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**Published in:** LIPIcs, Volume 230, 3rd Conference on Information-Theoretic Cryptography (ITC 2022)

Pseudorandom number generators with input (PRNGs) are cryptographic algorithms that generate pseudorandom bits from accumulated entropic inputs (e.g., keystrokes, interrupt timings, etc.). This paper studies in particular PRNGs that are secure against premature next attacks (Kelsey et al., FSE '98), a class of attacks leveraging the fact that a PRNG may produce an output (which could be seen by an adversary!) before enough entropy has been accumulated. Practical designs adopt either unsound entropy-estimation methods to prevent such attacks (as in Linux’s /dev/random) or sophisticated pool-based approaches as in Yarrow (MacOS/FreeBSD) and Fortuna (Windows).
The only prior theoretical study of premature next attacks (Dodis et al., Algorithmica '17) considers either a seeded setting or assumes constant entropy rate, and thus falls short of providing and validating practical designs. Assuming the availability of random seed is particularly problematic, first because this requires us to somehow generate a random seed without using our PRNG, but also because we must ensure that the entropy inputs to the PRNG remain independent of the seed. Indeed, all practical designs are seedless. However, prior works on seedless PRNGs (Coretti et al., CRYPTO '19; Dodis et al., ITC '21, CRYPTO'21) do not consider premature next attacks.
The main goal of this paper is to investigate the feasibility of theoretically sound seedless PRNGs that are secure against premature next attacks. To this end, we make the following contributions:
1) We prove that it is impossible to achieve seedless PRNGs that are secure against premature-next attacks, even in a rather weak model. Namely, the impossibility holds even when the entropic inputs to the PRNG are independent. In particular, our impossibility result holds in settings where seedless PRNGs are otherwise possible.
2) Given the above impossibility result, we investigate whether existing seedless pool-based approaches meant to overcome premature next attacks in practical designs provide meaningful guarantees in certain settings. Specifically, we show the following.
3) We introduce a natural condition on the entropic input and prove that it implies security of the round-robin entropy accumulation PRNG used by Windows 10, called Fortuna. Intuitively, our condition requires the input entropy "not to vary too wildly" within a given round-robin round.
4) We prove that the "root pool" approach (also used in Windows 10) is secure for general entropy inputs, provided that the system’s state is not compromised after system startup.

Sandro Coretti, Yevgeniy Dodis, Harish Karthikeyan, Noah Stephens-Davidowitz, and Stefano Tessaro. On Seedless PRNGs and Premature Next. In 3rd Conference on Information-Theoretic Cryptography (ITC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 230, pp. 9:1-9:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{coretti_et_al:LIPIcs.ITC.2022.9, author = {Coretti, Sandro and Dodis, Yevgeniy and Karthikeyan, Harish and Stephens-Davidowitz, Noah and Tessaro, Stefano}, title = {{On Seedless PRNGs and Premature Next}}, booktitle = {3rd Conference on Information-Theoretic Cryptography (ITC 2022)}, pages = {9:1--9:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-238-9}, ISSN = {1868-8969}, year = {2022}, volume = {230}, editor = {Dachman-Soled, Dana}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2022.9}, URN = {urn:nbn:de:0030-drops-164870}, doi = {10.4230/LIPIcs.ITC.2022.9}, annote = {Keywords: seedless PRNGs, pseudorandom number generators, PRNG, Fortuna, premature next} }

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**Published in:** LIPIcs, Volume 230, 3rd Conference on Information-Theoretic Cryptography (ITC 2022)

The question of building the most efficient tn-to-n-bit collision-resistant hash function H from a smaller (say, 2n-to-n-bit) compression function f is one of the fundamental questions in symmetric key cryptography. This question has a rich history, and was open for general t, until a recent breakthrough paper by Andreeva, Bhattacharyya and Roy at Eurocrypt'21, who designed an elegant mode (which we call ABR) achieving roughly 2t/3 calls to f, which matches the famous Stam’s bound from CRYPTO'08. Unfortunately, we have found serious issues in the claims made by the authors. These issues appear quite significant, and range from verifiably false statements to noticeable gaps in the proofs (e.g., omissions of important cases and unjustified bounds).
We were unable to patch up the current proof provided by the authors. Instead, we prove from scratch the security of the ABR construction for the first non-trivial case t = 11 (ABR mode of height 3), which was incorrectly handled by the authors. In particular, our result matches Stam’s bound for t = 11. While the general case is still open, we hope our techniques will prove useful to finally settle the question of the optimal efficiency of hash functions.

