A new construction of a pseudorandom generator based on a simple combination of two LFSRs is introduced. This construction allows users to generate a large family of sequences using the same initial states and the same characteristic feedback polynomials of the two combined LFSRs. The construction is related to the so-called shrinking generator that is a special case of this construction. The construction has attractive properties such as exponential period, exponential linear complexity, good statistical properties and security against correlation attacks. All these properties make it a suitable crypto-generator for stream cipher applications.

We investigate the possibility of building elliptic curves over finite prime fields having given small MOV-degrees. Using complex multiplication, we give many examples of such curves.

We describe a simple and novel cryptographic construction that we refer to as a {\em fuzzy vault}. A player Alice may place a secret value $\kappa$ in a fuzzy vault and ``lock'' it using a set $A$ of elements from some public universe $U$. If Bob tries to ``unlock'' the vault using a set $B$ of similar length, he obtains $\kappa$ only if $B$ is close to $A$, i.e., only if $A$ and $B$ overlap substantially. In constrast to previous constructions of this flavor, ours possesses the useful feature of {\em order invariance}, meaning that the ordering of $A$ and $B$ is immaterial to the functioning of the vault. As we show, our scheme enjoys provable security against a computationally unbounded attacker.

In this paper, we propose TMAC, Two-Key CBC Message Authentication Code. TMAC is a refinement of XCBC (which is a variant of CBC MAC) shown by Black and Rogaway. We use only $(k+n)$-bit key for TMAC while XCBC uses $(k+2n)$-bit key, where $k$ is the key length of the underlying block cipher and $n$ is its block length. The cost for reducing the size of secret keys is almost negligible; only one shift and one conditional XOR. Similarly to XCBC, our algorithm correctly and efficiently handles messages of arbitrary bit length.

The recently proposed multiplicative masking countermeasure against power analysis attacks on AES is interesting as it does not require the costly recomputation and RAM storage of S-boxes for every run of AES. This is important for applications where the available space is very limited such as the smart card applications. Unfortunately, it is here shown that this method is in fact inherently vulnerable to differential power analysis. Other possible random masking methods are also discussed.

We show how to efficiently transform any public coin honest verifier zero knowledge proof system into a proof system that is concurrent zero-knowledge with respect to any (possibly cheating) verifier via black box simulation. By efficient we mean that our transformation incurs only an additive overhead, both in terms of the number of rounds and the computational and communication complexity of each round, independently of the complexity of the original protocol. Moreover, the transformation preserves (up to negligible additive terms) the soundness and completeness error probabilities. The new proof system is proved secure based on the Decisional Diffie-Hellman (DDH) assumption, in the standard model of computation, i.e., no random oracles, shared random strings, or public key infrastructure is assumed. In addition to the introduction of a practical protocol, this construction provides yet another example of ideas in plausibility results that turn into ideas in the construction of practical protocols. We prove our main result by developing a mechanism for simulatable commitments that may be of independent interest. In particular, it allows a weaker result that is interesting as well. We present an efficient transformation of any honest verifier public-coin computational zero-knowledge proof into a (public coin) computational zero-knowledge proof secure against any verifier. The overhead of this second transformation is minimal: we only increase the number of rounds by 3, and increase the computational cost by 2 public key operations for each round of the original protocol. The cost of the more general transformation leading to concurrent zero knowledge is also close to optimal (for black box simulation), requiring only omega(log n) additional rounds (where n is a security parameter and omega(log n) can be any superlogarithmic function of n (e.g., log(n)log^*(n)), and omega(log n) additional public key operations for each round of the original protocol.

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.

Pairing-based cryptosystems depend on the existence of groups where the Decision Diffie-Hellman problem is easy to solve, but the Computational Diffie-Hellman problem is hard. Such is the case of elliptic curve groups whose embedding degree is large enough to maintain a good security level, but small enough for arithmetic operations to be feasible. However, the embedding degree is usually enormous, and the scarce previously known suitable elliptic groups had embedding degree $k \leqslant 6$. In this note, we examine criteria for curves with larger $k$ that generalize prior work by Miyaji et al. based on the properties of cyclotomic polynomials, and propose efficient representations for the underlying algebraic structures.

There is abundant literature on how to use linear approximations to break various stream ciphers. In this paper we show that it is possible to design an efficient attack based on higher degree approximations. We reduce the attack to solving an overdefined system of multivariate equations and use the XL algorithm from Eurocrypt 2000. The complexity of the XL algorithm is sometimes controversial, however in practice and for the cases relevant here (much more equations than variables), we show that the behaviour of XL is predictable with the utmost precision and 100% accuracy. Our new attack allows to break efficiently stream ciphers that are known to be immune to all the previously known attacks. It has also surprisingly small and loose requirements on the keystream needed, giving powerful attack scenarios, leading even to (almost) ciphertext-only attacks. For example, the new attack breaks the stream cipher Toyocrypt submitted to the Japanese government Cryptrec call for cryptographic primitives, and one of only two candidates accepted to the second phase of Cryptrec evaluation process. Toyocrypt is a 128-bit stream cipher and at the time of submission it was claimed to resist to all known attacks. Later, Mihaljevic and Imai showed that the effective key length in Toyocrypt is only 96 bits. Still Toyocrypt may be easily modified to avoid this attack and have full 128 bit key. Our best attack breaks both Toyocrypt and the modified versions taking exactly 2^92 CPU clocks for a 128-bit cipher. Moreover it works in much less restrictive conditions that the previous attack, for example knowing ONLY that the ciphertext is in English.

This paper presents results that show that there exist problems in that are provably hard in the generic group model but easy to solve whenever the random encoding function is replaced with a specific encoding function (or one drawn from a specific set of encoding functions). We also show that there exist cryptographic schemes that are provably hard in the generic group model but easy to break in practice.

In the {\em strong consensus} problem, $n$ players attempt to reach agreement on a value initially held by {\em one of the good players}, despite the (malicious) behavior of up to $t$ of them. Although the problem is closely related to the standard consensus problem (aka Byzantine agreement), the only known solution with the optimal number of players requires exponential computation and communication in the unconditional setting. In this paper we study this problem, and present {\em efficient} protocols and tight lower bounds for several standard distributed computation models --- unconditional, computational, synchronous, and asynchronous.

Certified exchange of messages is an essential mechanism for e-commerce; the timing aspects (timeouts and timestamps) are very important for practical applications. However existing formal methods for security analysis assume simplified completely synchronous or completely asynchronous models, and cannot deal with the timing aspects of these (and other e-commerce) protocols. We present model for realistic, Δ-synchronized adversarial settings. We then present a simple, efficient and provably-secure protocol for certified, time-stamped message delivery, providing precise guarantees of delay and timestamps. Our model and analysis use concrete (rather than asymptotic) notions of security.

We propose an elliptic curve scheme over the ring $\entq$, which is efficient and semantically secure in the standard model, and it has expansion factor 2 (previous schemes with similar features present expansion factors greater or equal than 4). Demytko's RSA type scheme has been used as an underlying primitive to obtain efficiency and probabilistic encryption. Semantic security of the scheme is based on a new decisional assumption, namely, the Decisional Small Root Assumption. Confidence on this assumption is also discussed.

Authentication is a well-studied area of classical cryptography: a sender A and a receiver B sharing a classical private key want to exchange a classical message with the guarantee that the message has not been modified or replaced by a dishonest party with control of the communication line. In this paper we study the authentication of messages composed of quantum states. We give a formal definition of authentication in the quantum setting. Assuming A and B have access to an insecure quantum channel and share a private, classical random key, we provide a non-interactive scheme that both enables A to encrypt and authenticate (with unconditional security) an m qubit message by encoding it into m+s qubits, where the probability decreases exponentially in the security parameter s. The scheme requires a private key of size 2m+O(s). To achieve this, we give a highly efficient protocol for testing the purity of shared EPR pairs. It has long been known that learning information about a general quantum state will necessarily disturb it. We refine this result to show that such a disturbance can be done with few side effects, allowing it to circumvent cryptographic protections. Consequently, any scheme to authenticate quantum messages must also encrypt them. In contrast, no such constraint exists classically: authentication and encryption are independent tasks, and one can authenticate a message while leaving it publicly readable. This reasoning has two important consequences: On one hand, it allows us to give a lower bound of 2m key bits for authenticating m qubits, which makes our protocol asymptotically optimal. On the other hand, we use it to show that digitally signing quantum states is impossible, even with only computational security.

