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arxiv: 1907.01317 · v1 · pith:XKXQVFNAnew · submitted 2019-07-02 · 💻 cs.CR

Padding Ain't Enough: Assessing the Privacy Guarantees of Encrypted DNS

Pith reviewed 2026-05-25 11:19 UTC · model grok-4.3

classification 💻 cs.CR
keywords encrypted DNSDNS over TLSDNS over HTTPStraffic analysismessage paddingwebsite fingerprintingprivacy attackdeanonymization
0
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The pith

Padding encrypted DNS messages fails to prevent website identification from traffic analysis.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that padding DNS messages in DoT and DoH, as recommended by RFC 8467, still leaves enough size and timing information for attackers to identify which websites a user visits. The authors build a classifier that models full sequences of DNS resolutions triggered by a site load rather than single queries, then matches observed padded traces against those models. In a closed-world test on the Alexa top-10k sites, the method correctly labels at least half the traces for 80.2 percent of sites and all traces for 32 percent of sites. Readers should care because current encrypted-DNS deployments rely on padding to deliver their privacy guarantees, yet this work indicates those guarantees do not hold against a passive network observer who can collect traces.

Core claim

The authors show that a classifier combining message sizes and timing information of padded DNS queries can deanonymize website visits. Using sequences of DNS resolutions instead of single queries, their attack succeeds in labeling at least half the test traces correctly for 80.2% of Alexa top-10k sites and all traces for 32% of them. They conclude that mitigations must remove entropy from inter-arrival timings between query responses.

What carries the argument

A size-and-timing classifier applied to modeled sequences of DNS resolutions triggered by website loads.

If this is right

  • Padding alone, as specified in RFC 8467, does not achieve the intended privacy protection for DoT and DoH.
  • Website fingerprinting remains feasible even when every DNS message is padded and encrypted.
  • Any effective defense must eliminate distinguishable inter-arrival timing patterns between responses.
  • DNS sequence modeling captures more identifying structure than single-query analysis.
  • The attack works against the full complexity of modern websites that trigger dozens of resolutions.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar size-timing leaks could appear in other padded encrypted protocols that carry variable-length objects.
  • Real deployments may see lower accuracy once unknown sites and changing network conditions are included.
  • Combining the DNS classifier with other side channels such as TCP or TLS fingerprints would likely raise success rates further.
  • Randomizing or constant-timing DNS response delivery would be a direct way to test the paper's timing-entropy claim.

Load-bearing premise

The attacker possesses a complete, up-to-date model of DNS query sequences for every website and real user traces match the training sequences closely enough for the classifier to succeed.

What would settle it

Collecting fresh padded DNS traces from visits to the same Alexa top-10k sites over varied networks and finding that the size-timing classifier drops to near-random accuracy would falsify the attack's reported effectiveness.

Figures

Figures reproduced from arXiv: 1907.01317 by Christian Rossow, Jonas Bushart.

Figure 1
Figure 1. Figure 1: shows the effect of k on the performance of the classification. The plot shows the percentage of correctly classified DNS sequences out of the 92 050 sequences in total. Irregardless of k, we can classify over 75 % of DNS sequences correctly. Since the accuracy of even k’s is even worse than for odd k’s, such that we exclude them from further analysis. The accuracy differs between 76.4 % for k = 1 to the b… view at source ↗
Figure 2
Figure 2. Figure 2: shows how many of the 10 traces per website we [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: ROC curve showing TPR and FPR when varying the maximum [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Average of classifying partially cached DNS traces using a model [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Classifying both partially cached and non-cached DNS traces using [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Comparison between our classifier (baseline; blue), a simulated [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Comparison of the overhead between AP and CR. The x-axis shows [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Overview over the distances between traces of the same domain. The [PITH_FULL_IMAGE:figures/full_fig_p016_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Analogue to [PITH_FULL_IMAGE:figures/full_fig_p016_10.png] view at source ↗
read the original abstract

DNS over TLS (DoT) and DNS over HTTPS (DoH) encrypt DNS to guard user privacy by hiding DNS resolutions from passive adversaries. Yet, past attacks have shown that encrypted DNS is still sensitive to traffic analysis. As a consequence, RFC 8467 proposes to pad messages prior to encryption, which heavily reduces the characteristics of encrypted traffic. In this paper, we show that padding alone is insufficient to counter DNS traffic analysis. We propose a novel traffic analysis method that combines size and timing information to infer the websites a user visits purely based on encrypted and padded DNS traces. To this end, we model DNS sequences that capture the complexity of websites that usually trigger dozens of DNS resolutions instead of just a single DNS transaction. A closed world evaluation based on the Alexa top-10k websites reveals that attackers can deanonymize at least half of the test traces in 80.2% of all websites, and even correctly label all traces for 32.0% of the websites. Our findings undermine the privacy goals of state-of-the-art message padding strategies in DoT/DoH. We conclude by showing that successful mitigations to such attacks have to remove the entropy of inter-arrival timings between query responses.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 1 minor

Summary. The paper claims that padding in DoT/DoH is insufficient to prevent traffic analysis. It introduces a sequence-based attack using size and timing features from multi-query DNS traces and reports, in a closed-world evaluation on Alexa top-10k sites, that at least half the test traces can be deanonymized for 80.2% of websites and all traces correctly labeled for 32.0% of websites. The authors conclude that effective mitigations must eliminate entropy in inter-arrival timings between responses.