Chandranan Dhar, Yevgeniy Dodis, and Mridul Nandi. Revisiting Collision and Local Opening Analysis of ABR Hash. In 3rd Conference on Information-Theoretic Cryptography (ITC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 230, pp. 11:1-11:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{dhar_et_al:LIPIcs.ITC.2022.11, author = {Dhar, Chandranan and Dodis, Yevgeniy and Nandi, Mridul}, title = {{Revisiting Collision and Local Opening Analysis of ABR Hash}}, booktitle = {3rd Conference on Information-Theoretic Cryptography (ITC 2022)}, pages = {11:1--11:22}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-238-9}, ISSN = {1868-8969}, year = {2022}, volume = {230}, editor = {Dachman-Soled, Dana}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2022.11}, URN = {urn:nbn:de:0030-drops-164890}, doi = {10.4230/LIPIcs.ITC.2022.11}, annote = {Keywords: ABR hash, collision resistance, local opening} }

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**Published in:** LIPIcs, Volume 215, 13th Innovations in Theoretical Computer Science Conference (ITCS 2022)

One of the ultimate goals of symmetric-key cryptography is to find a rigorous theoretical framework for building block ciphers from small components, such as cryptographic S-boxes, and then argue why iterating such small components for sufficiently many rounds would yield a secure construction. Unfortunately, a fundamental obstacle towards reaching this goal comes from the fact that traditional security proofs cannot get security beyond 2^{-n}, where n is the size of the corresponding component.
As a result, prior provably secure approaches - which we call "big-box cryptography" - always made n larger than the security parameter, which led to several problems: (a) the design was too coarse to really explain practical constructions, as (arguably) the most interesting design choices happening when instantiating such "big-boxes" were completely abstracted out; (b) the theoretically predicted number of rounds for the security of this approach was always dramatically smaller than in reality, where the "big-box" building block could not be made as ideal as required by the proof. For example, Even-Mansour (and, more generally, key-alternating) ciphers completely ignored the substitution-permutation network (SPN) paradigm which is at the heart of most real-world implementations of such ciphers.
In this work, we introduce a novel paradigm for justifying the security of existing block ciphers, which we call small-box cryptography. Unlike the "big-box" paradigm, it allows one to go much deeper inside the existing block cipher constructions, by only idealizing a small (and, hence, realistic!) building block of very small size n, such as an 8-to-32-bit S-box. It then introduces a clean and rigorous mixture of proofs and hardness conjectures which allow one to lift traditional, and seemingly meaningless, "at most 2^{-n}" security proofs for reduced-round idealized variants of the existing block ciphers, into meaningful, full-round security justifications of the actual ciphers used in the real world.
We then apply our framework to the analysis of SPN ciphers (e.g, generalizations of AES), getting quite reasonable and plausible concrete hardness estimates for the resulting ciphers. We also apply our framework to the design of stream ciphers. Here, however, we focus on the simplicity of the resulting construction, for which we managed to find a direct "big-box"-style security justification, under a well studied and widely believed eXact Linear Parity with Noise (XLPN) assumption.
Overall, we hope that our work will initiate many follow-up results in the area of small-box cryptography.

Yevgeniy Dodis, Harish Karthikeyan, and Daniel Wichs. Small-Box Cryptography. In 13th Innovations in Theoretical Computer Science Conference (ITCS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 215, pp. 56:1-56:25, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{dodis_et_al:LIPIcs.ITCS.2022.56, author = {Dodis, Yevgeniy and Karthikeyan, Harish and Wichs, Daniel}, title = {{Small-Box Cryptography}}, booktitle = {13th Innovations in Theoretical Computer Science Conference (ITCS 2022)}, pages = {56:1--56:25}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-217-4}, ISSN = {1868-8969}, year = {2022}, volume = {215}, editor = {Braverman, Mark}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2022.56}, URN = {urn:nbn:de:0030-drops-156527}, doi = {10.4230/LIPIcs.ITCS.2022.56}, annote = {Keywords: Block Ciphers, S-Box, Cryptography} }

