*21:17* [Pub][ePrint]
Packed Ciphertexts in LWE-based Homomorphic Encryption, by Zvika Brakerski and Craig Gentry and Shai Halevi
In this short note we observe that the Peikert-Vaikuntanathan-Waters (PVW) method of packing many plaintext elements in a single Regev-type ciphertext, can be used for performing SIMD homomorphic operations on packed ciphertext. This provides an alternative to the Smart-Vercauteren (SV) ciphertext-packing technique that relies on polynomial-CRT. While the SV technique is only applicable to schemes that rely on ring-LWE (or other hardness assumptions in ideal lattices), the PVW method can be used also for cryptosystems whose security is based on standard LWE (or more broadly on the hardness of ``General-LWE\'\').Although using the PVW method with LWE-based schemes leads to worse asymptotic efficiency than using the SV technique with ring-LWE schemes, the simplicity of this method may still offer some practical advantages. Also, the two techniques can be used in tandem with ``general-LWE\'\' schemes, suggesting yet another tradeoff that can be optimized for different settings.

*21:17* [Pub][ePrint]
Information Leakage of Continuous-Source Zero Secrecy Leakage Helper Data Schemes, by Joep de Groot and Boris Skoric and Niels de Vreede and Jean-Paul Linnartz
A Helper Data Scheme is a cryptographic primitive that extracts a high-entropy noise-free string from noisy data. Helper Data Schemes are used for privacy-preserving databases and for Physical Unclonable Functions.We refine the theory of Helper Data schemes with Zero Secrecy Leakage (ZSL), i.e. the mutual information between the helper data and the extracted secret is zero. We prove that ZSL necessitates particular properties of the helper data generating function, which also allows us to show the existence of `Sibling Points\'. In the special case that our generated secret is uniformly distributed (Fuzzy Extractors) our results coincide with the continuum limit of a recent construction by Verbiskiy et al. Yet our results cover secure sketches as well. Moreover we present an optimal reconstruction algorithm for this scheme, that not only provides the lowest possible reconstruction error rate but also yields an attractive, simple implementation of the verification.

Further, we introduce Diagnostic Category Leakage (DCL), which quantifies what an attacker can infer from helper data about a particular medical indication of the enrolled user, or reversely what probabilistic knowledge of a diagnose can leak about the secret. If the attacker has a priori knowledge about the enrolled user (medical indications, race, gender), then the ZSL property does not guarantee that there is no secrecy leakage from the helper data. However, this effect is typically very small.

*21:17* [Pub][ePrint]
Leakage Squeezing of Order Two, by Claude Carlet and Jean-Luc Danger and Sylvain Guilley and Houssem Maghrebi
In masking schemes, \\emph{leakage squeezing} is the study of the optimal shares\' representation, that maximizes the resistance order against high-order side-channel attacks.Squeezing the leakage of first-order Boolean masking has been problematized and solved previously in~\\cite{DBLP:conf/africacrypt/MaghrebiCGD12}.

The solution consists in finding a bijection $F$ that modifies the mask, in such a way that its graph, seen as a code, be of greatest dual distance.

This paper studies second-order leakage squeezing, \\emph{i.e.} leakage squeezing with two independent random masks.

It is proved that, compared to first-order leakage squeezing, second-order leakage squeezing at least increments (by one unit) the resistance against high-order attacks, such as high-order correlation power analyses (HO-CPA).

Now, better improvements over first-order leakage squeezing are possible by relevant constructions of the squeezing bijections pair.

We provide with linear bijections that improve by strictly more than one (instead of one) the resistance order.

Specifically,

when the masking is applied on bytes (which suits AES),

resistance against $1$st-order (resp. $2$nd-order) attacks is possible with one (resp. two) masks.

Optimal leakage squeezing with one mask resists HO-CPA of orders up to $5$.

In this paper, with two masks, we provide resistance against HO-CPA not only of order $5+1=6$, but also of order $7$.

