REVIEW 3 major objections 4 minor 46 references
Private Counterfactual Retrieval With Immutable Features
T0 review · 3 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read The paper proves that a rejected user can retrieve the index of the closest accepted counterfactual from a replicated database while keeping the feature vector, the immutable set, and the retrieved index information-theoretically private…
desk verdict The two I-PCR schemes are legitimate and the algebra holds under the stated non-colluding assumption, but the leakage table contradicts itself at |I|=0 and the 'private from the institution' claim outruns the per-server guarantee actually proven. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the $3\times 3$ Vandermonde matrix $M_3$ built from three distinct server evaluation points $\alpha_1,\alpha_2,\alpha_3$. Each server's answer is a polynomial in $\alpha_n$ whose constant coefficient is the quantity the user needs and whose higher coefficients are random interference terms; with three servers the user interpolates the constant term while each individual server sees only a masked query. The two-phase scheme uses binary indicator masks $h_1$ (immutable coordinates) and $h_2$ (matching records), and the single-phase scheme uses a weight vector $h$ with entries $1$ or $L$, where $L>R^2 d$, so that the value $(y_i-x)^T((y_i-x)\circ h)$ lies in $[0,(d-|I|)R^2]$ for matching immutable features and in disjoint higher intervals $[kL,kL+R^2(d-|I|)]$ when $k$ immutable features mismatch.
What would settle it
Take one execution of the two-phase scheme and place all three query tuples $Q_n = [h_1+\alpha_n Z_1,\, x\circ h_1+\alpha_n Z_2]$ in front of a single adversary who knows the distinct evaluation points $\alpha_1,\alpha_2,\alpha_3$; interpolating the degree-2 polynomial in $\alpha$ for each coordinate reveals the constant term $h_1$, the indicator vector of $I$, and $x\circ h_1$, the user's values on the immutable coordinates. This direct calculation shows the per-server privacy claim (5) fails if the non-collusion assumption is dropped.
Extended reading notes
Core claim
The central claim is that the exact counterfactual index $\theta^* = \arg\min_{i\in\Theta}\|y_i - x\|^2$ with $\Theta = \{i : y_{i,I} = x_I\}$ is retrievable with information-theoretic user privacy under both schemes, satisfying decodability $H(\theta^*|Q_{[N]}^{[x]}, A_{[N]}^{[x]}, x, I)=0$ and privacy $I(x,\theta^*,I; Q_n^{[x]}, A_n^{[x]}|D)=0$ for each server $n$, with $N=3$ replicated servers. In the two-phase scheme the user first learns the set $\Theta$ of records whose immutable coordinates match, then retrieves the squared distances only for those records and takes the minimum. In the single-phase scheme the server evaluates a weighted inner product $(y_i-x)^T((y_i-x)\circ h)$ with a weight vector $h$ that assigns a large factor $L$ to immutable coordinates, so the value falls in a low, distinguishable range exactly when the immutable features match. Both schemes leak some information about the database; the single-phase scheme leaks more because a record with $k$ mismatched immutable features produces a value in the overlapping interval $[kL, kL+R^2(d-|I|)]$, revealing partial information about non-matching records.
Load-bearing premise
The privacy guarantee rests on the three servers being genuinely non-colluding and non-communicating, with shared randomness that the user never sees; if one party controls all three servers and pools their query transcripts, the user's feature vector and immutable set can be recovered by interpolation.
Editorial extensions
If this is right
- If the schemes are correct, a rejected user can obtain the closest feasible counterfactual without ever revealing their input $x$, their immutable set $I$, or the retrieved index $\theta^*$ to any one server.
- The single-phase scheme strictly dominates on communication ($6d+3M$ versus $9(d+M)$) but leaks more about non-matching records, so the choice is a concrete privacy-efficiency tradeoff.
- Both schemes extend to weighted actionability on mutable features without leaking the immutable set; the two-phase version needs a fourth server and a larger field, while the single-phase version keeps three servers with a larger field.
- Because the distance metric is the squared $\ell_2$ norm, the same answer construction works for any even $\ell_k$ norm by raising the relevant terms to the power $k$.
- The schemes automatically inform the user when no feasible counterfactual exists ($\Theta=\emptyset$), which is itself useful and costs no extra communication.
Reading between the lines
- Beyond the paper: the three-server privacy guarantee is not robust against a single institution that controls all replicas; pooling the three masked queries allows interpolation of the user's immutable-mask vector, so the scheme is best understood as secure against independent server operators rather than against the database owner.
