Cited by 7,772Privacy-Preserving Technologies in DataStochastic Gradient Optimization TechniquesDomain Adaptation and Few-Shot Learning

The DOI matches its Crossref record; 497 of the 502 references it cites were found in a registry or an index; 188 of the 503 sources its claims cite were checked for support.

Advances and Open Problems in Federated Learning

Open access
Kairouz, Peter, McMahan, H. Brendan·2021·Foundations and Trends in Machine Learning

Abstract

Federated learning (FL) is a machine learning setting where many clients (e.g., mobile devices or whole organizations) collaboratively train a model under the orchestration of a central server (e.g., service provider), while keeping the training data decentralized. FL embodies the principles of focused data collection and minimization, and can mitigate many of the systemic privacy risks and costs resulting from traditional, centralized machine learning and data science approaches. Motivated by the explosive growth in FL research, this monograph discusses recent advances and presents an extensive collection of open problems and challenges.

Overview

AI snapshot

Written from a shortened version of the paper’s full text (it is longer than we read).

TL;DR

This paper is a broad survey of federated learning, and the provided text focuses on privacy, security, robustness, and fairness challenges rather than reporting a single empirical study. It argues that FL changes the attack surface and makes many standard defenses harder to deploy because of decentralization, limited client visibility, and communication constraints.

Key findings

  • Federated learning increases exposure to white-box evasion attacks because the model broadcast step means the model may be accessible to any malicious client; the text says this “increases the need for defenses against white-box evasion attacks.”
  • For model update poisoning, the paper notes that targeted attacks can be strong even with few adversaries: it reports that 10% of participating devices compromised can be enough to introduce a backdoor, even with anomaly detectors at the server.
  • Differential privacy is presented as a defense with two explicit strengths: it gives strong, worst-case protections and is closed under composition; but the paper also notes that in FL it can reduce accuracy and its hyperparameters are still hard to choose.
  • Under secure aggregation, the server sees only aggregates, which helps privacy, but the paper says the method has limitations: it assumes a semi-honest server in key setup, can still leak through per-round aggregates, is not efficient for sparse vectors, and does not enforce well-formed client inputs.

Methods

  • Not a single experimental study; this is a survey / open-problems paper organizing the FL literature across settings, efficiency, privacy, attacks, and fairness.
  • The text synthesizes prior work and uses qualitative analysis of threat models, privacy mechanisms (e.g., DP, secure aggregation, MPC, TEEs), and optimization results; it also reproduces convergence-rate tables for IID and non-IID federated optimization methods.
  • It discusses the FL setting using a cross-device / cross-silo / decentralized taxonomy and references typical FL workflows, datasets, and software frameworks rather than introducing new data collection.

Limitations

  • The paper explicitly states that many open questions remain: e.g., how to defend against inference-time attacks in FL, how to reconcile privacy with robust aggregation, and how to make secure aggregation compatible with non-linear defenses.
  • It says that formal analysis is often missing or incomplete for several areas, including split learning privacy guarantees, which are described as “still an open problem,” and the compatibility of some defenses with secure aggregation.
  • Evident from the text: because this is a survey, it does not present new experiments or a unified benchmark; conclusions are drawn from existing work and summarized tables, not from a new empirical evaluation.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1561/2200000083

Cites

497 of 502 references verified1 citation errors · 4 not checkable

Statistics

Nothing to recompute.

Verification

Claims the paper makes

188 of 503 cited sources checked · 8 supported · 4 flagged · accuracy not yet measured

Show all 195 claims

  • Probably not a substantial claim. 309
  • It may state two separate claims as one. 161
393 of 393
  • The paper relies on this without support.

    No single work directly addresses the full set of FL challenges.

    “Conversely, even since the introduction of the term federated learning, we are aware of no single work that directly addresses the full set of FL challenges.”
  • The paper relies on this without support.

    Cross-device federated learning involves a total population size of 10^6 to 10^10 devices.

    “Total population size | 10 6 - 10 10 devices”
  • The paper relies on this without support.

    In cross-device FL, only a fraction of clients are available at any one time, often with diurnal or other variations.

    “Only a fraction of clients are available at any one time, often with diurnal or other variations.”
  • The paper relies on this without support.

    In cross-device FL, communication is often the primary bottleneck.

    “Communication is often the primary bottleneck, though it depends on the task.”
  • The paper relies on this without support.

    In cross-device FL, clients cannot be indexed directly (no use of client identifiers).

    “Clients cannot be indexed directly (i.e., no use of client identifiers).”
  • The authors state this as an assumption.

    In cross-device FL, each client will likely participate only once in a task, so generally a fresh sample of never-before-seen clients in each round of computation is assumed.

