*04:59*[PhD][New] Johannes Buchmann

Name: Johannes Buchmann

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In the classical model of traitor tracing, one assumes that a traitor contributes its entire secret key to build a pirate decoder. However, new practical scenarios of pirate has been considered, namely Pirate Evolution Attacks at Crypto 2007 and Pirates 2.0 at Eurocrypt 2009, in which pirate decoders could be built from sub-keys of users. The key notion in Pirates 2.0 is the anonymity level of traitors: they can rest assured to remain anonymous when each of them only contributes a very small fraction of its secret information. This scenario encourages dishonest users to participate in collusion and the size of collusion could become very large, possibly beyond the considered threshold in the classical model. There are numerous attempts to deal with Pirates 2.0 each of which only considers a particular form of Pirates 2.0. In this paper, we propose a method for fighting Pirates 2.0 in any form.

Our method is based on the researches in key-leakage resilience. It thus gives an interesting and rather surprised connection between the rich domain of key-leakage resilient cryptography and Pirates 2.0. We first formalize the notion of key-leakage resilient revoke system and then identify sufficient conditions so that a key-leakage resilient revoke scheme can resist Pirates 2.0 in any form. We finally propose a construction of a secure key-leakage resilient identity-based revoke system that fulfills the required conditions. The main ingredient in the construction relies on the identity-based encryption with wildcards ($\\WIBE$) and our construction of key-leakage resilient $\\WIBE$ could be useful in its own right.

In this paper, we investigate on threshold proofs, a framework for distributing the prover\'s side of

interactive proofs of knowledge over multiple parties. Interactive proofs of knowledge (PoK) are widely used

primitives of cryptographic protocols, including important user-centric protocols, such as identification schemes,

electronic cash (e-cash), and anonymous credentials.

We present a security model for threshold proofs of knowledge and develop threshold versions of well-known

primitives such as range proofs, zero-knowledge proofs for preimages of homomorphisms (which generalizes PoKs

of discrete logarithms, representations, p-th roots, etc.), as well as OR statements. These building blocks are proven

secure in our model.

Furthermore, we apply the developed primitives and techniques in the context of user-centric protocols. In particular,

we construct distributed-user variants of Brands\' e-cash system and the bilinear anonymous credential scheme by

Camenisch and Lysyanskaya. Distributing the user party in such protocols has several practical advantages: First, the

security of a user can be increased by sharing secrets and computations over multiple devices owned by the user. In

this way, losing control of a single device does not result in a security breach. Second, this approach also allows

groups of users to jointly control an application (e.g., a joint e-cash account), not giving a single user full control.

The distributed versions of the protocols we propose in this paper are relatively efficient (when compared to a general

MPC approach). In comparison to the original protocols only the prover\'s (or user\'s) side is modified while the other

side stays untouched. In particular, it is oblivious to the other party whether it interacts with a distributed prover (or

user) or one as defined in the original protocol.

Broadcast encryption aims at sending a content to a large arbitrary group of users at once. Currently, the most efficient schemes provide constant-size headers, that encapsulate ephemeral session keys under which the payload is encrypted. However, in practice, and namely for pay-TV, providers have to send various contents to different groups of users. Headers are thus specific to each group, one for each channel: as a consequence, the global overhead is linear in the number of channels. Furthermore, when one wants to zap to and watch another channel, one has to get the new header and decrypt it to learn the new session key: either the headers are sent quite frequently or one has to store all the headers, even if one watches one channel only. Otherwise, the zapping time becomes unacceptably long.

In this paper, we consider encapsulation of several ephemeral keys, for various groups and thus various channels, in one header only, and we call this new primitive Multi-Channel Broadcast Encryption: one can hope for a much shorter global overhead and a short zapping time since the decoder already has the information to decrypt any available channel at once. Our candidates are private variants of the Boneh-Gentry-Waters scheme, with a constant-size global header, independently of the number of channels. In order to prove the CCA security of the scheme, we introduce a new dummy-helper technique and implement it in the random oracle model.

Verified security provides a firm foundation for cryptographic proofs

by means of rigorous programming language techniques and verification

methods. EasyCrypt is a framework that realizes the verified security

paradigm and supports the machine-checked construction and

verification of cryptographic proofs using state-of-the-art SMT

solvers, automated theorem provers and interactive proof assistants.

