*21:17* [Pub][ePrint]
Cryptanalysis and Security Enhancement of Two Advanced Authentication Protocols, by Sai Raghu Talluri and Swapnoneel Roy
In this work we consider two protocols for performing cryptanalysis and security enhancement. The first one by Jiang et al., is a password-based authentication scheme which does not use smart cards. We note that this scheme is an improvement over Chen et al.\'s scheme shown vulnerable to the off-line dictionary attack by Jiang et al. We perform a cryptanalysis on Jiang at al.\'s improved protocol and observe that it is prone to the clogging attack, a kind of denial of service (DoS) attack. We then suggest an improvement on the protocol to prevent the clogging attack.The other protocol we consider for analysis is by Wang et al. This is a smart card based authentication protocol. We again perform the clogging (DoS) attack on this protocol via replay. We observe that all smart card based authentication protocols which precede the one by Wang et al., and require the server to compute the computationally

intensive modular exponentiation are prone to the clogging attack. We suggest (another) improvement on the protocol to prevent the clogging attack, which also applies to the protocol by Jiang et. al.

*10:43* [Job][New]
Ph.D. students, *TELECOM-ParisTech*
TELECOM-ParisTech crypto group seeks 4 PhD studentsTELECOM-ParisTech crypto group develops prototype solutions to fight against cyber and physical penetration of embedded devices.

Our contributions in this field of research are:

We seek four PhD candidates on those subjects:

- \\\"Calculer dans les codes comme contremesure aux attaques physiques\\\", https://edite-de-paris.fr/spip/spip.php?page=phdproposal&id=10213751
- \\\"Native Protection of Processors against Cyber-Attacks\\\", https://edite-de-paris.fr/spip/spip.php?page=phdproposal&id=10253861
- \\\"Insertion automatique de contre-mesures dans des circuits de sécurité\\\", https://edite-de-paris.fr/spip/spip.php?page=phdproposal&id=10213779
- \\\"Attaques FIRE : Rétroconception de cryptographie secrète\\\", https://edite-de-paris.fr/spip/spip.php?page=phdproposal&id=10166999

Working language is French or English.

Positions are open until Aug 2014.

To ap

*21:17* [Pub][ePrint]
A Second Look at Fischlin\'s Transformation, by Özgür Dagdelen and Daniele Venturi
Fischlin\'s transformation is an alternative to the standard Fiat-Shamir transform to turn a certain class of public key identification schemes into digital signatures (in the random oracle model).We show that signatures obtained via Fischlin\'s transformation are existentially unforgeable even in case the adversary is allowed to get arbitrary (yet bounded) information on the entire state of the signer (including the signing key and the random coins used to generate signatures). A similar fact was already known for the Fiat-Shamir transform, however, Fischlin\'s transformation allows for a significantly higher leakage parameter than Fiat-Shamir.

Moreover, in contrast to signatures obtained via Fiat-Shamir, signatures obtained via Fischlin enjoy a tight reduction to the underlying hard problem. We use this observation to show (via simulations) that Fischlin\'s transformation, usually considered less efficient, outperforms the Fiat-Shamir transform in verification time for a reasonable choice of parameters. In terms of signing Fiat-Shamir is faster for equal signature sizes. Nonetheless, our experiments show that the signing time of Fischlin\'s transformation becomes, e.g., 22% of the one via Fiat-Shamir if one allows the signature size to be doubled.

*21:17* [Pub][ePrint]
Practical Receipt-Free Sealed-Bid Auction in the Coercive Environment, by Jaydeep Howlader, Sanjit Kumar Roy, Ashis Kumar Mal
Sealed-Bid auction is an efficient and rational method toestablish the price in open market. However sealed-bid auctions are sub-

ject to bid-rigging attack. Receipt-free mechanisms were proposed to

prevent bid-rigging. The prior receipt-free mechanisms are based on two

assumptions; firstly, existence of untappable channel between bidders

and auction authorities. Secondly, mechanisms assume the authorities

to be honest (not colluding). Moreover the bandwidth required to com-

municate the receipt-free bids is huge. This paper presents a sealed-bid

auction mechanism to resist bid-rigging. The proposed method does not

assume untappable channel nor consider the authorities to be necessarily

honest. The proposed mechanism also manages the bandwidth efficiently,

and improves the performance of the system.

*21:17* [Pub][ePrint]
Two-sources Randomness Extractors for Elliptic Curves, by Abdoul Aziz Ciss
This paper studies the task of two-sources randomness extractors for elliptic curves defined over finite fields $K$, where $K$ can be a prime or a binary field. In fact, we introduce new constructions of functions over elliptic curves which take in input two random points from two differents subgroups. In other words, for a ginven elliptic curve $E$ defined over a finite field $\\mathbb{F}_q$ and two random points $P \\in \\mathcal{P}$ and $Q\\in \\mathcal{Q}$, where $\\mathcal{P}$ and $\\mathcal{Q}$ are two subgroups of $E(\\mathbb{F}_q)$, our function extracts the least significant bits of the abscissa of the point $P\\oplus Q$ when $q$ is a large prime, and the $k$-first $\\mathbb{F}_p$ coefficients of the asbcissa of the point $P\\oplus Q$ when $q = p^n$, where $p$ is a prime greater than $5$. We show that the extracted bits are close to uniform. Our construction extends some interesting randomness extractors for elliptic curves, namely those defined in \\cite{op} and \\cite{ciss1,ciss2}, when $\\mathcal{P} = \\mathcal{Q}$. The proposed constructions can be used in any cryptographic schemes which require extraction of random bits from two sources over elliptic curves, namely in key exchange protole, design of strong pseudo-random number generators, etc.