- Beyond the paper: the same polynomial-masking template could be adapted to private nearest-neighbor search with arbitrary user-private predicates, not just fixed immutable coordinates, provided the predicate can be encoded as a masking vector.
- Beyond the paper: the leakage comparison suggests a possible improvement direction in which server-side randomization of distances trades exact nearest-neighbor optimality for lower database leakage, although that is not explored in the paper.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper introduces the immutable private counterfactual retrieval (I-PCR) problem, in which a user with a rejected feature vector x and a private immutable-feature set I wishes to retrieve theta* = argmin_{i in Theta} ||y_i - x||^2, with Theta = {i : y_{i,I} = x_I}, from M database records replicated on N = 3 non-colluding servers, while keeping x, I, and theta* private from each server. Two schemes are proposed: a two-phase scheme (first retrieve Theta, then retrieve distances only for i in Theta) with communication cost 9(d+M) symbols, and a single-phase scheme using an L-scaled weight vector h with cost 6d+3M symbols. The paper also extends the schemes to private actionability weights on mutable features and reports a numerical database-leakage comparison in Table I.
Significance. The immutable-feature constraint is a natural and useful generalization of the PCR problem of [20], and the algebraic constructions are mostly sound: Eqs. (12), (22), (29), and (33) decode correctly under the stated N=3 non-colluding assumption with the chosen field sizes. The explicit communication costs and the attempt to quantify database leakage are valuable contributions. However, the leakage comparison currently contains a concrete numerical inconsistency, and the advertised 'private from the institution' guarantee is stronger than the per-server privacy condition actually proved. With those issues repaired, the paper would be a solid incremental contribution to private counterfactual retrieval.
major comments (3)
- [Section V, Table I, row |I|=0] The two entries in this row cannot differ. With I = empty, the two-phase scheme has h1 = 0 and h2 = (1,...,1), so its second-phase decoded value is ||y_i - x||^2 for every i, while the single-phase scheme decodes (y_i - x)^T((y_i - x) ∘ 1) = ||y_i - x||^2 for every i. The remaining downloaded coefficients are masked by the independent server randomness Z'_1,...,Z'_4 and, in Phase 1, by the unknown nonzero scaling rho_i, so they contribute nothing to I(y[M]; A[N] | x, I). Hence the two schemes have exactly the same database leakage at |I|=0, yet Table I reports 1.1432 versus 0.9422. This internal inconsistency invalidates the numerical comparison and the statement in Section IV-D/V that the two-phase scheme 'is always less' than the single-phase scheme; at |I|=0 they are equal, and the single-phase |I|=0 and |I|=3 entries coincide for the same reason.
- [Abstract, Section I, and Eq. (5)] The privacy guarantee is stated per server n under the N=3 non-colluding assumption, but the abstract and introduction promise privacy 'from the institution.' A party that observes all three transcripts can interpolate the degree-1 query polynomials in (9), (18), and (27) to recover h1, x ∘ h1, h2, x, and hence I and theta*. Thus the schemes are private only against a single server, not against an institution that controls or monitors all replicas. Please either weaken the 'institution' claim throughout or extend the model to colluding servers; as written, the headline claim exceeds what Eq. (5) proves.
- [Sections IV-D and V] The analytical leakage comparison is informal at the point where Eq. (39) asserts that the query randomness 'align[s] on a subspace different from the database information subspace,' and the general conclusion that the two-phase leakage is always smaller is drawn from a single synthetic configuration. Please provide a formal derivation of which decoded coefficients actually depend on D and either prove the ordering under the stated distributions or present the comparison as an empirical observation for the tested setting.
minor comments (4)
- [Section III-A, Eq. (10)] Please state explicitly that rho_i is part of the servers' common randomness and is therefore the same rho_i for all N servers; otherwise the Vandermonde recovery in Eq. (13) does not follow, since the first column of the answer vector would be [rho_{i,1}, rho_{i,2}, rho_{i,3}]^T instead of [1,1,1]^T.
- [Section IV-D, J_k definition] The printed general formula J_k = [kL : kL + R^2(d-|I|)] is dimensionally inconsistent with the k=1 and k=2 expressions; it should be [kL : kL R^2 + R^2(d-|I|)], and even that is an outer bound because sums of k nonzero squares are not a contiguous integer range.
- [Section V, Table I] Please include the exact enumeration procedure or code used to compute the 'exact leakage values' in Table I; the text does not specify how the mutual information was evaluated, so the reported numbers are not reproducible from the paper alone.