*21:17* [Pub][ePrint]
On Transaction Pseudonyms with Implicit Attributes, by Stefan G. Weber
Transaction pseudonyms with implicit attributes are a novel approach to multilevel linkable transaction pseudonyms. We extend earlier work of Juels and Pappu on reencryption-based transaction pseudonyms, by developing new mechanisms for controlled pseudonym linkability.This includes mechanisms for cooperative, stepwise re-identication

as well as individual authentication of pseudonyms. Our proposal makes

use of efficient techniques from the area of secure multiparty computation and cryptographically secure PRNGs.

*15:17* [Pub][ePrint]
An Attack on a Fully Homomorphic Encryption Scheme, by Hu Yupu, Wang Fenghe
In this paper we present an attack on a fully homomorphic encryption scheme on PKC2010.We construct a modi¯ed secret key, a modi¯ed decryption algorithm and a subset of the ciphertext

space. When the ciphertext is from the subset, we can correctly decrypt it by our modi¯ed secret key

and modi¯ed decryption algorithm. We also discuss when our modi¯ed decryption algorithm is e±cient,

and when the subset is not negligible.

*15:17* [Pub][ePrint]
Aggregating CL-Signatures Revisited: Extended Functionality and Better Efficiency, by Kwangsu Lee and Dong Hoon Lee and Moti Yung
Aggregate signature is public-key signature that allows anyone to aggregate different signatures generated by different signers on different messages into a short aggregate signature. Although aggregate signatures have many applications like secure routing protocols and software module authentications, it is not easy to devise a suitable aggregate signature scheme that satisfies the conditions of real applications such that the size of public keys should be short, the size of aggregate signatures should be short, and the aggregate signing and aggregate verification should be efficient.In this paper, we propose two aggregate signature schemes based on the Camenisch-Lysyanskaya signature scheme that sufficiently satisfy the conditions of real applications. At first, we construct an efficient sequential aggregate signature scheme and prove its security under the LRSW assumption without random oracles. The proposed scheme has the shortest size of public keys among the sequential aggregate signature schemes and very short size of aggregate signatures. Additionally, the aggregate verification of this scheme is very efficient since it only requires constant number of pairing operations and $l$ number of exponentiations where $l$ is the number of signers in an aggregate signature. Next, we construct an efficient synchronized aggregate signature scheme and prove its security under the LRSW assumption in the random oracle model. The proposed scheme has very short size of public keys and the shortest size of aggregate signatures among the synchronized aggregate signature schemes. The signing and aggregate verification is also very efficient since these requires constant number of pairing operations and $l$ number of exponentiations.

Furthermore, a signer of our aggregate signature schemes can use two mode of aggregation ``sequential\'\' and ``synchronized\'\' at the same time just using the same private key and the public key since the private keys and the public keys of two schemes are the same.

*14:26* [Job][New]
Postdoc, *EPFL, Lausanne, Switzerland*
The Laboratory for Security and Cryptography (LASEC) at EPFL is hiring a post doctoral researcher. Applicants are encouraged to apply to *job_lasec (at) epfl.ch* by sending a detailed CV and a research plan.LASEC is active in research on cryptography and security. More specifically, our main interests span (but are not limited to) the following:

- Cryptographic analysis
- Design of cryptographic algorithms and protocols
- Lightweight cryptography
- Secure communication
- Wireless security
- Composability and setup assumptions
- Methodology and theory for cryptography
- Number theory and cryptography.

The selection of applicants will be made on a competitive basis.

Besides conducting top-quality research, postdocs are required to participate the the lab activities such as training students at all levels, running projects, fund raising, etc.

EPFL is a top-ranked research and teaching institution that attracts some of the best intellects in the world. EPFL offers excellent facilities, environment, and salaries. The EPFL campus is a multi cultural, idyllic spot overlooking Lake Geneva and facing the Alps.