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16:10 [Event][New] CHAE: Challenges in Authenticated Encryption

  Submission: 3 July 2015
From July 17 to July 17
Location: Utrecht, Netherlands
More Information:

09:17 [Pub][ePrint] Homomorphic Signature Schemes - A survey, by Giulia Traverso and Denise Demirel and Johannes Buchmann

  Homomorphic signature schemes are an important primitive for many applications and since their introduction numerous solutions have been presented. Thus, in this work we provide the first exhaustive, complete, and up-to-dated survey about the state of the art of homomorphic signature schemes. First, the general framework where homomorphic signatures are defined is described and it is shown how the currently available types of homomorphic signatures, these are the linearly homomorphic signature schemes, the homomorphic schemes supporting polynomial functions, the fully homomorphic signature schemes, and the homomorphic aggregate signature schemes, can then be derived from such a framework. In addition, this work also presents a description of each of the schemes presented so far together with the properties it provides. Furthermore, three use cases, electronic voting, smart grids, and electronic health records, where homomorphic signature schemes can be employed are described. For each of these applications the requirements that a homomorphic signature scheme should fulfill are defined and the suitable schemes already available are listed. This also highlights the shortcomings of current solutions. Thus, this work concludes with several ideas for future research in the direction of homomorphic signature schemes.

03:17 [Pub][ePrint] The Pythia PRF Service, by Adam Everspaugh and Rahul Chatterjee and Samuel Scott and Ari Juels and Thomas Ristenpart

  Conventional cryptographic services such as hardware-security modules and software-based key-management systems offer the ability to apply a pseudorandom function (PRF) such as HMAC to inputs of a client\'s choosing. These services are used, for example, to harden stored password hashes against offline brute-force attacks.

We propose a modern PRF service called PYTHIA designed to offer a level of flexibility, security, and ease- of-deployability lacking in prior approaches. The keystone of PYTHIA is a new cryptographic primitive called a verifiable partially-oblivious PRF that reveals a portion of an input message to the service but hides the rest. We give a construction that additionally supports efficient bulk rotation of previously obtained PRF values to new keys. Performance measurements show that our construction, which relies on bilinear pairings and zero-knowledge proofs, is highly practical. We also give accompanying formal definitions and proofs of security.

We implement PYTHIA as a multi-tenant, scalable PRF service that can scale up to hundreds of millions of distinct client applications on commodity systems. In our prototype implementation, query latencies are 15 ms in local-area settings and throughput is within a factor of two of a standard HTTPS server. We further report on implementations of two applications using PYTHIA, showing how to bring its security benefits to a new enterprise password storage system and a new brainwallet system for Bitcoin.

03:17 [Pub][ePrint] Improvements on Efficient Dynamic Provable Data Possession scheme with Public Verifiability and Data Privacy, by Cl\\\'{e}mentine Gritti, Willy Susilo, Thomas Plantard and Rongmao Chen

  An efficient Dynamic Provable Data Possession scheme with Public Verifiability and Data Privacy was recently published in ACISP\'15.

It appears that three attacks menace this scheme.

The first one enables the server to store only one block of a file $m$ and still pass the data integrity verification on any number of file blocks.

The second attack permits the server to keep the old version of a file block $m_{i}$ and the corresponding verification metadata $T_{m_{i}}$ after the client asked to modify them by sending the new version of these elements, and still pass the data integrity


The last attack allows the Third Party Auditor (TPA) to distinguish files when processing the data integrity checking.

In this paper, we propose several solution to overcome all the aforementioned issues.

For the two first attacks, we give two new constructions of the scheme, one using index-hash tables and the other based on the Merkle hash trees.

We compare the efficiency of these two new systems with the previous one.

For the third attack, we suggest a weaker security model for data privacy without modifying the current scheme and a new construction to enhance the security and to achieve the strongest data privacy notion.

03:17 [Pub][ePrint] Decomposition attack on SASASASAS, by Alex Biryukov and Dmitry Khovratovich

  We demonstrate the first attacks on the SPN ciphers with 6, 7, 8, and 9 secret components. In particular, we show a decomposition attack on the SASASASAS scheme when the S-box size $m$ and the block length $n$ satisfy the condition $m^2\\leq n$ (for example, 8-bit S-box and 128-bit block).

03:17 [Pub][ePrint] Efficient ephemeral elliptic curve cryptographic keys, by Andrea Miele and Arjen K. Lenstra

  We show how any pair of authenticated users can on-the-fly agree on an el- liptic curve group that is unique to their communication session, unpredictable to outside observers, and secure against known attacks. Our proposal is suitable for deployment on constrained devices such as smartphones, allowing them to efficiently generate ephemeral parameters that are unique to any single cryptographic application such as symmetric key agreement. For such applications it thus offers an alternative to long term usage of stan- dardized or otherwise pre-generated elliptic curve parameters, obtaining security against cryptographic attacks aimed at other users, and eliminating the need to trust elliptic curves generated by third parties.