    “Stateless -each client will likely par- ticipate only once in a task, so gener- ally a fresh sample of never-before-seen clients in each round of computation is assumed.”
  • The paper relies on this without support.

    In cross-device FL, 5% or more of the clients participating in a round of computation are expected to fail or drop out.

    “Highly unreliable -5%or more of the clients participating in a round of com- putation are expected to fail or drop out (e.g. because the device becomes ineli- gible when battery, network, or idleness requirements are violated).”
  • The paper relies on this without support.

    In cross-device FL, data is fixed partitioning by example (horizontal).

    “Fixed partitioning by example (horizon- tal).”
  • The paper relies on this without support.

    In cross-silo FL, data partition is fixed and could be example-partitioned (horizontal) or feature-partitioned (vertical).

    “Partition is fixed. Could be example-partitioned (horizontal) feature-partitioned (vertical).”
  • The paper relies on this without support.

    In cross-silo FL, all clients are almost always available.

    “All clients are almost always available.”
  • The paper relies on this without support.

    In cross-silo FL, there are typically 2 to 100 clients.

    “Typically 2 - 100 clients.”
  • The paper relies on this without support.

    Clients may worry that contributing their data to training federated learning models will benefit their competitors, who do not contribute as much but receive the same final model nonetheless (the free-rider problem).

    “Clients might worry that contributing their data to training federated learning models will benefit their competitors, who do not contribute as much but receive the same final model nonetheless (i.e. the free-rider problem).”
  • The paper relies on this without support.

    In many practical scenarios, the final trained model would be released only to those who participate in the training.

    “For example, in many practical scenarios, the final trained model would be released only to those who participate in the training, which makes the concerns about 'the rest of the world' less important.”
  • The paper relies on this without support.

    The potential threat from other clients is likely to be more important than threats from the rest of the world in cross-silo FL.

    “On the other hand, for a practically persuasive claim, we would usually need a notion of local differential privacy, as the potential threat from other clients is likely to be more important.”
  • The paper relies on this without support.

    Real-world federated learning datasets likely contain a mixture of non-IID effects.

    “Real-world federated learning datasets likely contain a mixture of these effects”
  • The paper relies on this without support.

    Devices typically meet eligibility requirements at night local time (when they are more likely to be charging, on free wi-fi, and idle).

    “Devices typically meet those requirements at night local time (when they are more likely to be charging, on free wi-fi, and idle)”
  • The paper relies on this without support.

    Practitioners often care about wall-clock convergence speed.

    “practitioners often care about wall-clock convergence speed.”
  • The paper relies on this without support.

    The costs (at least in wall-clock time) are small for increasing M.

    “The costs (at least in wall-clock time) are small for increasing M”
  • The paper relies on this without support.

    At different times within a day, client characteristics can vary significantly.

    “At different times within a day, the clients characteristics can vary significantly.”
  • The paper relies on this without support.

    Clients can perform different local steps because of heterogeneity in their computing capacities.

    “Clients can perform different local steps because of heterogeneity in their computing capacities.”
  • The paper relies on this without support.

    Communication can be a primary bottleneck for federated learning because wireless links and end-user internet connections typically operate at lower rates than datacenter links and can be expensive and unreliable.

    “It is now well-understood that communication can be a primary bottleneck for federated learning since wireless links and other end-user internet connections typically operate at lower rates than intra- or interdatacenter links and can be potentially expensive and unreliable.”
  • The paper relies on this without support.

    Most modern deep learning models cannot represent their uncertainty nor allow for a probability interpretation of parametric learning.

    “Most modern deep learning models cannot represent their uncertainty nor allow for a probability interpretation of parametric learning.”
  • The authors state this as an assumption.

    For the discussion in Section 4.3, a trusted server exists.

    “Section 4.3 assumes the existence of a trusted server and discusses the open problems and challenges in providing protections against adversarial clients and/or analysts.”
  • The paper relies on this without support.

    Secure aggregation can ensure that the central server obtains the aggregated result while guaranteeing that intermediate parameters of individual devices and participants are not revealed to the central server.

    “Secure aggregation can be used to ensure that the central server obtains the aggregated result, while guaranteeing that intermediate parameters of individual devices and participants are not revealed to the central server.”
  • The paper relies on this without support.

    A secure shuffler randomly permutes reports and sends the collection of shuffled reports without identifying information to the server for final analysis.

    “The shuffler randomly permutes the reports and sends the collection of shuffled reports (without any identifying information) to the server for final analysis.”
  • The paper relies on this without support.

    TEEs provide confidentiality features that can protect computations and data inside the enclave.

    “each client could run their local portion of federated learning inside a TEE providing confidentiality features”
  • The paper relies on this without support.

    MPC techniques can provide protections similar to TEE-based concealment for federated learning.

