REVIEW 3 major objections 5 minor 42 references
Sharing the Path: A Threshold Scheme from Isogenies and Error Correcting Codes
T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read A threshold scheme that treats missing shares as erasures in a binary code, with the SIDH attack as the recovery engine, claims to share an isogeny path among n participants with threshold t.
desk verdict The idea is fresh but the main theorem is impossible as stated: no binary code can encode a γn-bit message and correct erasures with no redundancy. 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 machinery is an erasure-correcting binary code C of length ell = gamma n together with the algorithmic pipeline that turns the SIDH attack into reconstruction: Robert's polynomial-time solution of the Supersingular Isogeny with Torsion problem, reached through the reduction of the one-torsion-point variant (SI-OTP) given by De Feo, Fouotsa, and Panny. The code's erasure-correction capacity matches exactly the number of missing gamma-bit blocks when t of n participants are present, and the encoding and decoding functions SESS and its inverse connect elliptic-curve points to the bit strings the code operates on.
What would settle it
Take any instance with n > t and compute the parameters in Theorem 4: w is a gamma n-bit string and c is a codeword of length gamma n, so the code has length ell = gamma n and must encode a message of length gamma n, forcing dimension k = gamma n and minimum distance d <= 1; a code with d <= 1 cannot correct a single erasure, contradicting the required gamma(n-t) > 0 erasure-correction capacity.
Extended reading notes
Core claim
The paper's central claim, stated as Theorem 4, is that a binary code C able to correct gamma(n-t) erasures lets one share a secret isogeny I:E0->E1 among n participants with threshold t. The dealer encodes the encoding of the start torsion point P and its image P'=I(P), concatenates them into a gamma n-bit word w, and encodes w into a codeword c of C; participant i receives the gamma-bit block y_i along with its index. Reconstruction fills the t known blocks into the codeword, treats the remaining n-t blocks as erasures, and decodes. The paper further claims, in Theorem 5, that under NIST security level I the scheme is a perfect threshold scheme whenever k/gamma <= t <= n - 128/gamma + 1, with the lower bound coming from the Singleton bound and the upper bound from a brute-force attack by t-1 participants.
Load-bearing premise
The scheme assumes that a binary code of length gamma n can encode the full gamma n-bit message w into a codeword of the same length while still correcting gamma(n-t) erasures; the Singleton bound makes this impossible unless the message is shorter than the codeword.
Editorial extensions
If this is right
- If Theorem 4 holds, any isogeny path can be shared among n parties with reconstruction by any t of them, using only the torsion-point images as auxiliary data.
- The NIST Level I bound gives a concrete recipe for choosing gamma and the code dimension k so that t-1 colluding participants face at least 2^128 guesses.
- The hyperoval subfield-code corollary gives explicit parameter ranges, such as t between 2/gamma or 3/gamma and (2^r - 126)/gamma + 1.
- The binary-representation Reed-Solomon variant handles burst erasures, with each gamma-bit burst affecting at most two RS symbols when r > gamma - 2.
- The construction would be the first isogeny threshold scheme that exploits the SIDH attack as a cryptographic tool rather than treating it only as a break.
Reading between the lines
- The proof of Theorem 4 appears to encode a gamma n-bit message into a codeword of the same length gamma n; if that is literal, the Singleton bound forces dimension k = gamma n and minimum distance d <= 1, so the code cannot correct any erasure. A workable version would need the message length to be strictly less than gamma n, with the extra length providing redundancy.
- The template generalizes: any secret that can be recovered from a pair of auxiliary values via a known polynomial-time attack could in principle be thresholdized by erasure-coding those auxiliary values.
- One testable extension is to replace the rate-1 encoding with a systematic [gamma n, k, d] code and set the message length to k; the threshold bounds would then shift, and one could measure the resulting share size against existing isogeny threshold schemes.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a threshold secret-sharing scheme for supersingular isogeny paths. The dealer encodes two torsion points P and P′ (auxiliary data for recovering the isogeny) into a bit string w = SP||SP′ of length γn, splits w into n blocks of γ bits, and distributes one block to each participant. Reconstruction works by treating the missing blocks from any t participants as erasures, using a binary error-correcting code C to recover w, then invoking the SIDH key-recovery machinery (Robert's algorithm together with the reduction of Feo, Fouotsa, and Panny) to compute the secret isogeny from the recovered points. The main theorems claim that any code C correcting γ(n−t) erasures yields a threshold-t scheme and that the scheme is perfect under the parameter bound k/γ ≤ t ≤ n−128/γ+1. Section 5 proposes instantiations from Reed-Solomon codes, subfield codes of hyperoval codes, and binary representations of RS codes with burst-erasure correction.
