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2013-06-03
15:17 [Pub][ePrint] Security Analysis of Pseudo-Random Number Generators with Input: /dev/random is not Robust, by Yevgeniy Dodis and David Pointcheval and Sylvain Ruhault and Damien Vergnaud and Daniel Wichs

  A pseudo-random number generator (PRNG) is a deterministic algorithm that produces numbers whose distribution is indistinguishable from uniform. A formal security model for PRNGs with input was proposed in 2005 by Barak and Halevi (BH). This model involves an internal state that is refreshed with a (potentially biased) external random source, and a cryptographic function that outputs random numbers from the continually internal state. In this work we extend the BH model to also include a new security property capturing how it should accumulate the entropy of the input data into the internal state after state compromise. This property states that a good PRNG should be able to eventually recover from compromise even if the entropy is injected into the system at a very slow pace, and expresses the real-life expected behavior of existing PRNG designs.

Unfortunately, we show that neither the model nor the specific PRNG construction proposed by Barak and Halevi meet this new property, despite meeting a weaker robustness notion introduced by BH. From a practical side, we also give a precise assessment of the security of the two Linux PRNGs, /dev/random and /dev/urandom. In particular, we show several attacks proving that these PRNGs are not robust according to our definition, and do not accumulate entropy properly. These attacks are due to the vulnerabilities of the entropy estimator and the internal mixing function of the Linux PRNGs. These attacks against the Linux PRNG show that it does not satisfy the \"robustness\" notion of security, but it remains unclear if these attacks lead to actual exploitable vulnerabilities in practice. Finally, we propose a simple and very efficient PRNG construction that is provably robust in our new and stronger adversarial model.

We therefore recommend to use this construction whenever a PRNG with input is used for cryptography.





2013-06-02
18:17 [Pub][ePrint] Anon-Pass: Practical Anonymous Subscriptions, by Michael Z. Lee and Alan M. Dunn and Jonathan Katz and Brent Waters and Emmett Witchel

  We present the design, security proof, and implementation of an anonymous subscription service. Users register for the service by providing some form of identity, which might or might not be linked to a real-world identity such as a credit card, a web login, or a public key. A user logs on to the system by presenting a credential derived from information received at registration. Each credential allows only a single login in any authentication window, or epoch. Logins are anonymous in the sense that the service cannot distinguish which user is logging in any better than random guessing. This implies unlinkability of a user across different logins.

We find that a central tension in an anonymous subscription service is the service provider\'s desire for a long epoch (to reduce server-side computation) versus users\' desire for a short epoch (so they can repeatedly \"re-anonymize\" their sessions). We balance this tension by having short epochs, but adding an efficient operation for clients who do not need unlinkability to cheaply re-authenticate themselves for the next time period.

We measure performance of a research prototype of our pro- tocol that allows an independent service to offer anonymous access to existing services. We implement a music service, an Android-based subway-pass application, and a web proxy, and show that adding anonymity adds minimal client latency and only requires 33 KB of server memory per active user.



18:17 [Pub][ePrint] Fully-Anonymous Functional Proxy-Re-Encryption, by Yutaka Kawai and Katsuyuki Takashima

  In this paper, we introduce a general notion of functional proxy-re-encryption (F-PRE), where a wide class of functional encryption (FE) is combined with proxy-re-encryption (PRE) mechanism. The PRE encryption system should reveal minimal information to a proxy, in particular, hiding parameters of re-encryption keys and of original ciphertexts which he manipulate is highly desirable. We first formulate such a fully-anonymous security notion of F-PRE including usual payload-hiding properties. We then propose the first fully-anonymous inner-product PRE (IP-PRE) scheme, whose security is proven under the DLIN assumption and the existence of a strongly unforgeable one-time signature scheme in the standard model. Also, we propose the first ciphertext-policy F-PRE scheme with the access structures of Okamoto-Takashima (CRYPTO 2010), which also has an anonymity property for re-encryption keys as well as payload-hiding for original and re-encrypted ciphertexts. The security is proven under the same assumptions as the above IP-PRE scheme in the standard model. For these results, we develop novel blind delegation and new hidden subspace generation techniques on the dual system encryption (DSE) technique and the dual pairing vector spaces (DPVS). These techniques seem difficult to be realized by a composite-order bilinear group DSE approach.