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**Published in:** LIPIcs, Volume 199, 2nd Conference on Information-Theoretic Cryptography (ITC 2021)

In this work we challenge the common misconception that information-theoretic (IT) privacy is too impractical to be used in the real-world: we propose to build simple and reusable IT-encryption solutions whose only efficiency penalty (compared to computationally-secure schemes) comes from a large secret key size, which is often a rather minor inconvenience, as storage is cheap. In particular, our solutions are stateless and locally computable at the optimal rate, meaning that honest parties do not maintain state and read only (optimally) small portions of their large keys with every use.
Moreover, we also propose a novel architecture for outsourcing the storage of these long keys to a network of semi-trusted servers, trading the need to store large secrets with the assumption that it is hard to simultaneously compromise too many publicly accessible ad-hoc servers. Our architecture supports everlasting privacy and post-application security of the derived one-time keys, resolving two major limitations of a related model for outsourcing key storage, called bounded storage model.
Both of these results come from nearly optimal constructions of so called doubly-affine extractors: locally-computable, seeded extractors Ext(X,S) which are linear functions of X (for any fixed seed S), and protect against bounded affine leakage on X. This holds unconditionally, even if (a) affine leakage may adaptively depend on the extracted key R = Ext(X,S); and (b) the seed S is only computationally secure. Neither of these properties are possible with general-leakage extractors.

Yevgeniy Dodis and Kevin Yeo. Doubly-Affine Extractors, and Their Applications. In 2nd Conference on Information-Theoretic Cryptography (ITC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 199, pp. 13:1-13:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{dodis_et_al:LIPIcs.ITC.2021.13, author = {Dodis, Yevgeniy and Yeo, Kevin}, title = {{Doubly-Affine Extractors, and Their Applications}}, booktitle = {2nd Conference on Information-Theoretic Cryptography (ITC 2021)}, pages = {13:1--13:23}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-197-9}, ISSN = {1868-8969}, year = {2021}, volume = {199}, editor = {Tessaro, Stefano}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2021.13}, URN = {urn:nbn:de:0030-drops-143320}, doi = {10.4230/LIPIcs.ITC.2021.13}, annote = {Keywords: extractors, information-theoretic privacy, everlasting privacy} }

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**Published in:** LIPIcs, Volume 199, 2nd Conference on Information-Theoretic Cryptography (ITC 2021)

In this work, we characterize linear online extractors. In other words, given a matrix A ∈ F₂^{n×n}, we study the convergence of the iterated process S ← AS⊕X, where X∼D is repeatedly sampled independently from some fixed (but unknown) distribution D with (min)-entropy k. Here, we think of S ∈ {0,1}ⁿ as the state of an online extractor, and X ∈ {0,1}ⁿ as its input.
As our main result, we show that the state S converges to the uniform distribution for all input distributions D with entropy k > 0 if and only if the matrix A has no non-trivial invariant subspace (i.e., a non-zero subspace V ⊊ F₂ⁿ such that AV ⊆ V). In other words, a matrix A yields a linear online extractor if and only if A has no non-trivial invariant subspace. For example, the linear transformation corresponding to multiplication by a generator of the field F_{2ⁿ} yields a good linear online extractor. Furthermore, for any such matrix convergence takes at most Õ(n²(k+1)/k²) steps.
We also study the more general notion of condensing - that is, we ask when this process converges to a distribution with entropy at least l, when the input distribution has entropy at least k. (Extractors corresponding to the special case when l = n.) We show that a matrix gives a good condenser if there are relatively few vectors w ∈ F₂ⁿ such that w, A^Tw, …, (A^T)^{n-k}w are linearly dependent. As an application, we show that the very simple cyclic rotation transformation A(x₁,…, x_n) = (x_n,x₁,…, x_{n-1}) condenses to l = n-1 bits for any k > 1 if n is a prime satisfying a certain simple number-theoretic condition.
Our proofs are Fourier-analytic and rely on a novel lemma, which gives a tight bound on the product of certain Fourier coefficients of any entropic distribution.

