Improved Condensers for Chor-Goldreich Sources with Long Blocks and Small Entropy Rate

Kernekoncepter

This research presents the first explicit condenser for Chor-Goldreich sources that effectively handles long blocks and small entropy rates, achieving near-optimal entropy preservation with a small entropy gap.

Resumé

• Bibliographic Information: Goodman, J., Li, X., & Zuckerman, D. (2024). Improved Condensers for Chor-Goldreich Sources. arXiv preprint arXiv:2410.08142.
• Research Objective: This paper investigates the deterministic condensation of Chor-Goldreich (CG) sources, particularly focusing on scenarios with a small number of long blocks and low entropy rates. The authors aim to design efficient condensers that can extract a significant portion of the source entropy while maintaining a small entropy gap.
• Methodology: The researchers develop a novel construction for a non-malleable condenser, a key component in their approach. This condenser leverages seeded extractors and a careful analysis of entropy propagation to merge rows of a somewhere high-entropy source derived from the CG source. By iteratively applying this merger, they reduce the source to a single string with high entropy and a controlled entropy gap.
• Key Findings: The paper presents the first explicit condenser capable of handling CG sources with arbitrary block length and entropy rate. Notably, the condenser achieves near-optimal entropy preservation (e.g., 99%) and maintains an entropy gap that is only polynomially larger than the gap of a single block in the CG source. The authors demonstrate two key instantiations of their condenser: one for constant entropy rate sources, achieving exponentially small error, and another for extremely low entropy rate sources (k=1), requiring only a polynomial number of blocks to achieve a high output entropy rate.
• Main Conclusions: This work significantly advances the understanding of deterministic randomness extraction from CG sources. The proposed condenser surpasses previous limitations, enabling efficient handling of long blocks and low entropy rates, which are particularly relevant for modeling real-world imperfect random sources.
• Significance: The development of efficient condensers for CG sources has significant implications for various domains, including cryptography, derandomization, and the simulation of randomized algorithms. The ability to handle long blocks and low entropy rates broadens the applicability of these techniques to more realistic scenarios.
• Limitations and Future Research: While the paper makes significant progress, it acknowledges limitations such as the lack of an online implementation and the focus on a specific notion of "almost" CG sources. Future research could explore online variants, extend the analysis to broader classes of almost CG sources, and investigate potential applications of the new condenser in specific areas like cryptography and derandomization.

Statistik

The condenser can preserve 99% of the min-entropy.
The condenser achieves an entropy gap that is only poly(1/δ) times larger than the gap g of a single block.
For constant entropy rate δ, the condenser requires a constant number of blocks t to produce an output with entropy rate 0.9.
In the low entropy regime, using t = poly(n) blocks, the condenser can achieve output entropy rate 0.9 even if each block has just 1 bit of min-entropy.
The condenser has exponentially small error.

Citater

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Improved Condensers for Chor-Goldreich Sources

by Jesse Goodma... kl. arxiv.org 10-11-2024

https://arxiv.org/pdf/2410.08142.pdf

Dybere Forespørgsler

How can the insights from this research be applied to develop more efficient randomness extractors for other types of weak random sources beyond Chor-Goldreich sources?

This research provides several valuable insights that could potentially be applied to develop more efficient randomness extractors for weak random sources beyond Chor-Goldreich sources:
1. Leveraging "Almost" Independence: The core observation that seeded extractors can tolerate a certain degree of "CG-correlation" between the source and the seed is powerful. This suggests exploring other weak source models exhibiting similar "almost" independence properties. For instance, sources with limited dependencies between blocks, like sources with bounded locality or sources with limited dependencies in their Fourier spectrum, could potentially be analyzed using similar techniques.
2. Adapting the "Collapsing Table" Paradigm: The high-level approach of expanding a source into a "somewhere high-entropy source" and then gradually collapsing it using mergers or non-malleable condensers could be adapted to other source models. The key would be to design mergers and non-malleable condensers tailored to the specific dependencies or weaknesses present in the target source model.
3. Exploiting Specific Source Structures: The paper demonstrates the effectiveness of carefully analyzing the structure of CG sources to design efficient condensers. This highlights the importance of understanding the specific properties and limitations of different weak source models. By identifying and exploiting unique structural characteristics, it might be possible to develop specialized extraction techniques that outperform generic approaches.
4. Combining Existential and Explicit Results: The paper combines strong existential results with explicit constructions. This approach could be fruitful for other source models as well. Existential results can guide the search for explicit constructions by providing achievable parameter trade-offs, while explicit constructions can inspire new techniques for proving existential results.
5. Exploring Alternative Primitives: While the paper focuses on seeded extractors as the main building block, exploring alternative cryptographic primitives or complexity-theoretic assumptions could lead to new condenser designs with different parameter trade-offs. For example, using pseudorandom generators (PRGs) or one-way functions in conjunction with specific source properties might yield interesting results.