18:17 [Pub][ePrint] On the use of continued fractions for stream ciphers, by Amadou Moctar Kane

  In this paper, we present a new approach to stream ciphers. This method draws its strength from public key algorithms such as RSA and the development in continued fractions of certain irrational numbers to produce a pseudo-random stream. Although the encryption scheme proposed in this paper is based on a hard mathematical problem, its use is fast.



18:17 [Pub][ePrint] Instantaneous Frequency Analysis, by Roman Korkikian and David Naccache and Guilherme Ozari de Almeida

  This paper investigated the use of instantaneous frequency (IF)

instead of power amplitude and power spectrum in side-channel analysis.

By opposition to the constant frequency used in Fourier Transform, instantaneous frequency reflects local phase differences and allows detecting frequency variations. These variations reflect the processed binary data and are hence cryptanalytically useful. IF exploits the fact that after higher power drops more time is required to restore power back to its nominal value. Whilst our experiments reveal IF does not bring specific benefits over usual power attacks when applied to unprotected designs, IF allows to obtain much better results in the presence of amplitude modification

countermeasures.



18:17 [Pub][ePrint] Generic Constructions of Secure-Channel Free Searchable Encryption with Adaptive Security, by Keita Emura and Atsuko Miyaji and Mohammad Shahriar Rahman and Kazumasa Omote

  For searching keywords against encrypted data, the public key encryption scheme with keyword search (PEKS), and its an extension called secure-channel free PEKS (SCF-PEKS) have been proposed.

In SCF-PEKS, a receiver makes a trapdoor for a keyword, and uploads it on a server. A sender computes an encrypted keyword, and sends it to the server. The server executes the searching procedure (called the test algorithm, which takes as inputs an encrypted keyword, trapdoor, and secret key of the server).

In this paper, we extend the security of SCF-PEKS, calling it adaptive SCF-PEKS, wherein an adversary (modeled as a ``malicious-but-legitimate\" receiver) is allowed to issue test queries \\emph{adaptively}, and show that adaptive SCF-PEKS can be generically constructed by anonymous identity-based encryption (anonymous IBE) only. That is, for constructing adaptive SCF-PEKS we need not require any additional cryptographic primitive when compared to the Abdalla et al. PEKS construction (J. Cryptology 2008), even though adaptive SCF-PEKS requires additional functionalities. Note that our generic construction needs to apply the KEM/DEM framework (a.k.a. hybrid encryption), where KEM stands for key encapsulation mechanism, and DEM stands for data encapsulation mechanism. We also show that there is a class of anonymous IBE that can be applied for constructing adaptive SCF-PEKS without using hybrid encryption, and propose an adaptive SCF-PEKS construction based on this IBE. Although our second construction is not fully generic, it is efficient compared to the first, since we can exclude the DEM part. Finally, we instantiate an adaptive SCF-PEKS scheme (via our second construction) that achieves a similar level of efficiency for the costs of the test procedure and encryption, compared to the (non-adaptive secure) SCF-PEKS scheme by Fang et al. (CANS2009).



18:17 [Pub][ePrint] BLAKE2: simpler, smaller, fast as MD5, by Jean-Philippe Aumasson and Samuel Neves and Zooko Wilcox-O\'Hearn and Christian Winnerlein

  We present the hash function BLAKE2, an improved version of the SHA-3 finalist BLAKE optimized for speed in software. Target applications include cloud storage, intrusion detection, or version control systems. BLAKE2 comes in two main flavors: BLAKE2b is optimized for 64-bit platforms, and BLAKE2s for smaller architectures. On 64-bit platforms, BLAKE2 is often faster than MD5, yet provides security similar to that of SHA-3: up to 256-bit collision resistance, immunity to length extension, indifferentiability from a random oracle, etc. We specify parallel versions BLAKE2bp and BLAKE2sp that are up to 4 and 8 times faster, by taking advantage of SIMD and/or multiple cores. BLAKE2 reduces the RAM requirements of BLAKE down to 168 bytes, making it smaller than any of the five SHA-3 finalists, and 32% smaller than BLAKE. Finally, BLAKE2 provides a comprehensive support for tree-hashing as well as keyed hashing (be it in sequential or tree mode).