- [Eqs. (7) and (40)] The indicator function is rendered as '/BD' in the typeset text; please fix the notation to a standard indicator symbol.
Circularity Check
No significant circularity; the I-PCR schemes are derived from explicit PIR query/answer algebra and evaluated without fitted predictions.
full rationale
The derivation chain is self-contained. The target quantity θ* is defined by (2), and both schemes present explicit queries and answers (eqs. (8)-(24) and (26)-(33)); the user decodes by the stated Vandermonde interpolation using N=3 distinct α_n, so neither scheme uses its own conclusion as an input. No parameter is fitted to data and then reported as a prediction: the leakage numbers in Section V are direct evaluations from explicitly stated synthetic distributions (R=3, d=3, M=3, uniform draws, q=757), not fitted outputs. The citation to the authors' prior PCR work [20] labels the baseline scheme and provides context, but the single-phase construction is fully specified in the paper and does not defer any load-bearing step to [20]. The only caveat is interpretive: privacy is proven per server under the N=3 non-colluding assumption in (5), so 'private from the institution' should be read in that threat-model sense; this is a modeling-scope issue, not a circular reduction.
Assumptions & free parameters
free parameters (2)
- L (single-phase immutable scaling factor) =
integer with L > R^2d; L = 28 in the numerical section
- L1 (actionability weight bound) =
positive integer; user-chosen
assumptions (6)
- domain assumption The N=3 servers are non-colluding and non-communicating, and share common randomness Z' that is independent of the database.
- domain assumption The user's sample x has been rejected, so x is not in the accepted-sample database D.
- domain assumption Each of the d features takes integer values in [0:R].
- domain assumption The squared l2 distance is the comparison metric; Remark 2 claims extension to any even lk without proof.
- standard math The Vandermonde matrix M3 with distinct alpha1, alpha2, alpha3 in F_q is invertible.
- standard math Field size q is chosen large enough that all true squared distances and weighted sums are distinct from zero and do not wrap modulo q.
Cite this review
Pith. "Pith review of Private Counterfactual Retrieval With Immutable Features." pith.science (2026). https://pith.science/paper/G5A2MTBF
@misc{pith2026241110429,
author = {Pith},
title = {Pith review of: Private Counterfactual Retrieval With Immutable Features},
year = {2026},
howpublished = {\url{https://pith.science/paper/G5A2MTBF}},
note = {Machine review of arXiv:2411.10429}
}
read the original abstract
In a classification task, counterfactual explanations provide the minimum change needed for an input to be classified into a favorable class. We consider the problem of privately retrieving the exact closest counterfactual from a database of accepted samples while enforcing that certain features of the input sample cannot be changed, i.e., they are \emph{immutable}. An applicant (user) whose feature vector is rejected by a machine learning model wants to retrieve the sample closest to them in the database without altering a private subset of their features, which constitutes the immutable set. While doing this, the user should keep their feature vector, immutable set and the resulting counterfactual index information-theoretically private from the institution. We refer to this as immutable private counterfactual retrieval (I-PCR) problem which generalizes PCR to a more practical setting. In this paper, we propose two I-PCR schemes by leveraging techniques from private information retrieval (PIR) and characterize their communication costs. Further, we quantify the information that the user learns about the database and compare it for the proposed schemes.
