International Association for Cryptologic Research

# IACR News Central

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2014-03-01
16:17 [Pub][ePrint]

We study the security of \\emph{key-alternating Feistel} ciphers, a class of key-alternating ciphers with a Feistel structure. Alternatively, this may be viewed as the study of Feistel ciphers where the pseudorandom round functions are of the form $F_i(x\\oplus k_i)$, where $k_i$ is the (secret) round key and $F_i$ is a \\emph{public} random function that the adversary is allowed to query in a black-box way. Interestingly, our results can be seen as a generalization of traditional results \\emph{à la} Luby-Rackoff in the sense that we can derive results for this model by simply letting the number of queries of the adversary to the public random functions $F_i$ be zero in our general bounds. We make an extensive use of the coupling technique. In particular (and as a result of independent interest), we improve the analysis of the coupling probability for balanced Feistel schemes previously carried out by Hoang and Rogaway (CRYPTO 2010).

16:17 [Pub][ePrint]

ide-channel attacks (SCAs) exploit leakage from the physical implementation of cryptographic algorithms to recover the otherwise secret information. In the last decade, popular SCAs like differential power analysis (DPA) and correlation power analysis (CPA) have been invented and demonstrated to be realistic threats to many critical embedded systems. However, there is still no sound and provable theoretical model that illustrates precisely what the success of these attacks depends on and how. Based on the maximum likelihood estimation (MLE) theory, this paper proposes a general statistical model for side-channel attack analysis that takes characteristics of both the physical implementation and cryptographic algorithm into consideration. The model establishes analytical relations between the success rate of attacks and the cryptographic system. For power analysis attacks, the side-channel characteristic of the physical implementation is modeled as signal-to-noise ratio (SNR), which is the ratio between the single-bit unit power consumption and the standard deviation of power distribution. The side-channel property of the cryptographic algorithm is extracted by a novel algorithmic confusion analysis. Experimental results of DPA and CPA on both DES and AES verify this model with high accuracy and demonstrate effectiveness of the algorithmic confusion analysis and SNR extraction. We expect the model to be extendable to other SCAs, like timing attacks, and would provide valuable guidelines for truly SCA-resilient system design and implementation.

16:17 [Pub][ePrint]

We formalize the notion of Verifiable Oblivious Storage (VOS), where a client outsources the storage of data to a server while ensuring data confidentiality, access pattern privacy, and integrity and freshness of data accesses. VOS generalizes the notion of Oblivious RAM (ORAM) in that it allows the server to perform computation, and also explicitly considers data integrity and freshness.

We show that allowing server-side computation enables us to

construct asymptotically more efficient VOS schemes whose bandwidth overhead cannot be matched by any ORAM scheme, due to a known lower bound by Goldreich and Ostrovsky. Specifically, for large block sizes

we can construct a VOS scheme with constant bandwidth per query; further, answering queries requires only poly-logarithmic

server computation. We describe applications of VOS to Dynamic Proofs of Retrievability, and RAM-model secure multi-party computation.

16:17 [Pub][ePrint]

Using recently developed techniques for program obfuscation, we show several constructions of non-interactive cryptosystems in the random-access machine (RAM) model of computation that are asymptotically more efficient than what would be obtained using generic RAM-to-circuit compilation. In particular, let $T$ denote the running time and $n$ the memory size of a RAM program. We show that using differing-inputs obfuscation, functional encryption for arbitrary RAM programs can be achieved with evaluation time $\\tilde{O}(T+n)$.

Additionally, we provide a number of RAM-model constructions assuming

the stronger notion of virtual black-box (VBB) obfuscation. We view these as initial feasibility results and leave instantiating similar protocols from weaker assumptions for future work. Specifically, using VBB obfuscation we show how to construct RAM-model functional encryption with function privacy, fully homomorphic encryption, and stateful, privacy-preserving verifiable computation in the memory-delegation model.

16:17 [Pub][ePrint]

We introduce {\\em honey encryption} (HE), a simple, general approach to encrypting messages using low min-entropy keys such as passwords. HE is designed to produce a ciphertext which, when decrypted with any of a number of {\\em incorrect} keys, yields plausible-looking but bogus plaintexts called {\\em honey messages}. A key benefit of HE is that it provides security in cases where too little entropy is available to withstand brute-force attacks that try every key; in this sense, HE provides security beyond conventional brute-force bounds. HE can also provide a hedge against partial disclosure of high min-entropy keys.

HE significantly improves security in a number of practical settings. To showcase this improvement, we build concrete HE schemes for password-based encryption of RSA secret keys and credit card numbers. The key challenges are development of appropriate instances of a new type of randomized message encoding scheme called a {\\em distribution-transforming encoder} (DTE), and analyses of the expected maximum loading of bins in various kinds of balls-and-bins games.