Significance. The high-level idea of turning the SIDH attack into a reconstruction tool for a shared isogeny path is conceptually interesting, and the paper correctly emphasizes the distinction between the broken SI-TP problem and the still-hard isogeny path problem. If the construction worked, it would be a novel combination of isogeny-based cryptography, point encodings, and coding-theoretic secret sharing. However, the central encoding/erasure-correction premise is infeasible as stated: the proof of Theorem 4 requires encoding a γn-bit message into a binary codeword of length γn while the same code is supposed to correct γ(n−t) erasures, which the Singleton bound forbids. The security proof of Theorem 5 also conflates brute-force work with perfect secrecy. The paper ships no implementation or machine-checked proof, and the main technical claims are not salvageable by local edits within the current framework.
major comments (3)
- [§3, Theorem 4 and Procedure 1] The proof sets w = SP||SP′ to be a word of length γn and encodes it as a codeword c = (c_1, ..., c_n) of length γn, so that the n participants collectively receive all γn bits. For this encoding to be injective on the whole space {0,1}^{γn}, the binary code C must have dimension k = γn; the Singleton bound then gives minimum distance d ≤ 1, so C cannot correct any erasure. This contradicts the theorem's assumption that C corrects γ(n−t) erasures for t<n and also contradicts the bound t ≥ k/γ in Theorem 5, which together with t<n forces k < γn. The proof never states that w is drawn from a k-dimensional subspace or specifies any code family that avoids this dimensional obstruction; the claimed threshold reconstruction therefore rests on an impossible encoding/erasure-correction premise.
- [§4, Theorem 5] The proof does not establish the perfect threshold property required by Definition 2. It argues that with t−1 shares the adversary must make 2^{γ(n−t+1)} brute-force attempts, but perfect secrecy has a precise information-theoretic meaning: any subset of t−1 shares must have zero mutual information with the secret. No entropy calculation, indistinguishability argument, or simulator is provided. In addition, 'NIST security level I' is used as if it were a formal condition; it is a computational heuristic, and the proof gives no reduction showing that an adversary with t−1 shares and bounded resources cannot learn the path. The derivation of t ≤ n−128/γ+1 from γ(n−t+1) ≥ 128 is presented as self-evidently sufficient for security, but no argument is given for that sufficiency.
- [§3, Procedure 2] The reconstruction algorithm is not well defined. Line 10 sets c ← E^{-1}(y) with the comment 'Correcting the erasures in y', but E in Procedure 1 is the encoder of the code, not a decoder; the correct operation would be a decoder that takes the received word with erasures and returns a codeword, followed by the inverse encoder to recover w. Neither such a decoder nor its erasure-correction guarantee is specified beyond the theorem's assumption. The notation is also inconsistent with the preceding text, where y_i is a block of γ bits but c is indexed as c_1, ..., c_n; the share formula y_i = (c_i, c_{i+1}, ..., c_{i+γ}) is dimensionally wrong. As a result, even if a suitable code existed, the paper would not provide a complete algorithm for reconstruction.
minor comments (5)
- [Throughout] The symbol E is used for both an elliptic curve and the encoding map; this ambiguity makes Procedures 1 and 2 hard to follow.
- [§2.2, Remark 1] The encoding function ψ is stated to map points to strings of length γn/2, but no parameter condition such as 2^{γn/2} ≥ #E(F_q) is given; without it, the encoding step may not be injective for all points on the chosen curve.
- [§2.1, Theorem 3] The condition 'N contains a large smooth square factor' is left undefined, and the proof then speaks of N being a power of a small prime ℓ, which is a different condition; the statement should be made precise.
- [§5.2, Corollary 2 proof] The proof jumps from Theorem 7 to the bound ǫ ≤ (γ−2)/r+2 without deriving the displayed inequality for r>γ−2; this step should be spelled out.
- [§3, Procedure 2] The loop 'for 0 ≤ j ≤ γn' should be a half-open interval to avoid an off-by-one error when processing the γn-bit word.
Circularity Check
No circularity: Theorem 4 reduces threshold recovery to the assumed erasure-correction capability of C, an external code primitive, with no self-citations, fitted parameters, or renamed known results.
full rationale
Walking the derivation chain: Theorem 4 assumes a binary code C correcting γ(n−t) erasures and then builds shares as codeword chunks; recovery is erasure decoding followed by SESS inversion and the external Feo–Fouotsa–Panny/Robert algorithms. The threshold property is exactly the assumed erasure-correction radius, but that is a standard reduction, not a circular derivation: the code property is an input, not something the paper claims to derive. Theorem 5 adds the Singleton bound to relate k, γ, n, and t; no fitted constant is renamed as a prediction. All cited theorems (Robert's SIDH attack, Feo et al.'s SI-OTP reduction, MacWilliams–Sloane coding facts) are independent external results, not self-citations, so the self-citation patterns do not apply. The serious concern raised by the skeptic — that no binary [γn,k,d] code can both encode an arbitrary γn-bit word and correct γ(n−t) erasures unless k=γn and d≤1 — is a feasibility/soundness objection to the construction, not an instance of the paper's conclusion being equivalent to its inputs. Under the circularity rubric, correctness gaps are out of scope. No step in the paper fits the enumerated circularity patterns.