In a threshold signature scheme, a group of players share a secret information in such a way that only those subsets with a minimum number of players can compute a valid signature. We propose methods to construct some useful and computationally secure distributed protocols from threshold signature schemes satisfying some suitable properties. Namely, we prove that any threshold signature scheme which is non-interactive can be used to construct a metering scheme. We also design a distributed key distribution scheme from any deterministic threshold signature scheme. The security of these news schemes is reduced to the security of the corresponding threshold signature schemes. Furthermore, the constructed protocols reach some desirable properties.

We study the problem of finding efficiently computable non-degenerate multilinear maps from $G_1^n$ to $G_2$, where $G_1$ and $G_2$ are groups of the same prime order, and where computing discrete logarithms in $G_1$ is hard. We present several applications to cryptography, explore directions for building such maps, and give some reasons to believe that finding examples with $n>2$ may be difficult.

Many bitstream generators are based on linear feedback shift registers. A widespread technique for the cryptanalysis of those generators is the linear consistency test (LCT). In this paper, we consider an application of the LCT in cryptanalysis of clock-controlled bitstream generators, called \textsl{clock control guessing}. We give a general and very simple method for estimating the efficiency of clock control guessing, yielding an upper bound on the effective key length of a whole group of bitstream generators. Finally, we apply the technique against a number of clock-controlled generators, such as the A5/1, alternating step generator, step1-step2 generator, cascade generator, and others.

The Secure Shell (SSH) protocol is one of the most popular cryptographic protocols on the Internet. Unfortunately, the current SSH authenticated encryption mechanism is insecure. In this paper, we propose several fixes to the SSH protocol and, using techniques from modern cryptography, we prove that our modified versions of SSH meet strong new chosen-ciphertext privacy and integrity requirements. Furthermore, our proposed fixes will require relatively little modification to the SSH protocol and to SSH implementations. We believe that our new notions of privacy and integrity for encryption schemes with stateful decryption algorithms will be of independent interest.

Cryptographic computations (decryption, signature generation, etc.) are often performed on a relatively insecure device (e.g., a mobile device or an Internet-connected host) which cannot be trusted to maintain secrecy of the private key. We propose and investigate the notion of \emph{key-insulated security} whose goal is to minimize the damage caused by secret-key exposures. In our model, the secret key(s) stored on the insecure device are refreshed at discrete time periods via interaction with a physically-secure --- but computationally-limited --- device which stores a ``master key''. All cryptographic computations are still done on the insecure device, and the public key remains unchanged. In a (t, N)-key-insulated scheme, an adversary who compromises the insecure device and obtains secret keys for up to t periods of his choice is unable to violate the security of the cryptosystem for \emph{any} of the remaining N-t periods. Furthermore, the scheme remains secure (for \emph{all} time periods) against an adversary who compromises \emph{only} the physically-secure device. We notice that key-insulated schemes significantly improve the security guarantee of forward-secure schemes [A97,BM99], in which exposure of the secret key at even a single time period (necessarily) compromises the security of the system for all future time periods. This improvement is achieved with minimal cost: infrequent key updates with a (possibly untrusted) secure device. We focus primarily on key-insulated public-key encryption. We construct a (t,N)-key-insulated encryption scheme based on any (standard) public-key encryption scheme, and give a more efficient construction based on the DDH assumption. The latter construction is then extended to achieve chosen-ciphertext security.

The article describes an attack on OpenPGP format, which leads to disclosure of the private signature keys of the DSA and RSA algorithms. The OpenPGP format is used in a number of applications including PGP, GNU Privacy Guard and other programs specified on the list of products compatible with OpenPGP, which is available at http://www.pgpi.org/products. Therefore all these applications must undergo the same revision as the actual program PGP. The success of the attack was practically verified and demonstrated on the PGP program, version 7.0.3 with a combination of AES and DH/DSS algorithms. As the private signature key is the basic information of the whole system which is kept secret, it is encrypted using the strong cipher. However, it shows that this protection is illusory, as the attacker has neither to attack this cipher nor user´s secret passphrase. A modification of the private key file in a certain manner and subsequent capturing of one signed message is sufficient for successful attack. Insufficient protection of the integrity of the public as well as private parts of signature keys in the OpenPGP format is analyzed in DSA and RSA algorithms and on the basis of this, a procedure of attacks is shown on both private signature keys. The attacks apply to all lengths of parameters (modules, keys) of RSA and DSA. In the end the cryptographic measures for correction of the OpenPGP format as well as PGP format are proposed.

In this paper we describe several fault attacks on the Advanced Encryption Standard (AES). First, using optical fault induction attacks as recently publicly presented by Skorobogatov and Anderson \cite{SA}, we present an implementation independent fault attack on AES. This attack is able to determine the complete $128$-bit secret key of a sealed tamper-proof smartcard by generating $128$ faulty cipher texts. Second, we present several implementation-dependent fault attacks on AES. These attacks rely on the observation that due to the AES's known timing analysis vulnerability (as pointed out by Koeune and Quisquater \cite{KQ}), any implementation of the AES must ensure a data independent timing behavior for the so called AES's {\tt xtime} operation. We present fault attacks on AES based on various timing analysis resistant implementations of the {\tt xtime}-operation. Our strongest attack in this direction uses a very liberal fault model and requires only $256$ faulty encryptions to determine a $128$-bit key.

The ESIGN signature scheme was provided with an inadequate proof of security. We propose two techniques to repair the scheme, which we name ESIGN-D and ESIGN-R. Another improvement of ESIGN is encouraged, where the public key is hashed together with the message. This allows to have a security proof in the multi key setting. Additionally, the lower security of ESIGN compared to RSA-PSS leads to suggest that a common public key is used for ESIGN and RSA-PSS, leaving to the signer the choice between fast signature or better security.

This article describes concrete results and practically approved countermeasures concerning differential fault attacks on RSA using the CRT. It especially investigates smartcards with a RSA coprocessor where any hardware countermeasure to defeat such fault attacks have been switched off. This scenario has been chosen in order to completely analyze the resulting effects and errors occurring inside the hardware. Using the results of this kind of physical stress attack enables the development of completely reliable software countermeasures. Although {\em successful\/} RSA attacks on the investigated hardware have been only possible with an expensive enhanced laboratory equipment, the effects on the unprotected hardware have been tremendously. This caused lots of analysis efforts to investigate what really happened during the attack. Indeed, this will be addressed in this paper. We first report on the nature of the resulting errors within the hardware due to the physical stress applied to the smartcard. Hereafter, we describe the experiments and results with a very efficient and prominent software RSA-CRT DFA countermeasure. This method could be shown to be insufficient, i.e., detected nearly no error, when we introduced stress at the right position during the computation. Naturally, a detailed error analysis model followed, specifying every failure point during the RSA-CRT operation. This model finally allowed to develop and present here new very practically oriented software countermeasures hedging the observed and characterized fault attacks. Eventually, we present the security analysis of our new developed software RSA-CRT DFA countermeasures. Thanks to their careful specification according to the observed and analyzed errors they resisted all kinds of physical stress attacks and were able to detect any subtle computation error, thus avoiding to break the smartcard by fault attacks. Nevertheless, we stress, that although our software countermeasures have been fully approved by practical experiments, we are convinced that only sophisticated hardware countermeasures like sensors and filters in combination with software countermeasures will be able to provide a secure and comfortable base to defeat in general any conceivable fault attacks scenario on smartcards properly.

Suppose Alice wishes to send a message to Bob using an identity-based encryption scheme (recall such a scheme is a public key cryptosystem where any string is a valid public key), but desires integrity as well as security. In other words, Alice wants Bob to know that only she could have sent the message. Furthermore, suppose she does not want the non-repudiation property that would necessarily be present if she simply used an identity-based signature scheme i.e. she does not want Bob to be able to prove to a third party that she is the sender. We augment the system of Boneh and Franklin to allow communication with integrity without nonrepudiation. We formalize notions of security and integrity for our scheme, and show that new encryption and decryption algorithms are more efficient, despite being equally secure and authenticated.

