol in the bounded retrieval model. The model employs a long shared private key to cope with an active adversary who can repeatedly compromise the user’s machine and perform any efficient computation on ..."
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Cited by 10 (1 self)
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Abstract. We construct an intrusionresilient symmetrickey authenticated key exchange (AKE) protocol in the bounded retrieval model. The model employs a long shared private key to cope with an active adversary who can repeatedly compromise the user’s machine and perform any efficient computation on the entire shared key. However, we assume that the attacker is communication bounded and unable to retrieve too much information during each successive breakin. In contrast, the users read only a small portion of the shared key, making the model quite realistic in situations where storage is much cheaper than bandwidth. The problem was first studied by Dziembowski [Dzi06a], who constructed a secure AKE protocol using random oracles. We present a general paradigm for constructing intrusionresilient AKE protocols in this model, and show how to instantiate it without random oracles. The main ingredients of our construction are UCsecure password authenticated key exchange and tools from the bounded storage model. 1
On RelatedSecret Pseudorandomness
 In Theory of Cryptography, LNCS 5978:255–272
, 2010
"... Abstract. Relatedkey attacks are attacks against constructions which use a secret key (such as a blockcipher) in which an attacker attempts to exploit known or chosen relationships among keys to circumvent security properties. Security against relatedkey attacks has been a subject of study in nume ..."
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Cited by 8 (0 self)
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Abstract. Relatedkey attacks are attacks against constructions which use a secret key (such as a blockcipher) in which an attacker attempts to exploit known or chosen relationships among keys to circumvent security properties. Security against relatedkey attacks has been a subject of study in numerous recent cryptographic papers. However, most of these results are attacks on specific constructions, while there has been little positive progress on constructing relatedkey secure primitives. In this paper, we attempt to address the question of whether relatedkey secure blockciphers can be built from traditional cryptographic primitives. We develop a theoretical framework of “relatedsecret secure” cryptographic primitives, a class of primitives which includes relatedkey secure blockciphers and PRFs. We show that while a single relatedsecret pseduorandom bit is sufficient and necessary to create relatedkey secure blockciphers, hardcore bits with typical proofs are not relatedsecret psuedorandom. Since the pseudorandomness of hardcore bits is the essential technique known to make pseudorandomness from assumptions of simple hardness, this presents a very strong barrier to the development of provably relatedkey secure blockciphers based on standard hardness assumptions. 1