Assumptions & free parameters
free parameters (3)
- γ (bit-block length per participant)
- Code C parameters [ℓ,k,d]
- Torsion order N
assumptions (4)
- ad hoc to paper There exists an injective encoding ψ (called SESS) that maps any elliptic-curve point to a fixed-length bit string and is efficiently invertible, for all relevant supersingular curves.
- ad hoc to paper A binary [γn,k,d] code can encode a message of length γn and correct γ(n-t) erasures with k<γn.
- domain assumption The SI-OTP problem with a large smooth square torsion order is solvable in polynomial time (Theorem 3 of [18]).
- domain assumption The SI-OTP problem is computationally hard for an adversary without the torsion images.
Cite this review
Pith. "Pith review of Sharing the Path: A Threshold Scheme from Isogenies and Error Correcting Codes." pith.science (2026). https://pith.science/paper/VP5ISWCA
@misc{pith2026241118844,
author = {Pith},
title = {Pith review of: Sharing the Path: A Threshold Scheme from Isogenies and Error Correcting Codes},
year = {2026},
howpublished = {\url{https://pith.science/paper/VP5ISWCA}},
note = {Machine review of arXiv:2411.18844}
}
read the original abstract
In 2022, a prominent supersingular isogeny-based cryptographic scheme, namely SIDH, was compromised by a key recovery attack. However, this attack does not undermine the isogeny path problem, which remains central to the security of isogeny-based cryptography. Following the attacks by Castryck and Decru, as well as Maino and Martindale, Robert gave a mature and polynomial-time algorithm that transforms the SIDH key recovery attack into a valuable cryptographic tool. In this paper, we combine this tool with advanced encoding techniques to construct a novel threshold scheme.
Reference graph
Works this paper leans on
-
[1]
S. Atapoor, K. Baghery, D. Cozzo, R. Pedersen (2023). CSI -SharK: CSI-FiSh with Sharing-friendly Keys. In: Simpson, L., Rezazadeh Baee, M.A. (eds) Information Security and Privacy. ACISP 2023. Lecture Notes in Computer Science, vol 13915. Springer, Cham. https://doi.org/10.1007/978-3-031- 35486-1_21
-
[2]
D. J. Bernstein, M. Hamburg, A. Krasnova, T. Lange.: Elli gator: elliptic-curve points indistinguishable from uniform random strings. In: Sadeghi, A.-R., Gligor, V.D., Yung, M., (eds.) ACM CCS 2013, pp. 967-980. ACM Press, November 2013
work page 2013
-
[3]
D. J. Bernstein and T. Lange. Safe curves for elliptic-cu rve cryptography Cryp- tology ePrint Archive, Paper 2024/1265, 2024 https://eprint.iacr.org/ 2024/1265
work page 2024
-
[4]
M. Bertilsson and I. Ingemarsson. A construction of prac tical secret sharing schemes using linear block codes. In J. Seberry and Y. Zheng, editors, Advances in Cryptology -AUSCRYPT’92, volume 718 of Lecture Notes in Compu t. Sci., pages 67-79. Springer, Berlin, Heidelberg, 1993
work page 1993
-
[5]
W. Beullens, L. Disson, R. Pedersen, F. Vercauteren.: CS I-RAShi: distributed key generation for CSIDH. In: Cheon, J.H., Tillich, J.-P. (eds.) PQCrypto 2021 2021. LNCS, vol. 12841, pp. 257–276. Springer, Cham (2021). https: //doi.org/10.1007/978-3-030-81293-5_14
-
[6]
W. Beullens, B. Preneel, A. Szepieniec, F. Vercauteren. : LUOV. Round 2 submission, NIST Post-Quantum Cryptography Standardization (2019). https: //www.esat.kuleuven.be/cosic/pqcrypto/luov/
work page 2019
-
[7]
G. R. Blakley. Safeguarding cryptographic keys. In Managing Requirements Knowledge, International Workshop on. IEEE Computer Society. 1979. 11
work page 1979
-
[8]