Yevgeniy Dodis, Siyao Guo, Noah Stephens-Davidowitz, and Zhiye Xie. Online Linear Extractors for Independent Sources. In 2nd Conference on Information-Theoretic Cryptography (ITC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 199, pp. 14:1-14:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{dodis_et_al:LIPIcs.ITC.2021.14, author = {Dodis, Yevgeniy and Guo, Siyao and Stephens-Davidowitz, Noah and Xie, Zhiye}, title = {{Online Linear Extractors for Independent Sources}}, booktitle = {2nd Conference on Information-Theoretic Cryptography (ITC 2021)}, pages = {14:1--14:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-197-9}, ISSN = {1868-8969}, year = {2021}, volume = {199}, editor = {Tessaro, Stefano}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2021.14}, URN = {urn:nbn:de:0030-drops-143339}, doi = {10.4230/LIPIcs.ITC.2021.14}, annote = {Keywords: feasibility of randomness extraction, randomness condensers, Fourier analysis} }

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**Published in:** LIPIcs, Volume 199, 2nd Conference on Information-Theoretic Cryptography (ITC 2021)

Given 2n-to-n compression functions h₁,h₂,h₃, we build a new 5n-to-n compression function T₅, using only 3 compression calls:
T₅(m₁, m₂, m₃, m₄, m₅) : = h₃(h₁(m₁, m₂)⊕ m₅ , h₂(m₃, m₄)⊕ m₅) ⊕ m₅
We prove that this construction matches Stam’s bound, by providing Õ(q²/2ⁿ) collision security and O(q³/2^{2n}+ nq/2ⁿ) preimage security (the latter term dominates in the region of interest, when q < 2^{n/2}). In particular, it provides birthday security for hashing 5 inputs using three 2n-to-n compression calls, instead of only 4 inputs in prior constructions. Thus, we get a sequential variant of the Merkle-Damgård (MD) hashing, where t message blocks are hashed using only 3t/4 calls to the 2n-to-n compression functions; a 25% saving over traditional hash function constructions. This time reduces to t/4 (resp. t/2) sequential calls using 3 (resp. 2) parallel execution units; saving a factor of 4 (resp. 2) over the traditional MD-hashing, where parallelism does not help to process one message. We also get a novel variant of a Merkle tree, where t message blocks can be processed using 0.75(t-1) compression function calls and depth 0.86 log₂ t, thereby saving 25% in the number of calls and 14% in the update time over Merkle trees. We provide two modes for a local opening of a particular message block: conservative and aggressive. The former retains the birthday security, but provides longer proofs and local verification time than the traditional Merkle tree. For the aggressive variant, we reduce the proof length to a 29% overhead compared to Merkle trees (1.29log₂ t vs log₂ t), but the verification time is now 14% faster (0.86log₂ t vs log₂ t). However, birthday security is only shown under a plausible conjecture related to the 3-XOR problem, and only for the (common, but not universal) setting where the root of the Merkle tree is known to correspond to a valid t-block message.

Yevgeniy Dodis, Dmitry Khovratovich, Nicky Mouha, and Mridul Nandi. T₅: Hashing Five Inputs with Three Compression Calls. In 2nd Conference on Information-Theoretic Cryptography (ITC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 199, pp. 24:1-24:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{dodis_et_al:LIPIcs.ITC.2021.24, author = {Dodis, Yevgeniy and Khovratovich, Dmitry and Mouha, Nicky and Nandi, Mridul}, title = {{T₅: Hashing Five Inputs with Three Compression Calls}}, booktitle = {2nd Conference on Information-Theoretic Cryptography (ITC 2021)}, pages = {24:1--24:23}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-197-9}, ISSN = {1868-8969}, year = {2021}, volume = {199}, editor = {Tessaro, Stefano}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2021.24}, URN = {urn:nbn:de:0030-drops-143430}, doi = {10.4230/LIPIcs.ITC.2021.24}, annote = {Keywords: hash functions, Merkle trees, Merkle-Damg\r{a}rd, collision resistance} }

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