This paper contains three parts. In the first part we present a new side channel attack on plaintext encrypted by EME-OAEP PKCS#1 v.2.1. In contrast with Manger´s attack, we attack that part of the plaintext, which is shielded by the OAEP method. In the second part we show that Bleichenbacher’s and Manger’s attack on the RSA encryption scheme PKCS#1 v.1.5 and EME-OAEP PKCS#1 v.2.1 can be converted to an attack on the RSA signature scheme with any message encoding (not only PKCS). This is a new threat for those implementations of PKI, in which the roles of signature and encryption keys are not strictly separated. This situation is often encountered in the SSL protocol used to secure access to web servers. In the third part we deploy a general idea of fault-based attacks on the RSA-KEM scheme and present two particular attacks as the examples. The result is the private key instead of the plaintext as with attacks on PKCS#1 v.1.5 and v.2.1. These attacks should highlight the fact that the RSA-KEM scheme is not an entirely universal solution to problems of RSAES-OAEP implementation and that even here the manner of implementation is significant.

The public key cryptosystem $MST_1$ has been introduced in~\cite{MaStTr00}. Its security relies on the hardness of factoring with respect to wild logarithmic signatures. To identify `wild-like' logarithmic signatures, the criterion of being totally-non-transversal has been proposed. We give tame totally-non-transversal logarithmic signatures for the alternating and symmetric groups of degree $\ge 5$. Hence, basing a key generation procedure on the assumption that totally-non-transversal logarithmic signatures are `wild like' seems critical. We also discuss the problem of recognizing `weak' totally-non-transversal logarithmic signatures, and demonstrate that another proposed key generation procedure based on permutably transversal logarithmic signatures may produce weak keys.

In 1999, Naor, Pinkas and Reingold introduced schemes in which some groups of servers distribute keys among a set of users in a distributed way. They gave some specific proposals both in the unconditional and in the computational security framework. Their computationally secure scheme is based on the Decisional Diffie-Hellman Assumption. This model assumes secure communication between users and servers. Furthermore it requires users to do some expensive computations in order to obtain a key. In this paper we modify the model introduced by Naor et al., requiring authenticated channels instead of assuming the existence of secure channels. Our model makes the user's computations easier, because most computations of the protocol are carried out by servers, keeping to a more realistic situation. We propose a basic scheme, that makes use of ElGamal cryptosystem, and that fits in with this model in the case of a passive adversary. We then add zero-knowledge proofs and verifiable secret sharing to prevent from the action of an active adversary. We consider general structures (not only the threshold ones) for those subsets of servers that can provide a key to a user and for those tolerated subsets of servers that can be corrupted by the adversary. We find necessary combinatorial conditions on these structures in order to provide security to our scheme.

The encryption system \(E_{0}\), which is the encryption system used in the Bluetooth specification, is a two level system where a key and a packet nonce is given to a level 1 key stream generator, which produces the key for a level 2 key stream generator, whose output is used to encrypt. We give a method for recovering the key for the level 1 key stream generator given the internal keys for two or three level 2 key stream generators. This method, combined with published methods for recovering keys for the level 2 key stream generator, can be used to recover the \(E_{0}\) second key with $O(2^{65})$ work, and $O(2^{80})$ precomputation time. Although this attack is of no advantage if \(E_{0}\) is used with the recommended security parameters (64 bit encryption key), it shows that no addition security would be made available by enlarging the encryption key, as discussed in the Bluetooth specification.

Most guidelines for implementation of the RC4 stream cipher recommend discarding the first 256 bytes of its output. This recommendation is based on the empirical fact that known attacks can either cryptanalyze RC4 starting at any point, or become harmless after these initial bytes are dumped. The motivation for this paper is to find a conservative estimate for the number of bytes that should be discarded in order to be safe. To this end we propose an idealized model of RC4 and analyze it applying the theory of random shuffles. Based on our analysis of the model we recommend dumping at least 512 bytes.

Preneel, Govaerts, and Vandewalle considered the 64 most basic ways to construct a hash function $H:\{0,1\}^*\rightarrow\{0,1\}^n$ from a block cipher $E:\{0,1\}^n\times\{0,1\}^n\rightarrow\{0,1\}^n$. They regarded 12 of these 64 schemes as secure, though no proofs or formal claims were given. The remaining 52 schemes were shown to be subject to various attacks. Here we provide a formal and quantitative treatment of the 64 constructions considered by PGV. We prove that, in a black-box model, the 12 schemes that PGV singled out as secure really \textit{are} secure: we give tight upper and lower bounds on their collision resistance. Furthermore, by stepping outside of the Merkle-Damgard approach to analysis, we show that an additional 8 of the 64 schemes are just as collision resistant (up to a small constant) as the first group of schemes. Nonetheless, we are able to differentiate among the 20 collision-resistant schemes by bounding their security as one-way functions. We suggest that proving black-box bounds, of the style given here, is a feasible and useful step for understanding the security of any block-cipher-based hash-function construction.

We consider communication sessions in which a pair of parties begin by running an authenticated key-exchange protocol to obtain a shared session key, and then secure successive data transmissions between them via an authenticated encryption scheme based on the session key. We show that such a communication session meets the notion of a secure channel protocol proposed by Canetti and Krawczyk if and only if the underlying authenticated encryption scheme meets two new, simple definitions of security that we introduce, and the key-exchange protocol is secure. In other words, we reduce the secure channel requirements of Canetti and Krawczyk to easier to use, stand-alone security requirements on the underlying authenticated encryption scheme.

A new framework for protection against key exposure was recently suggested by Dodis et. al.. We take its realization further towards practice by presenting simple new schemes that provide benefits over previous ones in terms of scalability, performance and security. Our first contribution is a simple, practical, scalable scheme called SKIE-OT that achieves the best possible security in their framework. SKIE-OT is based on the Boneh-Franklin identity-based encryption (IBE) scheme and exploits algebraic properties of the latter. We also show that the role of identity-based encryption is not coincidental by proving that IBE is equivalent to (not strongly) key-insulated encryption with optimal threshold and allowing random-access key updates.

Using more than two factors in the modulus of the RSA cryptosystem has the arithmetic advantage that the private key computations can be speeded up using Chinese remaindering. At the same time, with a proper choice of parameters, one does not have to work with a larger modulus to achieve the same level of security in terms of the difficulty of the integer factorization problem. However, numerous attacks on specific instances on the RSA cryptosystem are known that apply if, for example, the decryption or encryption exponent are chosen too small, or if partial knowledge of the private key is available. Little work is known on how such attacks perform in the multi-prime case. It turns out that for most of these attacks it is crucial that the modulus contains exactly two primes. They become much less effective, or fail, when the modulus factors into more than two distinct primes.

The author proposes a block cipher wich is easy to implement in software on modern 32 bit microprocessorsThe building blocks of the cipher are from the block ciphers MMB and SAFER. The cipher may be expanded for use with future 64 bit processors. Also a new diffusion layer, developed from the SAFER diffusion layer is proposed. It has complextity $\mathcal{O}(n \enspace log \enspace n)$ and the author conjectures that it is MDS. Diffusion layers currently known to be MDS are based on matrices and thus have complexity $\mathcal{O}(n^2)$.

Vaudenay [1] has presented an attack on the CBC mode of block ciphers, which uses padding according to the PKCS#5 standard. One of the countermeasures, which he has assumed, consisted of the encryption of the message M´= M || padding || hash(M || padding) instead of the original M. This can increase the length of the message by several blocks compared with the present padding. Moreover, Wagner [1] showed a security weakness in this proposal. The next correction, which Vaudenay proposed ("A Fix Which May Work") has a general character and doesn't solve practical problems with the real cryptographic interfaces used in contemporary applications. In this article we propose three variants of the CBC mode. From the external point of view they behave the same as the present CBC mode with the PKCS#5 padding, but they prevent Vaudenay's attack.