    “Similar protections can be achieved under the MPC model [351, 10].”
  • The paper relies on this without support.

    Homomorphic encryption allows computation on encrypted model parameters without decrypting them.

    “The client could then compute the encrypted model update using the homomorphic properties of the cryptosystem, without needing to decrypt the model parameters.”
  • The authors state this as an assumption.

    The distributed DP model assumes that participating clients remain connected to the server during a round.

    “The above model of distributed DP assumes that participating clients remain connected to the server during a round.”
  • The authors state this as an assumption.

    LDP assumes that a user's privacy comes solely from that user's addition of their own randomness.

    “LDP assumes that a user's privacy comes solely from that user's addition of their own randomness”
  • The authors state this as an assumption.

    Current work on the hybrid model assumes that regardless of the user trust preference, their data comes from the same distribution.

    “current work on the hybrid model typically assumes that regardless of the user trust preference, their data comes from the same distribution”
  • The authors state this as an assumption.

    Secure Aggregation assumes a semi-honest server (only in the private key infrastructure phase).

    “it assumes a semi-honest server (only in the private key infrastructure phase)”
  • The authors state this as an assumption.

    In the centralized setup, all users' data is assumed to be held by a single entity and no communication is required.

    “in the centralized setup where all the users' data is already assumed to be held by a single entity and hence no communication is required”
  • The authors state this as an assumption.

    The adversary directly manipulates model updates throughout Section 5.1.2.

    “We assume direct update manipulation throughout this section, as this strictly enhances the capability of the adversary.”
  • The authors state this as an assumption.

    The adversary (or adversaries) directly control some number of clients and can directly alter the outputs of these clients to bias the learned model towards their objective.

    “Thus, we assume that the adversary (or adversaries) directly control some number of clients, and that they can directly alter the outputs of these clients to try to bias the learned model towards their objective.”
  • The paper relies on this without support.

    Adversaries may be more effective if they can coordinate their poisoned updates than if they each acted individually.

    “Intuitively, the adversaries may be more effective if they are able to coordinate their poisoned updates than if they each acted individually.”
  • The paper relies on this without support.

    There is a current lack of demonstrably robust training methods in the federated setting.

    “Given the current lack of demonstrably robust training methods in the federated setting, this may not be a practical option.”
  • The paper relies on this without support.

    Even if robust training techniques existed, they may be too computation-intensive for many federated learning applications.

    “Moreover, even if such techniques existed, they may be too computation-intensive for many federated learning applications.”
  • The paper relies on this without support.

    Simulations of federated learning require dealing with multiple issues that do not arise in datacenter ML research, such as efficiently processing partitioned datasets with computations running on different simulated devices, each with a variable amount of data.

    “Simulations of federated learning require dealing with multiple issues that do not arise in datacenter ML research, for example, efficiently processing partitioned datasets, with computations running on different simulated devices, each with a variable amount of data.”
  • The paper relies on this without support.

    FL research requires different metrics such as the number of bytes upload or downloaded by device, as well as the ability to simulate issues like time-varying arrival of different clients or client drop-out that is potentially correlated with the nature of the local dataset.

    “FL research also requires different metrics such as the number of bytes upload or downloaded by device, as well as the ability to simulate issues like time-varying arrival of different clients or client drop-out that is potentially correlated with the nature of the local dataset.”
  • The paper relies on this without support.

    Federated learning is adopted when the data is decentralized and typically unbalanced and not identically distributed.

    “Federated learning is adopted when the data is decentralized and typically unbalanced (different clients have different numbers of examples) and not identically distributed (each client's data is drawn from a different distribution).”
41 of 41