Significance. If the closed-world results generalize, the work shows that existing padding strategies (per RFC 8467) fail to meet their privacy goals against sequence-aware adversaries and usefully highlights timing as the remaining leakage vector. The modeling of full DNS query sequences (rather than single transactions) is a methodological strength that better reflects real website behavior. The paper provides concrete, falsifiable accuracy numbers that can be tested against the described corpus.

major comments (3)
  1. [Closed-world evaluation (Section 5)] Closed-world evaluation (Section 5): The reported figures (80.2% of sites with ≥50% trace deanonymization; 32% with 100% accuracy) are obtained under the assumption that the attacker possesses a complete, up-to-date model of DNS sequences for every Alexa top-10k site. This assumption is load-bearing for the central claim that padding fails against practical adversaries, yet no open-world results (precision, recall against a background of unseen sites) are provided to quantify performance when the model is necessarily incomplete.
  2. [Classifier and feature details (Section 4)] Classifier and feature details (Section 4): The abstract states concrete accuracy numbers, but the manuscript provides insufficient information on the training/test split ratios, exact feature extraction for the size+timing classifier, choice of learning algorithm, and any cross-validation or significance testing. Without these, the 80.2% and 32.0% figures cannot be independently reproduced or assessed for sensitivity to post-hoc choices.
  3. [Mitigation conclusion (Section 6)] Mitigation conclusion (Section 6): The claim that 'successful mitigations have to remove the entropy of inter-arrival timings' is presented as following from the results, but the manuscript does not include an explicit ablation or countermeasure experiment demonstrating that timing removal defeats the attack while padding alone does not.
minor comments (1)
  1. [Abstract] The abstract should explicitly qualify the evaluation as closed-world to prevent readers from overgeneralizing the privacy implications.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive and detailed feedback. We address each major comment below, providing clarifications where the manuscript's scope limits new experiments while committing to revisions that strengthen the presentation without altering the core claims.

read point-by-point responses
  1. Referee: Closed-world evaluation (Section 5): The reported figures (80.2% of sites with ≥50% trace deanonymization; 32% with 100% accuracy) are obtained under the assumption that the attacker possesses a complete, up-to-date model of DNS sequences for every Alexa top-10k site. This assumption is load-bearing for the central claim that padding fails against practical adversaries, yet no open-world results (precision, recall against a background of unseen sites) are provided to quantify performance when the model is necessarily incomplete.

    Authors: We agree that open-world results would better quantify performance against incomplete attacker models. Our closed-world evaluation follows standard practice in traffic analysis to first establish the existence of leakage when an adversary has full knowledge of the target set; the high accuracies demonstrate that sequence and timing features leak website identity despite padding. We will add a new subsection discussing expected open-world degradation, referencing related work on how closed-world baselines inform practical threat models, while noting that collecting a representative background corpus is a substantial undertaking beyond the current scope. revision: partial

  2. Referee: Classifier and feature details (Section 4): The abstract states concrete accuracy numbers, but the manuscript provides insufficient information on the training/test split ratios, exact feature extraction for the size+timing classifier, choice of learning algorithm, and any cross-validation or significance testing. Without these, the 80.2% and 32.0% figures cannot be independently reproduced or assessed for sensitivity to post-hoc choices.

    Authors: We acknowledge the need for greater reproducibility. The revised manuscript will expand Section 4 to specify the train/test split ratios, the exact construction of the size and timing feature vectors from multi-query sequences, the learning algorithm and hyperparameters, and the cross-validation procedure with any significance testing performed. These additions will allow independent verification of the reported figures. revision: yes

  3. Referee: Mitigation conclusion (Section 6): The claim that 'successful mitigations have to remove the entropy of inter-arrival timings' is presented as following from the results, but the manuscript does not include an explicit ablation or countermeasure experiment demonstrating that timing removal defeats the attack while padding alone does not.

    Authors: The conclusion is deductive from the experimental design: all traces were already padded per RFC 8467, so the attack's success with combined size+timing features isolates timing as the residual entropy source. Prior literature has already shown size-only attacks are defeated by padding; we will revise Section 6 to make this logical chain explicit, including a brief contrast with size-only baselines, rather than adding a new ablation experiment. revision: partial

Circularity Check

0 steps flagged

No circularity: standard held-out empirical evaluation on collected traces

full rationale

The paper reports results from a supervised classifier (size+timing features on modeled DNS sequences) trained on one subset of Alexa top-10k traces and evaluated on held-out test traces. No equations, parameter fits, or derivations are presented that reduce the reported deanonymization rates to the inputs by construction. No self-citations are invoked as load-bearing uniqueness theorems or ansatzes. The closed-world setup is an explicit modeling choice whose limitations are acknowledged in the skeptic reading, but the measurement itself is not circular.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review supplies no explicit free parameters, axioms, or invented entities; the evaluation implicitly rests on a closed-world assumption and the representativeness of Alexa top-10k traces.

axioms (1)
  • domain assumption Closed-world assumption: attacker knows every possible website the user might visit and has representative training traces for each.
    Evaluation is performed on the Alexa top-10k; success rates are reported only under this assumption.

pith-pipeline@v0.9.0 · 5743 in / 1261 out tokens · 21592 ms · 2026-05-25T11:19:46.850729+00:00 · methodology

discussion (0)

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