Cryptographic computations are often carried out on insecure devices for which the threat of key exposure represents a serious and realistic concern. In an effort to mitigate the damage caused by exposure of secret data stored on such devices, the paradigm of \emph{forward security} was introduced. In this model, secret keys are updated at regular intervals throughout the lifetime of the system; furthermore, exposure of a secret key corresponding to a given interval does not enable an adversary to ``break'' the system (in the appropriate sense) for any \emph{prior} time period. A number of constructions of forward-secure digital signature schemes and symmetric-key schemes are known. We present the first construction of a forward-secure public-key encryption scheme whose security is based on the bilinear Diffie-Hellman assumption in the random oracle model. Our scheme can be extended to achieve chosen-ciphertext security at minimal additional cost. The construction we give is quite efficient: all parameters of the scheme grow (at most) poly-logarithmically with the total number of time periods.

Recently, Canetti and Krawczyk (Eurocrypt 2001) formulated a notion of security for key-exchange (KE) protocols, called SK-security, and showed that this notion suffices for constructing secure channels. Their model and proofs, however, do not suffice for proving more general composability properties of SK-secure KE protocols. We show that while the notion of SK-security is strictly weaker than a fully-idealized notion of key exchange security, it is sufficiently robust for providing secure composition with arbitrary protocols. In particular, SK-security guarantees the security of the key for any application that desires to set-up secret keys between pairs of parties. We also provide new definitions of secure-channels protocols with similarly strong composability properties, and show that SK-security suffices for obtaining these definitions. To obtain these results we use the recently proposed framework of "universally composable (UC) security." We also use a new tool, called "non-information oracles," which will probably find applications beyond the present case. These tools allow us to bridge between seemingly limited indistinguishability-based definitions such as SK-security and more powerful, simulation-based definitions, such as UC-security, where general composition theorems can be proven. Furthermore, based on such composition theorems we reduce the analysis of a full-fledged multi-session key-exchange protocol to the (simpler) analysis of individual, stand-alone, key-exchange sessions.

We present a binary tree based parallel algorithm for extending the domain of a UOWHF. The key length expansion is $2m$ bits for $t=2$; $m(t+1)$ bits for $3\leq t\leq 6$ and $m\times(t+\lfloor\log_2 (t-1)\rfloor)$ bits for $t\geq 7$, where $m$ is the length of the message digest and $t\geq 2$ is the height of the binary tree.

In this paper we present a simpler construction of an encryption scheme that achieves adaptive chosen ciphertext security (CCA2), assuming the existence of trapdoor permutations. We build on previous works of Sahai and De Santis et al. and construct a scheme that we believe is the easiest to understand to date. In particular, it is only slightly more involved than the Naor-Yung encryption scheme that is secure against passive chosen-ciphertext attacks (CCA1). We stress that the focus of this paper is on simplicity only.

We present hierarchical identity-based encryption schemes and signature schemes that have total collusion resistance on an arbitrary number of levels and that have chosen ciphertext security in the random oracle model assuming the difficulty of the Bilinear Diffie-Hellman problem.

We consider the problem of constructing Concurrent Zero Knowledge Proofs, in which the fascinating and useful ``zero knowledge'' property is guaranteed even in situations where multiple concurrent proof sessions are executed with many colluding dishonest verifiers. Canetti et al. show that black-box concurrent zero knowledge proofs for non-trivial languages require $\tilde\Omega(\log k)$ rounds where $k$ is the security parameter. Till now the best known upper bound on the number of rounds for NP languages was $\omega(\log^2 k)$, due to Kilian, Petrank and Richardson. We establish an upper bound of $\omega(\log k)$ on the number of rounds for NP languages, thereby closing the gap between the upper and lower bounds, up to a $\omega(\log\log k)$ factor.

We propose a new notion of intrusion-resilient signature schemes, which generalizes and improves upon both forward-secure [And97,BM99] and key-insulated [DKXY02] signature schemes. Specifically, as in the prior notions, time is divided into predefined time periods (e.g., days); each signature includes the number of the time time period in which it was generated; while the public key remains the same, the secret keys evolve with time. Also, as in key-insulated schemes, the user has two modules, signer and home base: the signer generates signatures on his own, and the base is needed only to help update the signer's key from one period to the next. The main strength of intrusion-resilient schemes, as opposed to prior notions, is that they remain secure even after arbitrarily many compromises of both modules, as long as the compromises are not simultaneous. Moreover, even if the intruder does compromise both modules simultaneously, she will still be unable to generate any signatures for the previous time periods. We provide an efficient intrusion-resilient signature scheme, provably secure in the random oracle model based on the strong RSA assumption. We also discuss how such schemes can eliminate the need for certificate revocation in the case of on-line authentication.

A broadcast protocol allows a sender to distribute a value among a set of players such that it is guaranteed that all players receive the same value (consistency), and if the sender is honest, then all players receive the sender's value (validity). Classical broadcast protocols for $n$ players provide security with respect to a fixed threshold $t<n/3$, where both consistency and validity are guaranteed as long as at most $t$ players are corrupted, and no security at all is guaranteed as soon as $t+1$ players are corrupted. Depending on the environment, validity or consistency may be the more important property. We generalize the notion of broadcast by introducing an additional threshold $t^+\ge t$. In a {\em broadcast protocol with extended validity}, both consistency and validity are achieved when no more than $t$ players are corrupted, and validity is achieved even when up to $t^+$ players are corrupted. Similarly, we define {\em broadcast with extended consistency}. We prove that broadcast with extended validity as well as broadcast with extended consistency is achievable if and only if $t+2t^+<n$ (or $t=0$). For example, six players can achieve broadcast when at most one player is corrupted (this result was known to be optimal), but they can even achieve consistency (or validity) when two players are corrupted. Furthermore, our protocols achieve {\em detection} in case of failure, i.e., if at most $t$ players are corrupted then broadcast is achieved, and if at most $t^+$ players are corrupted then broadcast is achieved or every player learns that the protocol failed. This protocol can be employed in the precomputation of a secure multi-party computation protocol, resulting in {\em detectable multi-party computation}, where up to $t$ corruptions can be tolerated and up to $t^+$ corruptions can either be tolerated or detected in the precomputation, for any $t,t^+$ with $t+2t^+<n$.

The Cramer-Shoup cryptosystem for groups of prime order is a practical public-key cryptosystem, provably secure in the standard model under standard assumptions. This paper extends the cryptosystem for groups of unknown order, namely the group of quadratic residues modulo a composed N. Two security results are: In the standard model, the scheme is provably secure if both the Decisional Diffie-Hellman assumption for QR_N *and* the factorisation assumption for N hold. In the random oracle model, the security of the scheme is provable by a quite efficient reduction.

In a proxy signature scheme, a potential signer delegates his signing capability to a proxy entity, who signs a message on behalf of the original signer. All the proposals of proxy signature schemes made until now have been based on Schnorr's signature scheme. Threshold versions of these schemes have also been proposed, in which the power of the proxy signer is distributed among a group of players, in such a way that any subset with a minimum number (threshold) of players can sign a message on behalf of the original signer. We consider a model that is fully distributed, because we want to distribute not only the power of the proxy signer, but also the original signer ability to delegate his signing capability. Furthermore, we consider general structures, instead of only the threshold ones, for both the tolerated subsets of dishonest players and the subsets of honest players authorized to execute a valid instance of the protocol, and in both the original and the proxy signer entities. We find sufficient combinatorial conditions that these structures must satisfy in order to design a fully distributed, secure and robust proxy signature scheme for this general scenario. We propose such a scheme for this setting. It is also based on Schnorr's signature scheme.

In this paper we study secret sharing schemes whose access structure has three or four minimal qualified subsets. The ideal case is completely characterized and for the non-ideal case we provide bounds on the optimal information rate.

We introduce a tensor transform for Boolean functions that covers the algebraic normal and Walsh transforms but which also allows for the definition of new, probabilistic and weight transforms, relating a function to its bias polynomial and to the weights of its subfunctions respectively. Our approach leads to easy proofs for some known results and to new properties of the aforecited transforms. Several new results about algebraic and correlation properties that depend on the weight transform of Boolean functions are proved. Finally, we present a new probabilistic characteristic of a Boolean function that is defined by its algebraic normal and probabilistic transforms over the reals.