03:17 [Pub][ePrint] Adaptive Proofs of Knowledge in the Random Oracle Model, by David Bernhard and Marc Fischlin and Bogdan Warinschi

  We formalise the notion of adaptive proofs of knowledge in the random oracle model,

where the extractor has to recover witnesses for multiple, possibly adaptively chosen

statements and proofs. We also discuss extensions to simulation soundness, as typically

required for the ``encrypt-then-prove\'\' construction of strongly secure encryption

from IND-CPA schemes.

Utilizing our model we show three results:

(1) Simulation-sound adaptive proofs exist.

(2) The ``encrypt-then-prove\'\' construction with a simulation-sound

adaptive proof yields CCA security. This appears to be a ``folklore\'\' result

but which has never been proven in the random oracle model. As a corollary, we

obtain a new class of CCA-secure encryption schemes.

(3) We show that the

Fiat-Shamir transformed Schnorr protocol is _not_ adaptively secure and

discuss the implications of this limitation.

Our result not only separates

adaptive proofs from proofs of knowledge, but also gives a strong hint why

Signed ElGamal as the most prominent encrypt-then-prove example has not been

proven CCA-secure without making further assumptions.

03:17 [Pub][ePrint] On the Hardness of Proving CCA-security of Signed ElGamal, by David Bernhard and Marc Fischlin and Bogdan Warinschi

  The well-known Signed ElGamal scheme consists of ElGamal

encryption with a non-interactive Schnorr proof of knowledge. While this

scheme should be intuitively secure against chosen-ciphertext attacks

in the random oracle model, its security has not yet been proven nor

disproven so far, without relying on further non-standard assumptions

like the generic group model. Currently, the best known positive result

is that Signed ElGamal is non-malleable under chosen-plaintext attacks.

In this paper we provide evidence that Signed ElGamal may not be CCA

secure in the random oracle model. That is, building on previous work of

Shoup and Gennaro (Eurocrypt\'98), Seurin and Treger (CT-RSA 2013),

and Bernhard et al. (PKC 2015), we exclude a large class of potential

reductions that could be used to establish CCA security of the scheme.

03:17 [Pub][ePrint] A New Encryption Standard of Ukraine: The Kalyna Block Cipher, by Roman Oliynykov and Ivan Gorbenko and Oleksandr Kazymyrov and Victor Ruzhentsev and Oleksandr Kuznetsov and Yurii Gorbenko and Oleksan

  The Kalyna block cipher was selected during Ukrainian National Public Cryptographic Competition (2007-2010) and its slight modification was approved as the new encryption standard of Ukraine in 2015. Main requirements for Kalyna were both high security level and high performance of software implementation on general-purpose 64-bit CPUs. The cipher has SPN-based (Rijndael-like) structure with increased MDS matrix size, a new set of four different S-boxes, pre- and postwhitening using modulo 2^{64} addition and a new construction of the key schedule. Kalyna supports block size and key length of 128, 256 and 512 bits (key length can be either equal or double of the block size). On the time of this paper publishing, no more effective cryptanalytic attacks than exhaustive search are known. In this paper we present the adapted English translated specification of Kalyna as it is given in the national standard of Ukraine.

03:17 [Pub][ePrint] Secure Execution Architecture based on PUF-driven Instruction Level Code Encryption, by Stephan Kleber and Florian Unterstein and Matthias Matousek and Frank Kargl and Frank Slomka and Matthias Hiller

  A persistent problem with program execution, despite numerous mitigation attempts, is its inherent vulnerability to the injection of malicious code. Equally unsolved is the susceptibility of firmware to reverse engineering, which undermines the manufacturer\'s code confidentiality. We propose an approach that solves both kinds of security problems employing instruction-level code encryption combined with the use of a physical unclonable function (PUF). Our novel Secure Execution PUF-based Processor (SEPP) architecture is designed to minimize the attack surface, as well as performance impact, and requires no significant changes to the development process. This is possible based on a tight integration of a PUF directly into the processor\'s instruction pipeline. Furthermore, cloud scenarios and distributed embedded systems alike inherently depend on remote execution; our approach supports this, as the secure execution environment needs not to be locally available at the developers site. We implemented an FPGA-based prototype based on the OpenRISC Reference Platform. To assess our results, we performed a security analysis of the processor and evaluated the performance impact of the encryption. We show that the attack surface is significantly reduced compared to previous approaches while the performance penalty is at a reasonable factor of about 1.5.

03:17 [Pub][ePrint] Modelling ciphersuite and version negotiation in the TLS protocol, by Benjamin Dowling and Douglas Stebila

  Real-world cryptographic protocols such as the widely used Transport Layer Security (TLS) protocol support many different combinations of cryptographic algorithms (called ciphersuites) and simultaneously support different versions. Recent advances in provable security have shown that most modern TLS ciphersuites are secure authenticated and confidential channel establishment (ACCE) protocols, but these analyses generally focus on single ciphersuites in isolation. In this paper we extend the ACCE model to cover protocols with many different sub-protocols, capturing both multiple ciphersuites and multiple versions, and define a security notion for secure negotiation of the optimal sub-protocol. We give a generic theorem that shows how secure negotiation follows, with some additional conditions, from the authentication property of secure ACCE protocols. Using this framework, we analyse the security of ciphersuite and three variants of version negotiation in TLS, including a recently proposed mechanism for detecting fallback attacks.