18:17 [Pub][ePrint] Encryption Schemes with Post-Challenge Auxiliary Inputs, by Tsz Hon Yuen and Ye Zhang and Siu-Ming Yiu

  In this paper, we tackle the open problem of proposing a leakage-resilience encryption model that can capture leakage from both the secret key owner and the encryptor, in the auxiliary input model. Existing models usually do not allow adversaries to query more leakage

information after seeing the challenge ciphertext of the security games. On one hand, side-channel attacks on the random factor (selected by the encryptor) are already shown to be feasible. Leakage from the encryptor should not be overlooked. On the other hand, the technical challenge for allowing queries from the adversary after he sees the ciphertext is to avoid a trivial attack to the system since he can then embed the decryption function as the leakage function (note that we consider the auxiliary input model in which the leakage is modeled as computationally hard-to-invert functions). We solve this problem by defining the post-challenge auxiliary input model in which the family of leakage functions must be defined before the adversary is given the public key. Thus the adversary cannot embed the decryption function as a leakage function after seeing the challenge ciphertext while is allowed to make challenge-dependent queries. This model is able to capture a wider class of real-world side-channel attacks.

To realize our model, we propose a generic transformation from the auxiliary input model to our new post-challenge auxiliary input model for both public key encryption (PKE) and identity-based encryption (IBE). Furthermore, we extend Canetti et al.\'s technique, that converts CPA-secure IBE to CCA-secure PKE, into the leakage-resilient setting. More precisely, we construct a CCA-secure PKE in the post-challenge auxiliary input model, by using strong one-time signatures and strong extractor with hard-to-invert auxiliary inputs, together with a CPA-secure IBE in the auxiliary input model. Moreover, we extend our results to signatures, to obtain fully leakage-resilient signatures with auxiliary inputs using standard signatures and strong extractor with hard-to-invert auxiliary inputs. It is more efficient than the existing fully leakage-resilient signature schemes.



18:17 [Pub][ePrint] Sieve-in-the-Middle: Improved MITM Attacks (Full Version), by Anne Canteaut and Maria Naya-Plasencia and Bastien Vayssière

  This paper presents a new generic technique, named sieve-in-the-middle, which improves meet-in-the-middle attacks in the sense that it provides an attack on a higher number of rounds. Instead of selecting the key candidates by searching for a collision in an intermediate state which can be computed forwards and backwards, we here look for the existence of valid transitions through some middle sbox. Combining this technique with short bicliques allows to freely add one or two more rounds with the same time complexity. Moreover, when the key size of the cipher is larger than its block size, we show how to build the bicliques by an improved technique which does not require any additional data (on the contrary to previous biclique attacks). These techniques apply to PRESENT, DES, PRINCE and AES, improving the previously known results on these four ciphers. In particular, our attack on PRINCE applies to 8 rounds (out of 12), instead of 6 in the previous cryptanalyses. Some results are also given for theoretically estimating the sieving probability provided by some subsets of the input and output bits of a given sbox.



18:17 [Pub][ePrint] Elligator: Elliptic-curve points indistinguishable from uniform random strings, by Daniel J. Bernstein and Anna Krasnova and Tanja Lange

  Censorship-circumvention tools are in an arms race against censors. The censors study all traffic passing into and out of their controlled sphere, and try to disable censorship-circumvention tools without completely shutting down the Internet. Tools aim to shape their traffic patterns to match unblocked programs, so that simple traffic profiling cannot identify the tools within a reasonable number of traces; the censors respond by deploying firewalls with increasingly sophisticated deep-packet inspection.

Cryptography hides patterns in user data but does not evade censorship if the censor can recognize patterns in the cryptography itself. In particular, elliptic-curve cryptography often transmits points on known elliptic curves, and those points are easily distinguishable from uniform random strings of bits.

This paper introduces high-security high-speed elliptic-curve systems in which elliptic-curve points are encoded so as to be indistinguishable from uniform random strings.



18:17 [Pub][ePrint] Key-Versatile Signatures and Applications: RKA, KDM and Joint Enc/Sig, by Mihir Bellare and Sarah Meiklejohn and Susan Thomson

  Given an *arbitrary* one-way function F, is it possible to design a signature scheme where the secret key is an input x to F and the public key is y = F(x)? We show that signatures that are \"key-versatile\" in this sense, while also meeting stronger-than-usual security conditions we define, enable us to add signature-based integrity that is \"for-free\" in terms of key material, meaning we can sign with keys already in use for another purpose without impacting the security of the original purpose or in turn being impacted by it. We show applications across diverse areas including (1) security against related-key attack (RKA) (2) security for key-dependent messages (KDM), and (3) joint encryption and signing. We show how to build key versatile signature schemes and then obtain new results in all these application domains in a modular way.