Figures
Reference graph
Works this paper leans on
-
[20]
Private Counterfactual Retrieval
M. Nomeir, P . Dissanayake, S. Meel, S. Dutta, and S. Uluk us. Private counterfactual retrieval. arXiv preprint arXiv:2410.13812 , 2024
work page Pith review arXiv 2024
-
[1]
P . V oigt and A. Bussche. The EU general data protection re gulation (GDPR). A Practical Guide, 1st Ed., Cham: Springer International Publishing, 10(3152676):10–5555, 2017
work page 2017
-
[2]
S. Wachter, B. Mittelstadt, and C. Russell. Counterfact ual explanations without opening the black box: Automated decisions and the g dpr. Cybersecurity, 2017
work page 2017
-
[3]
D. Brughmans, P . Leyman, and D. Martens. Nice: an algorit hm for nearest instance counterfactual explanations. Data Mining and Knowledge Discovery, 38, April 2023
work page 2023
-
[4]
S. Upadhyay, S. Joshi, and H. Lakkaraju. Towards robust a nd reliable algorithmic recourse. Advances in Neural Information Processing Systems, 34:16926–16937, 2021
work page 2021
- [5]
- [6]
- [7]
Show all 46 references
-
[8]
Poyiadzi, K
R. Poyiadzi, K. Sokol, R. Santos-Rodriguez, T. De Bie, an d P . Flach. Face: Feasible and actionable counterfactual explanation s. Association for Computing Machinery, 2020
2020
-
[9]
Mothilal, A
R. Mothilal, A. Sharma, and C. Tan. Explaining machine le arn- ing classifiers through diverse counterfactual explanatio ns. CoRR, abs/1905.07697, 2019
1905 arXiv
-
[10]
V erma, J
S. V erma, J. Dickerson, and K. Hines. Counterfactual ex planations for machine learning: A review. arXiv preprint arXiv:2010.10596, 2:1, 2020
2010 arXiv
-
[11]
Guidotti
R. Guidotti. Counterfactual explanations and how to fin d them: literature review and benchmarking. Data Mining and Knowledge Discovery , 38(5), 2024
2024
-
[12]
Mishra, S
S. Mishra, S. Dutta, J. Long, and D. Magazzeni. A survey o n the robustness of feature importance and counterfactual expla nations. arXiv preprint arXiv:2111.00358, 2021
2021 arXiv
-
[13]
Pawelczyk, H
M. Pawelczyk, H. Lakkaraju, and S. Neel. On the privacy r isks of algorithmic recourse. In International Conference on Artificial Intelligence and Statistics . PMLR, 2023
2023
-
[14]
Y ang, Q
F. Y ang, Q. Feng, K. Zhou, J. Chen, and X. Hu. Differentia lly private counterfactuals via functional mechanism. arXiv preprint arXiv:2208.02878, 2022
2022 arXiv
-
[15]
Goethals, K
S. Goethals, K. S¨ orensen, and D. Martens. The privacy i ssue of counterfactual explanations: Explanation linkage attack s. ACM Trans. Intell. Syst. Technol. , 14, October 2023
2023
-
[16]
Pentyala, S
S. Pentyala, S. Sharma, S. Kariyappa, F. Lecue, and D. Ma g- azzeni. Privacy-preserving algorithmic recourse. arXiv preprint arXiv:2311.14137, 2023
2023 arXiv
-
[17]
A¨ ıvodji, A
U. A¨ ıvodji, A. Bolot, and S. Gambs. Model extraction fr om counter- factual explanations. arXiv preprint arXiv:2009.01884 , 2020
2009 arXiv
-
[18]
Y . Wang, H. Qian, and C. Miao. Dualcf: Efficient model ext raction attack from counterfactual explanations. In Proceedings of the 2022 ACM Conference on Fairness, Accountability, and Transpare ncy, 2022
2022
-
[19]
Dissanayake and S
P . Dissanayake and S. Dutta. Model reconstruction usin g counterfactual explanations: A perspective from polytope theory. In Advances in Neural Information Processing Systems (NeurIPS) , 2024
2024
-
[21]
B. Chor, E. Kushilevitz, O. Goldreich, and M. Sudan. Pri vate information retrieval. Jour . of the ACM, 45(6):965–981, November 1998
1998
-
[22]
Sun and S
H. Sun and S. A. Jafar. The capacity of private informati on retrieval. IEEE Trans. Info. Theory , 63(7):4075–4088, July 2017
2017
-
[23]
Sun and S
H. Sun and S. A. Jafar. The capacity of symmetric private information retrieval. IEEE Trans. Info. Theory , 65(1):322–329, June 2018
2018
-
[24]
X. Y ao, N. Liu, and W. Kang. The capacity of private infor mation retrieval under arbitrary collusion patterns for replicat ed databases. IEEE Trans. Info. Theory , 67(10):6841–6855, July 2021
2021
-
[25]
Banawan and S
K. Banawan and S. Ulukus. Private information retrieva l through wiretap channel II: Privacy meets security. IEEE Trans. Info. Theory , 66(7):4129–4149, February 2020
2020
-
[26]
Banawan and S