Previous experiments have shown that EasyCrypt is effective for a

posteriori validation of cryptographic systems. In this paper, we

report on the first application of verified security to a novel

cryptographic construction, with strong security properties and

interesting practical features. Specifically, we use EasyCrypt to

prove the IND-CCA security of a redundancy-free public-key encryption

scheme based on trapdoor one-way permutations. Somewhat surprisingly,

we show that even with a zero-length redundancy, Boneh\'s SAEP scheme

(an OAEP-like construction with a single-round Feistel network rather

than two) converts a trapdoor one-way permutation into an

IND-CCA-secure scheme, provided the permutation satisfies two

additional properties. We then prove that the Rabin function and RSA

with short exponent enjoy these properties, and thus can be used to

instantiate the construction we propose to obtain efficient encryption

schemes. The reduction that justifies the security of our construction

is tight enough to achieve practical security with reasonable key

sizes.

Elliptic curve cryptosystems have improved greatly in speed over the past few years. In this paper we outline a new elliptic curve signature and key agreement implementation which achieves record speeds while remaining relatively compact. For example, on Intel Sandy Bridge, a curve with about $2^{250}$ points produces a signature in just under 60k clock cycles, verifies in under 169k clock cycles, and computes a Diffie-Hellman shared secret in under 153k clock cycles. Our implementation has a small footprint: the library is under 55kB. We also post competitive timings on ARM processors, verifying a signature in under 626k Tegra-2 cycles. We introduce faster field arithmetic, a new point compression algorithm, an improved fixed-base scalar multiplication algorithm and a new way to verify signatures without inversions or coordinate recovery. Some of these improvements should be applicable to other systems.

In this paper, we specify a class of mathematical problems, which we refer to as ``Function Density Problems\'\' (FDPs, in short), and point out novel connections of FDPs to the following two cryptographic topics; theoretical security evaluations of keyless hash functions (such as SHA-1), and constructions of provably secure pseudorandom generators (PRGs) with some enhanced security property introduced by Dubrov and Ishai [STOC 2006]. Our argument aims at proposing new theoretical frameworks for these topics (especially for the former) based on FDPs, rather than providing some concrete and practical results on the topics. We also give some examples of mathematical discussions on FDPs, which would be of independent interest from mathematical viewpoints. Finally, we discuss possible directions of future research on other cryptographic applications of FDPs and on mathematical studies on FDPs themselves.

We construct the first public-key encryption scheme whose chosen-ciphertext (i.e., IND-CCA) security can be proved under a standard assumption and does not degrade in either the number of users or the number of ciphertexts. In particular, our scheme can be safely deployed in unknown settings in which no a-priori bound on the number of encryptions and/or users is known.

As a central technical building block, we devise the first structure-preserving signature scheme with a tight security reduction. (This signature scheme may be of independent interest.) Combining this scheme with Groth-Sahai proofs yields a tightly simulation-sound non-interactive zero-knowledge proof system for group equations. If we use this proof system in the Naor-Yung double encryption scheme, we obtain a tightly IND-CCA secure public-key encryption scheme from the Decision Linear assumption.

We point out that our techniques are not specific to public-key encryption security. Rather, we view our signature scheme and proof system as general building blocks that can help to achieve a tight security reduction.

Recently, Chien et al. proposed a gateway-oriented password-based authenticated key exchange (GPAKE) protocol, through which a client and a gateway could generate a session key for future communication with the help of an authentication server. They also demonstrated that their scheme is provably secure in a formal model. However, in this letter, we will show that Chien et al.\'s protocol is vulnerable to the off-line password guessing attack. To overcome the weakness, we also propose an efficient countermeasure.

The concept of proxy signature was introduced in 1996, up to now many proxy signature schemes have been proposed. In order to protect the proxy signer\'s privacy, the concept of anonymous proxy signature, which is also called proxy ring signature, was introduced in 2003. Some anonymous proxy signature schemes, which are provable secure in the random oracle model, have been proposed. However, provable security in the random oracle model is doubtful when the random oracles are instantiated with hash functions in their implementation. Hence, we propose the first secure anonymous proxy signature scheme without random oracles.

The nonlinear feedback shift registers (NLFSR) are used to construct pseudorandom generators for stream ciphers. Their theory is not so complete as that of the linear feedback shift registers (LFSR). In general, it is not known how to construct NLFSRs with maximum period. The direct method is to search for such registers with suitable properties. We used the implementation of NLFSRs in Field Programmable Gate Arrays (FPGA) to perform a corresponding search. We also investigated local statistical properties of the binary sequences ganerated by NLFSRs of order 25 and 27.