The public key cryptosystems $MST_1$ and $MST_2$ make use of certain kinds of factorizations of finite groups. We show that generalizing such factorizations to infinite groups allows a uniform description of several proposed cryptographic primitives. In particular, a generalization of $MST_2$ can be regarded as a unifying framework for several suggested cryptosystems including the ElGamal public key system, a public key system based on braid groups and the MOR cryptosystem.

Cryptographic systems often involve running multiple concurrent instances of some protocol, where the instances have some amount of joint state and randomness. (Examples include systems where multiple protocol instances use the same public-key infrastructure, or the same common reference string.) Rather than attempting to analyze the entire system as a single unit, we would like to be able to analyze each such protocol instance as stand-alone, and then use a general composition theorem to deduce the security of the entire system. However, no known composition theorem applies in this setting, since they all assume that the composed protocol instances have disjoint internal states, and that the internal random choices in the various instances are independent. We propose a new composition operation that can handle the case where different components have some amount of joint state and randomness, and demonstrate sufficient conditions for when the new operation preserves security. The new operation, which is called {\em universal composition with joint state} (and is based on the recently proposed universal composition operation), turns out to be very useful in a number of quite different scenarios such as those mentioned above.

We formally study the notion of a joint signature and encryption in the public-key setting. We refer to this primitive as {\em signcryption}, adapting the terminology of Zheng [Zhe97]. We present wo definitions for the security of signcryption depending on whether the adversary is an outsider or a legal user of the system. We then examine generic sequential composition methods of building signcryption from a signature and encryption scheme. Contrary to what recent results in the symmetric setting [BN00,Kra01] might lead one to expect, we show that classical ``encrypt-then-sign'' (EtS) and ``sign-then-encrypt'' (StE) methods are both {\em secure} composition methods in the public-key setting. We also present a new composition method which we call ``commit-then-encrypt-and-sign'' (CtE&S). Unlike the generic sequential composition methods, CtE&S applies the expensive signature and encryption operations {\em in parallel}, which could imply a gain in efficiency over the StE and EtS schemes. We also show that the new CtE&S method elegantly combines with the recent ``hash-sign-switch'' technique of Shamir and Tauman [ST01], leading to efficient {\em on-line/off-line} signcryption. Finally and of independent interest, we discuss the {\em definitional} inadequacy of the standard notion of chosen ciphertext (CAA) security. Motivated by our applications to signcryption, we show that the notion of CAA-security is syntactically ill-defined, and leads to artificial examples of ``secure'' encryption schemes which do not meet the formal definition of CCA-security. We suggest a natural and very slight relaxation of CAA-security, which we call generalized CCA-security (gCCA). We show that gCCA-security suffices for all known uses of CCA-secure encryption, while no longer suffering from the definitional shortcomings of the latter.

This paper describes an effort to attack S-DES using differential cryptanalysis and linear cryptanalysis. S-DES is a reduced version of the Data Encryption Standard (DES). It also includes a discussion on the subject of cryptology and a literature survey of useful papers regarding cryptography and cryptanalysis. This paper is meant as a tutorial on the fundamentals of differential cryptanalysis and linear cryptanalysis of a Feistel cipher.

Several recently proposed ciphers are built with layers of small S-boxes, interconnected by linear key-dependent layers. Their security relies on the fact, that the classical methods of cryptanalysis (e.g. linear or differential attacks) are based on probabilistic characteristics, which makes their security grow exponentially with the number of rounds Nr. In this paper we study the security of such ciphers under an additional hypothesis: the S-box can be described by an overdefined system of algebraic equations (true with probability 1). We show that this hypothesis is true for both Serpent (due to a small size of S-boxes) and Rijndael (due to unexpected algebraic properties). We study general methods known for solving overdefined systems of equations, such as XL from Eurocrypt'00, and show their inefficiency. Then we introduce a new method called XSL that uses the sparsity of the equations and their specific structure. The XSL attack is very powerful, but heuristic and it is very difficult to evaluate its complexity. The XSL attack has a parameter P, and in theory we show that P should be a constant. The XSL attack would then be polynomial in Nr, with a huge constant that is double-exponential in the size of the S-box. We demonstrated by computer simulations that the XSL attack works well enough on a toy cipher. It seems however that P will rather increase very slowly with Nr. More simulations are needed for bigger ciphers. Our optimistic evaluation shows that the XSL attack might be able to break Rijndael 256 bits and Serpent for key lengths 192 and 256 bits. However if only $P$ is increased by 2 (respectively 4) the XSL attack on Rijndael (respectively Serpent) would become slower than the exhaustive search. At any rate, it seems that the security of these ciphers does NOT grow exponentially with the number of rounds.

The notion of efficient computation is usually identified in cryptography and complexity with (strict) probabilistic polynomial time. However, until recently, in order to obtain *constant-round* zero-knowledge proofs and proofs of knowledge, one had to allow simulators and knowledge-extractors to run in time that is only polynomial *on the average* (i.e., *expected* polynomial time). Recently Barak gave the first constant-round zero-knowledge argument with a *strict* (in contrast to expected) polynomial-time simulator. The simulator in his protocol is a *non-black-box* simulator (i.e., it makes inherent use of the description of the *code* of the verifier). In this paper, we further address the question of strict polynomial-time in constant-round zero-knowledge proofs and arguments of knowledge. First, we show that there exists a constant-round zero-knowledge *argument of knowledge* with a *strict* polynomial-time *knowledge extractor*. As in the simulator of Barak's zero-knowledge protocol, the extractor for our argument of knowledge is not black-box and makes inherent use of the code of the prover. On the negative side, we show that non-black-box techniques are *essential* for both strict polynomial-time simulation and extraction. That is, we show that no (non-trivial) constant-round zero-knowledge proof or argument can have a strict polynomial-time *black-box* simulator. Similarly, we show that no (non-trivial) constant-round zero-knowledge proof or argument of knowledge can have a strict polynomial-time *black-box* knowledge extractor.

We introduce a new methodology for achieving security against adaptive chosen-ciphertext attack (CCA) for public-key encryption schemes, which we call the {\em oblivious decryptors model}. The oblivious decryptors model generalizes both the two-key model of Naor and Yung, as well the Cramer--Shoup encryption schemes. The key ingredient in our new paradigm is Sahai's notion of Simulation-Sound NIZK proofs. Our methodology is easy to use: First, construct an encryption scheme which satisfies the ``bare'' oblivious-decryptors model: This can be done quite easily, with simple proofs of security. Then, by adding a Simulation-Sound NIZK proof, the scheme becomes provably CCA-secure. Note that this paradigm allows for the use of {\em efficient} special-purpose Simulation-Sound NIZK proofs, such as those recently put forward by Cramer and Shoup. We also show how to present all known efficient (provably secure) CCA-secure public-key encryption schemes as special cases of our model.

The boomerang attack is a new and very powerful cryptanalytic technique. However, due to the adaptive chosen plaintext and ciphertext nature of the attack, boomerang key recovery attacks that retrieve key material on both sides of the boomerang distinguisher are hard to mount. We also present a method for using a boomerang distinguisher, which enables retrieving subkey bits on both sides of the boomerang distinguisher. The rectangle attack evolved from the boomerang attack.In this paper we present a new algorithm which improves the results of the rectangle attack. Using these improvements we can attack 3.5-round SC2000 with $2^{67}$ adaptive chosen plaintexts and ciphertexts, and 10-round Serpent with time complexity of $2^{173.8}$ memory accesses (which are equivalent to $2^{165.3}$ Serpent encryptions) with data complexity of $2^{126.3}$ chosen plaintexts.

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.

In (Partial) Key Escrow, how to monitoring a user securitly and efficiently is a very important problem. This paper initially proposes a monitoring scheme of a typical partial key escrow scheme. In this scheme, the escrowed key is not compromised even if the user is monitored for many times.