Could there be alternative approaches, perhaps based on different cryptographic primitives or complexity-theoretic assumptions, that yield even better condensers for CG sources with different trade-offs between parameters?

Yes, alternative approaches based on different cryptographic primitives or complexity-theoretic assumptions could potentially lead to improved condensers for CG sources with different parameter trade-offs. Here are a few possibilities:
1. PRG-Based Approaches: Instead of relying solely on seeded extractors, one could explore using pseudorandom generators (PRGs) to expand the randomness of the CG source. For instance, one could use a portion of the CG source to generate a pseudorandom sequence, which is then used as a seed for a seeded extractor applied to the remaining part of the CG source. This approach might offer better seed length or output length trade-offs, depending on the PRG's properties.
2. One-Way Function-Based Approaches: One-way functions, which are easy to compute but hard to invert, could potentially be used to construct condensers. For example, one could hash the CG source using a one-way function and then apply a randomness extractor to the output. The one-wayness property might provide some guarantee about the entropy of the hashed output, even if the CG source has significant correlations.
3. Lattice-Based Cryptography: Techniques from lattice-based cryptography, which are believed to be resistant to quantum attacks, could potentially be leveraged to design condensers. Lattice-based constructions often rely on the hardness of certain lattice problems, and these hardness assumptions might translate into guarantees about the randomness properties of the output.
4. Computational Assumptions: Instead of relying on information-theoretic security, one could explore using computational assumptions to design condensers. For instance, assuming the existence of one-way functions or other cryptographic primitives, one could potentially construct condensers that are secure against computationally bounded adversaries, even if they don't provide the same level of information-theoretic guarantees.

What are the practical implications of these findings for real-world systems that rely on randomness, such as cryptographic protocols or Monte Carlo simulations, particularly in scenarios where the available randomness might be biased or correlated?

These findings have significant practical implications for real-world systems that rely on randomness, especially in scenarios where the available randomness might be biased or correlated:
1. Improved Randomness Sources in Cryptography: Cryptographic protocols often rely on high-quality randomness for tasks like key generation, encryption, and digital signatures. In practice, obtaining truly random bits can be challenging, and physical randomness sources might exhibit biases or correlations. The development of efficient condensers for CG sources provides a practical way to extract nearly uniform randomness from such imperfect sources, enhancing the security of cryptographic systems.
2. More Robust Monte Carlo Simulations: Monte Carlo simulations, used extensively in fields like finance, physics, and engineering, rely heavily on random sampling. However, if the underlying random number generator produces biased or correlated samples, the simulation results can be inaccurate or misleading. By using condensers to purify the randomness used in these simulations, one can improve the reliability and trustworthiness of the results.
3. Handling Real-World Imperfections: Real-world randomness sources often deviate from the ideal uniform distribution. These deviations can arise from hardware imperfections, environmental factors, or limitations in the measurement process. The ability to condense CG sources, which model a specific type of imperfection, provides a practical tool for dealing with these real-world limitations and ensuring the proper functioning of systems that rely on randomness.
4. Reducing Reliance on External Randomness: In some applications, obtaining external randomness can be costly or impractical. The development of deterministic condensers for CG sources opens up the possibility of relying more on internal, potentially imperfect randomness sources, reducing the need for external randomness and simplifying system design.
5. Designing More Resilient Systems: By understanding the limitations of imperfect randomness sources and developing techniques to mitigate their impact, we can design more resilient systems that are less susceptible to failures or attacks arising from randomness imperfections. This is particularly important in security-critical applications where the consequences of randomness failures can be severe.