International Association for Cryptologic Research

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2013-11-25
22:17 [Pub][ePrint]

Rabbit stream cipher is one of the finalists of eSTREAM

project which uses 128-bit secret keys. Prior to us, the attacks on Rabbit

has been all focused on the bias analysis and the best result showed the

distinguishing attack with complexity 2136. Our analysis in this paper,

is based on chosen IV analysis on reduced N-S round of Rabbit though

using multi cube tester. For this purpose we show for a mature cube

we could easily identify weak subcubes which increase the probability of

distinguishing for an unknown secret key. We also represent with 225

complexity, using one iteration of next state function the keystream is

completely distinguishable from random.

22:17 [Pub][ePrint]

We define a notion of semantic security of multi-linear

(a.k.a. graded) encoding schemes: roughly speaking, we require that if

an algebraic attacker (obeying the multi-linear restrictions) cannot tell

apart two constant-length sequences $\\vec{m}_0$, $\\vec{m}_1$ in the

presence of some other elements $\\vec{z}$, then

encodings of these sequences should be indistinguishable.

Assuming the existence of semantically secure multi-linear encodings

and the LWE assumption, we demonstrate the existence of

indistinguishability obfuscators for all polynomial-size circuits.

Additionally, if we assume an strengthening of

semantic security, our construction yields extractatability

obfuscators for all polynomial-size circuits.

We rely on the beautiful candidate obfuscation constructions

of Garg et al (FOCS\'13), Brakerski and Rothblum (TCC\'14) and Barak et

al (ePrint\'13) that were proven secure only in idealized generic

multilinear encoding models,

and develop new techniques for demonstrating security in the standard model, based only on

semantical security of multi-linear encoding (which trivially holds in

the generic multilinear encoding model).

22:17 [Pub][ePrint]

The Bitcoin scheme is one of the most popular and talked about alternative payment schemes. It was conceived in 2008 by the mysterious Satoshi Nakamoto, whose real identity remains unknown even

though his bitcoin holdings are believed to be worth several hundred

million dollars. One of the most active parts of the Bitcoin ecosystem was the Silk Road marketplace, in which highly illegal substances and services were traded. It was run by another mysterious person who called himself Dread Pirate Roberts (DPR), whose bitcoin holdings are also estimated to be worth hundreds of millions of dollars at today\'s exchange rate. On October 1-st 2013, the FBI arrested a 29 year old person named Ross William Ulbricht, claiming that he is DPR, and seizing a small fraction of his bitcoin wealth. In this paper we use the publicly available record to trace the evolution of his holdings in order to find how he acquired and how he tried to hide them from the authorities. For example, we show that all his income from the months of May, June and September 2013, along with numerous other amounts, were not seized by the FBI. One of the most surprising discoveries we made during our analysis was the existence of a recent substantial transfer (which was worth more than 60,000 dollars when made on March 20-th 2013, and close to a million dollars at today\'s exchange rate) which may link these two mysterious figures.

19:17 [Pub][ePrint]

In this paper we propose a new approach to code-based signatures that

makes use in particular of rank metric codes. When the classical approach consists in finding the unique

preimage of a syndrome through a decoding algorithm, we propose to introduce the notion

of mixed decoding of erasures and errors for building signature schemes.

In that case the difficult problem becomes, as is the case in lattice-based cryptography,

finding a preimage of weight above the Gilbert-Varshamov bound (case where

many solutions occur) rather than finding a unique preimage of weight below

the Gilbert-Varshamov bound. The paper describes RankSign: a

new signature algorithm for the rank metric

based on a new mixed algorithm for decoding erasures and errors for

the recently introduced Low Rank Parity Check (LRPC) codes.

We explain how it is possible (depending on choices

of parameters) to obtain a full decoding algorithm which is able

to find a preimage of reasonable rank weight for any random syndrome

with a very strong probability. We study the semantic security

of our signature algorithm and show how it is possible to reduce

the unforgeability to direct attacks on the public matrix, so that

no information leaks through signatures. Finally, we give several examples of parameters

for our scheme, some of which with public key of size $5760$ bits and signature of size $1728$ bits.

Moreover the scheme can be very fast for small base fields.

05:40 [Event][New]

Submission: 15 March 2014
From June 24 to June 26
Location: Putrajaya, Malaysia

2013-11-21
22:17 [Pub][ePrint]

Elliptic Curve Cryptography can be vulnerable to Side-Channel Attacks, such as the Zero Power Analysis (ZPA).

This attack takes advantage of the occurrence of special points that bring a zero-value when computing a doubling or an addition of points.

This paper consists in analysing this attack.

Some properties of the said special points are explicited.

A novel dynamic countermeasure is described.

The elliptic curve formul\\ae{} are updated depending on the elliptic curve and the provided base point.

22:17 [Pub][ePrint]

While the hybrid public key encryption scheme of Kurosawa and Desmedt (CRYPTO 2004) is provably secure against chosen ciphertext attacks (namely, IND-CCA-secure), its associated key encapsulation mechanism (KEM) is not IND-CCA-secure (Herranz et al. 2006, Choi et al. 2009). In this paper, we show a simple twist, yet neglected in the literature, on the Kurosawa-Desmedt KEM turning it into a scheme with IND-CCA security under the decisional Diffie-Hellman assumption. Our KEM beats the standardized version of Cramer-Shoup KEM in ISO/IEC 18033-2 by margins of around 30% in encapsulation speed, and 20% ~ 60% in decapsulation speed. Moreover, the public and secret key sizes in our schemes are at least 160-bit smaller than those of the Cramer-Shoup KEM. We then generalize the technique into hash proof systems, proposing several KEM schemes with IND-CCA security under decision linear and decisional composite residuosity assumptions respectively. All the KEMs are in the standard model, and use standard, computationally secure symmetric building blocks.