In a distributed digital signature scheme, a set of participants shares a secret information that allows them to compute a valid signature for a given message. These systems are said to be robust if they can tolerate the presence of some dishonest players. Up to now, all the proposed schemes consider only threshold structures: the tolerated subsets of corrupted players as well as the subsets of players who can sign a message are defined according to their cardinality. We propose a framework that is more general than the threshold one, considering a general access structure of players allowed to sign and a general family of dishonest players that the scheme can tolerate. If these general structures satisfy some combinatorial conditions, we can design a distributed and secure RSA signature scheme for this setting. Our construction is based on the threshold scheme of Shoup.

We propose an elliptic curve scheme over the ring $\entq$, which is efficient and semantically secure in the standard model. There appears to be no previous elliptic curve cryptosystem based on factoring that enjoys both of these properties. KMOV scheme has been used as an underlying primitive to obtain efficiency and probabilistic encryption. Semantic security of the scheme is based on a new decisional assumption, namely, the Decisional Small-$x$ $e$-Multiples Assumption. Confidence on this assumption is also discussed.

A {\em black-box} secret sharing scheme for the threshold access structure $T_{t,n}$ is one which works over any finite Abelian group $G$. Briefly, such a scheme differs from an ordinary linear secret sharing scheme (over, say, a given finite field) in that distribution matrix and reconstruction vectors are defined over the integers and are designed {\em independently} of the group $G$ from which the secret and the shares are sampled. This means that perfect completeness and perfect privacy are guaranteed {\em regardless} of which group $G$ is chosen. We define the black-box secret sharing problem as the problem of devising, for an arbitrary given $T_{t,n}$, a scheme with minimal expansion factor, i.e., where the length of the full vector of shares divided by the number of players $n$ is minimal. Such schemes are relevant for instance in the context of distributed cryptosystems based on groups with secret or hard to compute group order. A recent example is secure general multi-party computation over black-box rings. In 1994 Desmedt and Frankel have proposed an elegant approach to the black-box secret sharing problem based in part on polynomial interpolation over cyclotomic number fields. For arbitrary given $T_{t,n}$ with $0<t<n-1$, the expansion factor of their scheme is $O(n)$. This is the best previous general approach to the problem. Using low degree integral extensions of the integers over which there exists a pair of sufficiently large Vandermonde matrices with co-prime determinants, we construct, for arbitrary given $T_{t,n}$ with $0<t<n-1$ , a black-box secret sharing scheme with expansion factor $O(\log n)$, which we show is minimal.

Joux's protocol is a one round, tripartite key agreement protocol that is more bandwidth-efficient than any previous three-party key agreement protocol. But it is insecure, suffering from a simple man-in-the-middle attack. This paper shows how to make Joux's protocol secure, presenting several tripartite, authenticated key agreement protocols that still require only one round of communication. A pass-optimal authenticated and key confirmed tripartite protocol that generalises the station-to-station protocol is also presented. The security properties of the new protocols are studied using provable security methods and heuristic approaches. Applications for the protocols are also discussed.

We introduce the OAEP++ encoding method, which is an adaptation of the OAEP encoding method, replacing the last step of the encoding operation with an application of a block cipher such as AES. We demonstrate that if $f$ is a one-way trapdoor function that is hard to invert, then OAEP++ combined with $f$ is secure against an IND-CCA2 adversary in the random oracle model. Moreover, the security reduction is tight; an adversary against $f$-OAEP++ can be extended to an $f$-inverter with a running time linear in the number of oracle queries.

The aim of this work is to examine the relation between the notions of semantic security and indistinguishability against chosen ciphertext attacks. For this purpose, a new security notion called non-dividability is introduced independent of attack models, and is shown to be equivalent to both of the two notions. This result is expected to provide a clearer understanding of the equivalence between semantic security and indistinguishability under any form of attack.

In this paper we describe an elementary criterion to determine supersingular hyperelliptic curve of genus $2$, using only the given Weierstrass equation. Furthermore, we show that the discrete logarithm problem defined on any supersingular abelian variety of dimension $2$ over ${\mathbb F}_p, p>16,$ can be embedded to that over the extension field ${\mathbb F}_{p^{k}}$, with $k \leq 6.$ This implies that supersingular hyperelliptic curves are cryptographically weaker than the general case due to the Frey-R$\ddot{u}$ck attack. A family of the hyperelliptic curve $H/{\mathbb F}_p$ of the type $v^2=u^5+a$ and $v^2 = u^5 + au$ have been studied and further examples are listed.

We describe a parallel design principle for hash functions. Given a secure hash function $h:\{0,1\}^n\rightarrow \{0,1\}^m$ with $n\geq 2m$, and a binary tree of $2^t$ processors we show how to construct a secure hash function $h^{*}$ which can hash messages of lengths less than $2^{n-m}$ and a secure hash function $h^{\infty}$ which can hash messages of arbitrary length. The number of parallel rounds required to hash a message of length $L$ is $\lfloor\frac{L}{2^t}\rfloor+t+2$. Further, our algorithm is incrementally parallelizable in the following sense : given a digest produced using a binary tree of $2^t$ processors, we show that the same digest can also be produced using a binary tree of $2^{t^{\prime}}$ $(0\leq t^{\prime}\leq t)$ processors.

We address the problem of testing the hypothesis H_0 that the letters from some alphabet A= {a_1,a_2,..., a_k }, are distributed uniformly against the alternative hypothesis H_1 that the true distribution is not uniform, in case k is large. (It is typical for random number testing and some cryptographic problems where k= 2^{10} - 2^{30} and more). In such a case it is difficult to use the chi-square test because the sample size must be greater than k. We suggest the adaptive chi-square test which can be successfully applied for testing some kinds of H_1 even in case when the sample size is much less than k. This statement is confirmed theoretically and experimentally. The theoretical proof is based on the consideration of one kind of the alternative hypothesis H_1 where the suggested test rejects the null hypothesis when the sample size is O( \sqrt{k} ) (instead of const k for the usual chi-square test ). For experimental investigation of the suggested test we consider a problem of testing ciphered Russian texts. It turns out that the suggested test can distinguish the ciphered texts from random sequences basing on a sample which is much smaller than that required for the usual chi-square test.

We present a new protocol for efficient distributed computation modulo a shared secret. We further present a protocol to distributively generate a random shared prime or safe prime that is much more efficient than previously known methods. This allows to distributively compute shared RSA keys, where the modulus is the product of two safe primes, much more efficiently than was previously known.

In this paper we propose a universal forgery attack of Hess's second ID-based signature scheme against the known-message attack.

We describe very efficient protocols for non-malleable (interactive) proofs of plaintext knowledge for the RSA, Rabin, Paillier, and El-Gamal encryption schemes whose security can be proven in the standard model. We also highlight some important applications of these protocols, where we take care to ensure that our protocols remain secure when run in an asynchronous, concurrent environment: --- Chosen-ciphertext-secure, interactive encryption: In some settings where both parties are on-line (e.g., SSL), an interactive encryption protocol may be used. We construct chosen-ciphertext-secure interactive encryption schemes based on any of the schemes above. In each case, the improved scheme requires only a small overhead beyond the original, semantically-secure scheme. --- Password-based authenticated key exchange: We provide efficient protocols for password-based authenticated key exchange in the public- key model \cite{HK98,B99}. Security of our protocols may be based on any of the cryptosystems mentioned above. --- Deniable authentication: We demonstrate deniable authentication protocols satisfying the strongest notion of security. These are the first efficient constructions based on, e.g., the RSA or computational Diffie-Hellman assumptions. Our techniques provide a general methodology for constructing efficient \emph{non-malleable} (zero-knowledge) proofs of knowledge when shared parameters are available (for our intended applications, these parameters can simply be included as part of users' public keys). Thus, non-malleable proofs of knowledge are easy to achieve ``in practice''.