K. Banawan and S. Ulukus. Multi-message private inform ation retrieval: Capacity results and near-optimal schemes. IEEE Trans. Info. Theory , 64(10):6842–6862, April 2018
2018
-
[27]
Banawan and S
K. Banawan and S. Ulukus. The capacity of private inform ation retrieval from coded databases. IEEE Trans. Info. Theory , 64(3):1945–1956, January 2018
1945
-
[28]
Banawan, B
K. Banawan, B. Arasli, Y .-P . Wei, and S. Ulukus. The capa city of private information retrieval from heterogeneous uncoded caching databases. IEEE Trans. Info. Theory , 66(6):3407–3416, June 2020
2020
-
[29]
Banawan and S
K. Banawan and S. Ulukus. The capacity of private inform ation retrieval from Byzantine and colluding databases. IEEE Trans. Info. Theory , 65(2):1206–1219, September 2018
2018
-
[30]
Q. Wang, H. Sun, and M. Skoglund. The capacity of private information retrieval with eavesdroppers. IEEE Transactions on Information Theory , 65(5):3198–3214, December 2018
2018
-
[31]
C. Tian, H. Sun, and J. Chen. Capacity-achieving privat e information retrieval codes with optimal message size and upload cost. IEEE Trans. Info. Theory , 65(11):7613–7627, November 2019
2019
-
[32]
Holzbaurand, R
L. Holzbaurand, R. Freij-Hollanti, and C. Hollanti. On the capacity of private information retrieval from coded, colluding, an d adversarial servers. In IEEE ITW , August 2019
2019
-
[33]
Sun and S
H. Sun and S. A. Jafar. The capacity of robust private inf ormation re- trieval with colluding databases. IEEE Trans. Info. Theory , 64(4):2361– 2370, April 2018
2018
-
[34]
Z. Jia, H. Sun, and S. A. Jafar. Cross subspace alignment and the asymptotic capacity of X-secure T -private information retrieval. IEEE Trans. Info. Theory , 65(9):5783–5798, May 2019
2019
-
[35]
Y ang, W
H. Y ang, W. Shin, and J. Lee. Private information retrie val for secure distributed storage systems. IEEE Trans. Info. F oren. Security , 13(12):2953–2964, May 2018
2018
-
[36]
Y .-P . Wei, K. Banawan, and S. Ulukus. Fundamental limit s of cache- aided private information retrieval with unknown and uncod ed prefetch- ing. IEEE Trans. Info. Theory , 65(5):3215–3232, November 2018
2018
-
[37]
Wang and S
Z. Wang and S. Ulukus. Symmetric private information re trieval at the private information retrieval rate. IEEE Jour . on Selected Areas in Info. Theory, 3(2):350–361, June 2022
2022
-
[38]
Ulukus, S
S. Ulukus, S. Avestimehr, M. Gastpar, S. A. Jafar, R. Tan don, and C. Tian. Private retrieval, computing, and learning: Recen t progress and future challenges. IEEE Journal on Selected Areas in Communications , 40(3):729–748, March 2022
2022
-
[39]
M. A. Attia, D. Kumar, and R. Tandon. The capacity of priv ate information retrieval from uncoded storage constrained da tabases. IEEE Trans. Info. Theory , 66(11):6617–6634, September 2020
2020
-
[40]
Wang and M
Q. Wang and M. Skoglund. Symmetric private information retrieval from MDS coded distributed storage with non-colluding and c olluding servers. IEEE Trans. Info. Theory , 65(8):5160–5175, March 2019
2019
-
[41]
Woolsey, R
N. Woolsey, R. Chen, and M. Ji. Uncoded placement with li near sub-messages for private information retrieval from stora ge constrained databases. IEEE Trans. Commun. , 68(10):6039–6053, October 2020
2020
-
[42]
Z. Chen, Z. Wang, and S. A. Jafar. The capacity of T -private information retrieval with private side information. IEEE Trans. Info. Theory , 66(8):4761–4773, March 2020
2020
-
[43]
Q. Wang, H. Sun, and M. Skoglund. The capacity of private information retrieval with eavesdroppers. IEEE Trans. Inf. Theory, 65(5):3198–3214, December 2018
2018
-
[44]
Vithana, K
S. Vithana, K. Banawan, and S. Ulukus. Semantic private information retrieval. IEEE Trans. Info. Theory , 68(4):2635–2652, December 2021
2021
-
[45]
Tajeddine, O
R. Tajeddine, O. Gnilke, and S. El Rouayheb. Private inf ormation retrieval from MDS coded data in distributed storage system s. IEEE Trans. Info. Theory , 64(11):7081–7093, March 2018
2018
-
[46]
Cheng, N
J. Cheng, N. Liu, W. Kang, and Y . Li. The capacity of symme tric private information retrieval under arbitrary collusion a nd eavesdropping patterns. IEEE Trans. Info. F oren. Security, 17:3037–3050, August 2022
2022
Reviewed August 12, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.