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Citations

Cites

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Show all 451 references

Research works printed but not linked: 44

  1. McMahan, 2017, Federated learning: Collaborative machine learning without centralized training data
  2. Differential Privacy Team, 2017, Learning with privacy at scale, Apple Machine Learning Journal, 1
  3. Pichai, 2019, Google’s Sundar Pichai: Privacy should not be a luxury good
  4. Apple , Private federated learning (NeurIPS 2019 Expo Talk Abstract), https://nips.cc/ExpoConferences/2019/schedule?talk_id=40, 2019.
  5. Abadi, 2015, Tensor Flow: Large-scale machine learning on heterogeneous systems
  6. Tang, D2: Decentralized training over decentralized data
  7. Koloskova, Decentralized deep learning with arbitrary communication compression
  8. Jaderberg, Decoupled neural interfaces using synthetic gradients, 1627
  9. Wang, Adaptive communication strategies for best error-runtime trade-offs in communication-efficient distributed SGD
  10. He, Milenas: Efficient neural architecture search via mixed-level reformulation
  11. Barnes, 2020, Lower bounds for learning distributions under communication constraints via Fisher information, J. Mach. Learn. Res., 21, 1
  12. Han, Geometric lower bounds for distributed parameter estimation under communication constraints, 1
  13. Bell, Secure single-server aggregation with (poly)logarithmic overhead, 1253
  14. Andrew, How to generate and exchange secrets (extended abstract), 162
  15. Nikolaenko, Privacy-preserving ridge regression on hundreds of millions of records, 334
  16. Barak, 2019, Secure evaluation of quantized neural networks, IACR Cryptology ePrint Archive, 2019, 131
  17. 2020, Lattigo 2.0.0
  18. 2020, Microsoft SEAL (release 3.6)
  19. 2020, SHELL
  20. Roth, Honeycrisp: Large-scale differentially private aggregation without a trusted core, 196
  21. Intel, 2012, Architecture instruction set extensions programming reference
  22. Shi, Privacy-preserving aggregation of time-series data
  23. Lie, Glimmers: Resolving the privacy/trust quagmire, 94
  24. Sabater, 2020, Distributed differentially private averaging with improved utility and robustness to malicious parties
  25. Koeberl, TrustLite: A security architecture for tiny embedded devices, 10:1
  26. Amin, Bounding user contributions: A bias-variance trade-off in differential privacy, 263
  27. Feldman, Privacy amplification by iteration, 521
  28. Balle, Advances in Neural Information Processing Systems, 4623
  29. Riazi, 2019, HEAX: High-performance architecture for computation on homomorphically encrypted data in the cloud
  30. Cummings, Differentially private change-point detection, 10825
  31. Cummings, Differential privacy for growing databases, 8864
  32. Tramèr, Stealing machine learning models via prediction APIs, 601
  33. Bassily, 2020, Practical locally private heavy hitters, J. Mach. Learn. Res., 21, 1
  34. Cummings, 2019, Bringing differential privacy into the experimental economics lab: Theory and an application to a public-good game
  35. Goodfellow, Explaining and harnessing adversarial examples
  36. Chen, DRACO: Byzantine-resilient distributed training via redundant gradients
  37. Xie, Zeno: Distributed stochastic gradient descent with suspicion-based fault-tolerance, 6893
  38. Chen, ZOO: Zeroth order optimization based black-box attacks to deep neural networks without training substitute models, 15
  39. Wang, 2020, Attack of the tails: Yes, you really can backdoor federated learning
  40. Barocas, 2019, Fairness and Machine Learning
  41. Richardson, 2019, Dirty data, bad predictions: How civil rights violations impact police data, predictive policing systems, and justice
  42. Sambasivan, ‘privacy is not for me, it’s for those rich women’: Performative privacy practices on mobile phones by women in South Asia, 127
  43. Madras, Learning adversarially fair and transferable representations
  44. Ramage, 2020, Federated analytics: Collaborative data science without data collection

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Metadata

Authors
Kairouz, P., & McMahan, H. B.
Affiliations
  • Google Research, USA
  • Google (United States) (US)
Venue
Foundations and Trends in Machine Learning
Publisher
Emerald
Published
2021-06-23
Volume
14
Issue
1-2
Pages
1-210
ISSN
  • 1935-8237 (ISSN-L)
  • 1935-8245
OpenAlex
W2995022099
Cited by
7,772
References
451
Citations by year
2026: 1786 · 2025: 1736 · 2024: 1359 · 2023: 1149 · 2022: 781 · 2021: 665
FWCI
282.57
Type
journal article
Language
en
Open access
Locations
  • Foundations and Trends® in Machine Learning (publishedVersion)
  • Warwick Research Archive Portal (University of Warwick) (acceptedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • HAL (submittedVersion, other-oa)
  • HAL (submittedVersion, cc-by)
  • Infoscience (Ecole Polytechnique Fédérale de Lausanne) (submittedVersion, cc-by-nc-nd)
  • LillOA (Université de Lille (University Of Lille)) (submittedVersion, other-oa)
  • Oskar-Bordeaux (Universite de Bordeaux) (submittedVersion)
  • Rare & Special e-Zone (The Hong Kong University of Science and Technology) (submittedVersion)
  • arXiv (Cornell University)
Funders
Agence Nationale de la Recherche (ANR-16-CE23-0016, ANR-20-CE23-0015)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Privacy-Preserving Technologies in Data
  • Physical Sciences › Computer Science › Artificial Intelligence › Stochastic Gradient Optimization Techniques
  • Physical Sciences › Computer Science › Artificial Intelligence › Domain Adaptation and Few-Shot Learning
Keywords
federated learning, decentralized data, privacy-preserving machine learning, data minimization, open problems, mobile devices, data collection

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