Proved here is the sufficiency of certain conditions to ensure the Elliptic Curve Digital Signature Algorithm (ECDSA) existentially unforgeable by adaptive chosen-message attacks. The sufficient conditions include (i) a uniformity property and collision-resistance for the underlying hash function, (ii) pseudo-randomness in the private key space for the ephemeral private key generator, (iii) generic treatment of the underlying group, and (iv) a further condition on how the ephemeral public keys are mapped into the private key space. For completeness, a brief survey of necessary security conditions is also given. Some of the necessary conditions are weaker than the corresponding sufficient conditions used in the security proofs here, but others are identical. Despite the similarity between DSA and ECDSA, the main result is not appropriate for DSA, because the fourth condition above seems to fail for DSA. (The corresponding necessary condition is plausible for DSA, but is not proved here nor is the security of DSA proved assuming this weaker condition.) Brickell et al., Jakobsson et al. and Pointcheval et al. only consider signature schemes that include the ephemeral public key in the hash input, which ECDSA does not do, and moreover, assume a condition on the hash function stronger than the first condition above. This work seems to be the first advance in the provable security of ECDSA.

We propose a new technique for making mix nets robust, called randomized partial checking (RPC). The basic idea is that rather than providing a proof of completely correct operation, each server provides strong evidence of its correct operation by revealing a pseudo-randomly selected subset of its input/output relations. Randomized partial checking is exceptionally efficient compared to previous proposals for providing robustness; the evidence provided at each layer is shorter than the output of that layer, and producing the evidence is easier than doing the mixing. It works with mix nets based on any encryption scheme (i.e., on public-key alone, and on hybrid schemes using public-key/symmetric-key combinations). It also works both with Chaumian mix nets where the messages are successively encrypted with each servers' key, and with mix nets based on a single public key with randomized re-encryption at each layer. Randomized partial checking is particularly well suited for voting systems, as it ensures voter privacy and provides assurance of correct operation. Voter privacy is ensured (either probabilistically or cryptographically) with appropriate design and parameter selection. Unlike previous work, our work provides voter privacy as a global property of the mix net rather than as a property ensured by a single honest server. RPC-based mix nets also provide very high assurance of a correct election result, since a corrupt server is very likely to be caught if it attempts to tamper with even a couple of ballots.

In this paper we investigate the timed release of standard digital signatures, and demonstrate how to do it for RSA, Schnorr and DSA signatures. Such signatures, once released, cannot be distinguished from signatures of the same type obtained without a timed release, making it transparent to an observer of the end result. While previous work has allowed timed release of signatures, these have not been standard, but special-purpose signatures. Building on the recent work by Boneh and Naor on timed commitments, we introduce the notion of a reusable time-line, which, besides allowing the release of standard signatures, lowers the session costs of existing timed applications.

We introduce a novel technique for computation of consecutive preimages of hash chains. Whereas traditional techniques have a memory-times-computation complexity of $O(n)$ per output generated, the complexity of our technique is only $O(log^2 \, n)$, where $n$ is the length of the chain. Our solution is based on the same principal amortization principle as \cite{J01}, and has the same asymptotic behavior as this solution. However, our solution decreases the real complexity by approximately a factor of two. Thus, the computational costs of our solution are approximately $ {1 \over 2} log_2 \, n$ hash function applications, using only a little more than $log_2 \, n$ storage cells. A result of independent interest is the lower bounds we provide for the optimal (but to us unknown) solution to the problem we study. The bounds show that our proposed solution is very close to optimal. In particular, we show that there exists no improvement on our scheme that reduces the complexity by more than an approximate factor of two.

The Fiat-Shamir paradigm for transforming identification schemes into signature schemes has been popular since its introduction because it yields efficient signature schemes, and has been receiving renewed interest of late as the main tool in deriving forward-secure signature schemes. We find minimal (meaning necessary and sufficient) conditions on the identification scheme to ensure security of the signature scheme in the random oracle model, in both the usual and the forward-secure cases. Specifically we show that the signature scheme is secure (resp. forward-secure) against chosen-message attacks in the random oracle model if and only if the underlying identification scheme is secure (resp. forward-secure) against impersonation under passive (i.e.. eavesdropping only) attacks, and has its commitments drawn at random from a large space. An extension is proven incorporating a random seed into the Fiat-Shamir transform so that the commitment space assumption may be removed.

For independent binary random variables x_1,...,x_n and a Boolean function f(x), x=(x_1,...,x_n), we suppose that |1/2 - P{x_i = 1}|<=e, 1<=i<=n. Under these conditions we present new characteristics D_F(f(),e) = max{|1/2 - P{y=1}|} of the probability properties of Boolean functions, where y = F(x), and F(x) being equal to 1) F(x)=f(x), 2) F(x)=f(x)+(a,x), 3) F(x)=f(x)+f(x+a), and investigate their properties. Special attention is paid to the classes of balanced and correlation immune functions, bent functions, and second order functions, for which upper estimates of D_F(f(),e) are found and statements on behaviour of sequences f^{(n)}(x) of functions of n arguments are made.

We describe a cryptanalytical technique for distinguishing some stream ciphers from a truly random process. Roughly, the ciphers to which this method applies consist of a ``non-linear process'' (say, akin to a round function in block ciphers), and a ``linear process'' such as an LFSR (or even fixed tables). The output of the cipher can be the linear sum of both processes. To attack such ciphers, we look for any property of the ``non-linear process'' that can be distinguished from random. In addition, we look for a linear combination of the linear process that vanishes. We then consider the same linear combination applied to the cipher's output, and try to find traces of the distinguishing property. In this report we analyze two specific ``distinguishing properties''. One is a linear approximation of the non-linear process, which we demonstrate on the stream cipher SNOW. This attack needs roughly $2^{95}$ words of output, with work-load of about $2^{100}$. The other is a ``low-diffusion'' attack, that we apply to the cipher Scream-0. The latter attack needs only about $2^{43}$ bytes of output, using roughly $2^{50}$ space and $2^{80}$ time.

We report on the design of Scream, a new software-efficient stream cipher, which was designed to be a ``more secure SEAL''. Following SEAL, the design of Scream resembles in many ways a block-cipher design. The new cipher is roughly as fast as SEAL, but we believe that it offers a significantly higher security level. In the process of designing this cipher, we re-visit the SEAL design paradigm, exhibiting some tradeoffs and limitations.

In this paper we propose an identity(ID)-based signature scheme using gap Diffie-Hellman (GDH) groups. Our scheme is proved secure against existential forgery on adaptively chosen message and ID attack under the random oracle model. Using GDH groups obtained from bilinear pairings, as a special case of our scheme, we obtain an ID-based signature scheme that shares the same system parameters and the same private/public key pairs with the ID-based encryption scheme (BF-IBE) by Boneh and Franklin, and is as efficient as the BF-IBE. Combining our signature scheme with the BF-IBE yields a complete solution of an ID-based public key system. It can be an alternative for certificate-based public key infrastructures, especially when efficient key management and moderate security are required.

We discuss a modification of the Cramer-Shoup strong-RSA signature scheme. Our proposal also presumes the strong RSA assumption (and a collision-intractable hash function for long messages), but -without loss in performance- the size of a signature is almost halved compared to the original scheme. We also show how to turn the signature scheme into a "lightweight" anonymous (but linkable) group identification protocol without random oracles.

Motivated by emerging needs in online interactions, we define a new type of digital signature called a `Content Extraction Signature' (CES). A CES allows the owner, Bob, of a document signed by Alice, to produce an `extracted signature' on selected extracted portions of the original document, which can be verified to originate from Alice by any third party Cathy, while hiding the unextracted (removed) document portions. The new signature therefore achieves verifiable content extraction with minimal multi-party interaction. We specify desirable functional and security requirements for a CES (including an efficiency requirement: a CES should be more efficient in either computation or communication than the simple multiple signature solution). We propose and analyze four CES constructions which are provably secure with respect to known cryptographic assumptions and compare their performance characteristics.

In time of internet attacks is important to use cryptographic protcols and algorithms to secure private data that are sent via Internet. But using of such protocol is not enough. To really secure our data we must know that used protocol is secure. For this purpose where a lot of methods design such as well-known BAN logic. In this articel we want to present DLA (Database and Logic Abduction)- method that is used to prove that a security or cryptographic protocol is secure or it is possible perform an attack.

One-time signature schemes have found numerous applications: in ordinary, on-line/off-line, and forward-secure signatures. More recently, they have been used in multicast and broadcast authentication. We propose a one-time signature scheme with very efficient signing and verifying, and short signatures. Our scheme is well-suited for broadcast authentication, and, in fact, can be viewed as an improvement of the BiBa one-time signature (proposed by Perrig in CCS 2001 for broadcast authentication).