2014-02-28
01:17 [Pub][ePrint]

In this paper we consider TLS MITM attacks in the context of web applications, where the attacker\'s goal is to impersonate the user to the legitimate server, and thus gain access to the user\'s online account. We describe in detail why the recently proposed TLS Channel ID-based client authentication, as well as client web authentication in general, cannot fully prevent such attacks.

We then leverage TLS Channel ID-based authentication and combine it with the concept of sender invariance to create a novel mechanism that we call SISCA: Server Invariance with Strong Client Authentication. SISCA resists user impersonation via TLS MITM attacks even if the attacker has obtained the private key of the legitimate server. We analyze our proposal and show how it can be integrated in today\'s web infrastructure.

2014-02-27
22:17 [Pub][ePrint]

In this paper, we study the discrete logarithm problem in medium and

high characteristic finite fields. We propose a variant of the Number Field Sieve (NFS) based on numerous number fields. Our improved algorithm computes discrete logarithms in $\\mathbb{F}_{p^n}$ for the whole range of applicability of NFS and lowers the asymptotic complexity from $L_{p^n}(1/3, (128/9)^{1/3})$ to $L_{p^n}(1/3, (2^{13} /3^6)^{1/3})$ in the medium characteristic case, and from $L_{p^n} (1/3, (64/9)^{1/3})$ to $L_{p^n}(1/3,((92 + 26\\sqrt{13})/27))^{1/3})$ in the high characteristic case.

22:17 [Pub][ePrint]

We construct the first schemes that allow a client to privately outsource arbitrary program executions to a remote server while ensuring that: (I) the client\'s work is small and essentially independent of the complexity of the computation being outsourced, and (II) the server\'s work is only proportional to the run-time of the computation on a random access machine (RAM), rather than its potentially much larger circuit size. Furthermore, our solutions are non-interactive and have the structure of reusable garbled RAM programs, addressing an open question of Lu and Ostrovsky (Eurocrypt 2013). We also construct schemes for an augmented variant of the above scenario, where the client can initially outsource a large private and persistent database to the server, and later outsource arbitrary program executions with read/write access to this database.

Our solutions are built from non-reusable garbled RAM in conjunction with new types of reusable garbled circuits that are more efficient than prior solutions but only satisfy weaker security. For the basic setting without a persistent database, we can instantiate the required reusable garbled circuits using indistinguishability obfuscation. For the more complex setting with a persistent database we need stronger notions of obfuscation. Our basic solution also requires the client to perform a one-time preprocessing step to garble a program at the cost of its RAM run-time, and we can avoid this cost using stronger notions of obfuscation. It remains an open problem to instantiate these new types of reusable garbled circuits under weaker assumptions, possibly avoiding obfuscation altogether.

22:17 [Pub][ePrint]

We describe a general Multi-Party Computation (MPC) protocol for arithmetic circuits that is secure against a static malicious adversary corrupting up to a 1/10 fraction of the parties. The protocol requires each party to send an average of soft-O(m/n) bits, and compute soft-O(m/n) operations in a network of size n, where m is the size of circuit. This is achieved by increasing latency from constant to O(d) , where d is the depth of the circuit. Our protocol has a setup phase that is independent of the circuit and relies on Threshold Fully Homomorphic Encryption (TFHE). The setup requires each party to send soft-O(k^2) messages and compute soft-O(k^2) operations, where k is the security parameter. We provide results from microbenchmarks conducted over a sorting network showing that our protocol may be practical for deployment in large networks. For example, we consider a network of size 2^25 (over 33 million), where each party has an input item of size 20 bytes. To securely sort the items, our protocol requires each party on average to send only 5 kilobytes per item sorted.

04:17 [Pub][ePrint]

We illustrate a vulnerability introduced to elliptic curve cryptographic protocols when implemented using a function of the OpenSSL cryptographic library. For the given implementation using an elliptic curve E over a binary field with a point G \\in E, our attack recovers the majority of the bits of a scalar k when kG is computed using the OpenSSL implementation of the Montgomery ladder. For the Elliptic Curve Digital Signature Algorithm (ECDSA) the scalar k is intended to remain secret. Our attack recovers the scalar k and thus the secret key of the signer and would therefore allow unlimited forgeries. This is possible from snooping on only one signing process and requires computation of less than one second on a quad core desktop when the scalar k (and secret key) is around 571 bits.

04:17 [Pub][ePrint]

Signcryption is a relatively new cryptographic technique that is supposed to fulfill the functionalities of digital signature and encryption in a single logical step and can effectively decrease the computational costs and communication overheads in comparison with the traditional signature-then-encryption schemes, and aggregate signcryption scheme allows individual signcryption ciphertexts intended for the same recipient to be aggregated into a single (shorter) combined ciphertext without losing any of the security guarantees. In this paper, we present a new identity-based aggregate signcryption scheme using multilinear maps. To the best of my knowledge, our new scheme is the first identity-based aggregate signcryption scheme that admits unrestricted aggregation.