19:17 [Pub][ePrint]

This paper describes software optimization for the stream Cipher ChaCha. We leverage the wide vectorization capabilities of the new AVX2 architecture, to speed up ChaCha encryption (and decryption) on the latest x86_64 processors. In addition, we show how to apply vectorization for the future AVX512 architecture, and get further speedup. This leads to significant performance gains. For example, on the latest Intel Haswell microarchitecture, our AVX2 implementation performs at 1.43 cycles per byte (on a 4KB message), which is ~2x faster than the current implementation in the Chromium project.

19:17 [Pub][ePrint]

We explain the origins of Boolean feedback functions of nonlinear feedback shift registers (NLFSRs) of fixed order n generating de Bruijn binary sequences. They all come into existence by cross joining operations starting from one maximum period feedback shift register, e.g., a linear one which always exists for any order n. The result obtained yields some constructions of NLFSRs generating maximum period $2^n-1$ binary sequences.

19:17 [Pub][ePrint]

In this paper, we investigate the multi-user setting both in public-key and in secret-key cryptanalytic applications. In this setting, the adversary tries to recover keys of many users in parallel more efficiently than with classical attacks, \\textit{i.e.}, the number of recovered keys multiplied by the time complexity to find a single key, by amortizing the cost among several users. One possible scenario is to recover a single key in a large set of users more efficiently than to recover a key in the classical model. Another possibility is, after some shared precomputation, to be able to learn individual keys very efficiently. This latter model is close to traditional time/memory tradeoff attacks with precomputation. With these goals in mind, we introduce two new algorithmic ideas to improve collision-based attacks in the multi-user setting. Both ideas are derived from the parallelizable collision search as proposed by van~Oorschot and Wiener.

We recall that this collision search uses precomputed chains obtained by iterating some basic function. In our cryptanalytic application, each pair of merging chains can be used to correlate the key of two distinct users. The first idea is to construct a graph, whose vertices are keys and whose edges are these correlations. When the graph becomes connected, we simultaneously recover all the keys. Thanks to random graph analysis techniques, we can show that the number of edges that are needed to make this event occurs is small enough to obtain some improved attacks.

The second idea modifies the basic technique of van~Oorschot and Wiener: instead of waiting for two chains to merge, we now require that they become {\\it parallel}.

We first show that, using the first idea alone, we can recover the discrete logs of $L$ users in a group of size $N$ in time $\\widetilde{O}(\\sqrt{NL})$, without any special restriction on the value of $L$. As a first application of these two ideas put together, we show that in the multi-user Even-Mansour scheme, \\textit{all} the keys of $L=N^{1/3}$ users can be found with $N^{1/3+\\epsilon}$ queries for each user (where $N$ is the domain size). Finally, we consider the PRINCE block cipher (with 128-bit keys and 64-bit blocks) and find the keys of $2$ users among a set of $2^{32}$ users in time $2^{65}$. We also describe a new generic attack in the classical model for PRINCE that is better than all published attacks.

19:17 [Pub][ePrint]

Revocation and key evolving paradigms are central issues in cryptography, and in PKI in particular. A novel concern related to these areas was raised in the recent work of Sahai, Seyalioglu, and Waters (Crypto 2012) who noticed that revoking past keys should at times (e.g., the scenario of cloud storage) be accompanied by revocation of past ciphertexts (to prevent unread ciphertexts from being read by revoked users). They introduced revocable-storage attribute-based encryption (RS-ABE) as a good access control mechanism for cloud storage. RS-ABE protects against the revoked users not only the future data by supporting key-revocation but also the past data by supporting ciphertext-update, through which a ciphertext at time $T$ can be updated to a new ciphertext at time $T+1$ using only the public key.

Motivated by this pioneering work, we ask whether it is possible to have a modular approach, which includes a primitive for time managed ciphertext update as a primitive. We call encryption which supports this primitive a self-updatable encryption\'\' (SUE). We then suggest a modular cryptosystems design methodology based on three sub-components: a primary encryption scheme, a key-revocation mechanism, and a time-evolution mechanism which controls the ciphertext self-updating via an SUE method, coordinated with the revocation (when needed). Our goal in this is to allow the self-updating ciphertext component to take part in the design of new and improved cryptosystems and protocols in a flexible fashion. Specifically, we achieve the following results:

- We first introduce a new cryptographic primitive called self-updatable encryption (SUE), realizing a time-evolution mechanism. In SUE, a ciphertext and a private key are associated with time. A user can decrypt a ciphertext if its time is earlier than that of his private key. Additionally, anyone (e.g., a cloud server) can update the ciphertext to a ciphertext with a newer time. We also construct an SUE scheme and prove its full security under static assumptions.

- Following our modular approach, we present a new RS-ABE scheme with shorter ciphertexts than that of Sahai et al. and prove its security. The length efficiency is mainly due to our SUE scheme and the underlying modularity.

- We apply our approach to predicate encryption (PE) supporting attribute-hiding property, and obtain a revocable-storage PE (RS-PE) scheme that is selectively-secure.

- We further demonstrate that SUE is of independent interest, by showing it can be used for timed-release encryption (and its applications), and for augmenting key-insulated encryption with forward-secure storage.