We study the problem of root extraction in finite Abelian groups, where the group order is unknown. This is a natural generalization of the problem of decrypting RSA ciphertexts. We study the complexity of this problem for generic algorithms, that is, algorithms that work for any group and do not use any special properties of the group at hand. We prove an exponential lower bound on the generic complexity of root extraction, even if the algorithm can choose the "public exponent" itself. In other words, both the standard and the strong RSA assumption are provably true w.r.t. generic algorithms. The results hold for arbitrary groups, so security w.r.t. generic attacks follows for any cryptographic construction based on root extracting. As an example of this, we revisit Cramer-Shoup signature scheme. We modify the scheme such that it becomes a generic algorithm. This allows us to implement it in RSA groups without the original restriction that the modulus must be a product of safe primes. It can also be implemented in class groups. In all cases, security follows from a well defined complexity assumption (the strong root assumption), without relying on random oracles, and the assumption is shown to be true w.r.t. generic attacks.

We describe general exponent group signature schemes and show how these naturally give rise to identity based signature schemes if pairings are used. We prove these schemes to be secure in the random oracle model. Furthermore we describe a particular identity based signature scheme which is quite efficient in terms of bandwidth and computing time, and we develop a further scheme which is not derived from a general exponent group signature scheme. The realization of these schemes uses supersingular elliptic curves and the Tate pairing, which is more efficient than the Weil pairing. Finally we show that these schemes have a more natural solution, than Shamir's original scheme, to the ``escrow'' property that all identity based signature schemes suffer from.

This paper considers arbitrary-length chosen-ciphertext secure asymmetric encryption, thus addressing what is actually needed for a practical usage of strong public-key cryptography in the real world. We put forward two generic constructions, gem-1 and gem-2, which apply to explicit fixed-length weakly secure primitives and provide a strongly secure (IND-CCA2) public-key encryption scheme for messages of unfixed length (typically computer files). Our techniques optimally combine a single call to any one-way trapdoor function with repeated encryptions through some weak block-cipher (a simple xor is fine) and hash functions of fixed-length input so that a minimal number of calls to these functions is needed. Our encryption/decryption throughputs are comparable to the ones of standard methods (asymmetric encryption of a session key + symmetric encryption with multiple modes). In our case, however, we formally prove that our designs are secure in the strongest sense and provide complete security reductions holding in the random oracle model.

This paper describes malicious applets that use Java's sophisticated graphic features to rectify the browser's padlock area and cover the address bar with a false https domain name. The attack was successfully tested on Netscape's Navigator and Microsoft's Internet Explorer; we consequently recommend to neutralize Java whenever funds or private data transit via these browsers and patch the flaw in the coming releases. The degree of novelty of our attack is unclear since similar (yet non-identical) results can be achieved by spoofing as described by Felten; however our scenario is much simpler to mount as it only demands the inclusion of an applet in the attacker's web page. In any case, we believe that the technical dissection of our malicious Java code has an illustrative value in itself.

Secure and reliable group communication is an active area of research. Its popularity is caused by the growing importance of group-oriented and collaborative applications. The central research challenge is secure and efficient group key management. While centralized methods are often appropriate for key distribution in large multicast-style groups, many collaborative group settings require distributed key agreement techniques. This work investigates a novel group key agreement approach which blends so-called key trees with Diffie-Hellman key exchange. It yields a secure protocol suite (TGDH) that is both simple and fault-tolerant. Moreover, the efficiency of TGDH appreciably surpasses that of prior art.

We describe fast new algorithms to implement recent cryptosystems based on the Tate pairing. In particular, our techniques improve pairing evaluation speed by a factor of about 55 compared to previously known methods in characteristic 3, and attain performance comparable to that of RSA in larger characteristics. We also propose faster algorithms for scalar multiplication in characteristic 3 and square root extraction over $\GF{p^m}$, the latter technique being also useful in contexts other than that of pairing-based cryptography.

For speeding up elliptic curve scalar multiplication and making it secure against side-channel attacks such as timing or power analysis, various methods have been proposed using specifically chosen elliptic curves. We show that both goals can be achieved simultaneously even for conventional elliptic curves over $\mathbb{F}_p$. This result is shown via two facts. First, we recall the known fact that every elliptic curve over $\mathbb{F}_p$ admits a scalar multiplication via a (Montgomery ladder) Lucas chain. As such chains are known to be resistant against timing- and simple power/electromagnetic radiation analysis attacks, the security of our scalar multiplication against timing and simple power/electromagnetic radiation analysis follows. Second, we show how to parallelize the 19 multiplications within the resulting \lq\lq double" and \lq\lq add" formulas of the Lucas chain for the scalar multiplication. This parallelism together with the Lucas chain results in 10 parallel field multiplications per bit of the scalar. Finally, we also report on a concrete successful implementation of the above mentioned scalar multiplication algorithm on a very recently developed and commercially available coprocessor for smart cards.

For certain security applications, including identity based encryption and short signature schemes, it is useful to have abelian varieties with security parameters that are neither too small nor too large. Supersingular abelian varieties are natural candidates for these applications. This paper determines exactly which values can occur as the security parameters of supersingular abelian varieties (in terms of the dimension of the abelian variety and the size of the finite field), and gives constructions of supersingular abelian varieties which are optimal for use in cryptography.

Filiol and Fontaine recently proposed a family of stream ciphers named COS. COS is based on nonlinear feedback shift registers and was claimed to be with high cryptographic strength. Babbage showed that COS $(2,128)$ Mode II is extremely weak. But Babbage's attack is too expensive to break the COS $(2,128)$ Mode I (the complexity is around $2^{52}$). In this paper, we show that the COS $(2,128)$ Mode I is too weak. With about $2^{16}$-bit known plaintext, the secret information could be recovered with small amount of memory and computation time (less than one second on a Pentium IV Processor).

We present an efficient identity-based signature scheme which makes use of bilinear pairings on elliptic curves. Our scheme is similar to the generalized ElGamal signature scheme. We consider the security of our scheme.

This report surveys on a series of Square attacks on reduced-round versions of the Skipjack block cipher. {\bf Skipjack} is an iterated block cipher encrypting 64-bit plaintext blocks into 64-bit ciphertext blocks, using an 80-bit key. Its design is based on a generalized Feistel Network making up 32 rounds of two different types. This cipher was developed by the National Security Agency for the Clipper chip and Fortezza PC card.

In the literature, voting protocols are considered secure if they satisfy requirements such as privacy, accuracy, robustness, etc. It can be time consuming to evaluate a voting protocol with respect to all these requirements and it is not clear that the list of known requirements is complete. Perhaps because of this many papers on electronic voting do not offer any security proof at all. As a solution to this, we suggest evaluating voting schemes in the universal composability framework. We investigate the popular class of voting schemes based on homomorphic threshold encryption. It turns out that schemes in this class realize an ideal voting functionality that takes the votes as input and outputs the result. This ideal functionality corresponds closely to the well-known ballot box model used today in manual voting. Security properties such as privacy, accuracy and robustness now follow as easy corollaries. We note that some security requirements, for instance incoercibility, are not addressed by our solution. Security holds in the random oracle model against a non-adaptive adversary. We show with a concrete example that the schemes are not secure against adaptive adversaries. We proceed to sketch how to make them secure against adaptive adversaries in the erasure model with virtually no loss of efficiency. We also sketch how to achieve security against adaptive adversaries in the erasure-free model.

We introduce a novel amortization technique for computation of consecutive preimages of hash chains, given knowledge of the seed. While all previously known techniques have a memory-times-computational complexity of $O(n)$ per chain element, the complexity of our technique can be upper bounded at $O(log^2 n)$, making it a useful primitive for low-cost applications such as authentication, signatures and micro-payments. Our technique uses a logarithmic number of {\em pebbles} associated with points on the hash chain. The locations of these pebbles are modified over time. Like fractals, where images can be found within images, the pebbles move within intervals and sub-intervals according to a highly symmetric pattern.