W. Castryck, T. Decru.: An efficient key recovery attack on SIDH. In: Hazay, C., Stam, M. (eds.): EUROCRYPT 2023, Part V, LNCS, vol. 140 08. Springer, Heidelberg (2023) pp. 423–447. https://doi.org/10.1007/978-3- 031-30589-4_15
doi:10.1007/978-3- 2023
Show all 42 references
-
[9]
Castryck, T
W. Castryck, T. Lange, C. Martindale, L. Panny, J. Renes. CSIDH: an efficient post-quantum commutative group action. In: Peyrin, T., Galbraith, S. (eds.) ASIACRYPT 2018. LNCS, vol. 11274, pp. 395-427. Springer, Cham (2018). https://doi.org/10.1007/978-3-030-03332-315
2018 doi
-
[10]
Chavez-Saab, F
J. Chavez-Saab, F. Rodriguez-Henriquez, M. Tibouchi. : SwiftEC: Shallue- van de Woestijne Indifferentiable Function to Elliptic Curve s. In: Agrawal, S., Lin, D. (eds) Advances in Cryptology - ASIACRYPT 2022. ASIACRYP T
2022
-
[11]
Chen, T.M
L. Chen, T.M. Laing, K.M. Martin.: Efficient, XOR-based, ideal (t,n )- thresh- old schemes. In: Foresti, S., Persiano, G. (eds.) CANS 2016. LNCS, vol. 10052, pp. 467-483. Springer, Cham (2016). https://doi.org/10.1007/978- 3-319-48965-0_28
2016 doi
-
[12]
Hard homogeneous spaces 1997
Jean-Marc Couveignes. Hard homogeneous spaces 1997. IACR Cryptology ePrint Archive 2006/291. https://ia.cr/2006/291
1997
-
[13]
Cozzo, N
D. Cozzo, N. P. Smart, N.P. (2019). Sharing the LUOV: Thr eshold Post- quantum Signatures. In: Albrecht, M. (eds) Cryptography and Coding. IMACC
2019
-
[14]
Cozzo, N
D. Cozzo, N. P. Smart.: Sashimi: cutting up CSI-FiSh sec ret keys to produce an actively secure distributed signing protocol. In: Ding, J., Tillich, J.-P. (eds.) PQCrypto 2020. LNCS, vol. 12100, pp. 169–186. Springer, Cham (2020). https://doi.org/10.1007/978-3-030-44223-1_10
2020 doi
-
[15]
Desmedt.: Threshold cryptosystems
Y. Desmedt.: Threshold cryptosystems. In: Seberry, J., Zheng, Y. (eds.) AUSCRYPT 1992. LNCS, vol. 718, pp. 1-14. Springer, Heidelberg (1993). https://doi.org/10.1007/3-540-57220-1_47
1993 doi
-
[16]
van Dijk
M. van Dijk. A linear construction of perfect secret sha ring schemes. In A. DeSantis, editor, Advances in Cryptology - EUROCRYPT’94, volu me 950 of Lecture Notes in Comput. Sci., pages 23-34. Springer, Berlin, Heidelberg, 1995
1995
-
[17]
J. Ding, M. S. Chen, A. Petzoldt, D. Schmidt, B. Y. Yang.: Rainbow. Round 2 submission, NIST Post-Quantum Cryptography Standardization (2019). https://csrc.nist.gov/projects/post-quantum-cryptog raphy/ round-2-submissions
2019
-
[18]
De Feo, T.B
L. De Feo, T.B. Fouotsa, L. Panny (2024). Isogeny Proble ms with Level Struc- ture. In: Joye, M., Leander, G. (eds) Advances in Cryptology - EUROCRY PT
2024
-
[19]
De Feo, M
L. De Feo, M. Meyer.: Threshold schemes from isogeny ass umptions. In: Kiayias, A., Kohlweiss, M., Wallden, P., Zikas, V. (eds.) PK C 2020. LNCS, vol. 12111, pp. 187–212. Springer, Cham (2020). https://doi.org/10.1007/ 978-3-030-45388-6_7
2020
-
[20]
Moriya, C
T.B Fouotsa, T. Moriya, C. Petit. (2023). M-SIDH and MD- SIDH: Coun- tering SIDH Attacks by Masking Information. In: Hazay, C., Stam, M. (eds) Advances in Cryptology – EUROCRYPT 2023. EUROCRYPT 2023. Lecture Notes in Computer Science, vol 14008. Springer, Cham. https: //doi.or...
2023 doi
-
[21]
Heng and C
Z. Heng and C. Ding. The subfield codes of hyperoval and co nic codes, Finite Fields Their Appl., vol. 56, pp. 308-331, Mar. 2019
2019
-
[22]
Heng and C
Z. Heng and C. Ding. The Subfield Codes of Some [ q + 1, 2,q ] MDS Codes, IEEE TRANSACTIONS ON INFORMATION THEORY, VOL. 68, NO. 6, JUNE 2022
2022
-
[23]
Hineman and M
A. Hineman and M. Blaum, A Modified Shamir Secret Sharing Scheme With Efficient Encoding, in IEEE Communications Letters, vol. 26, no. 4, pp. 758- 762, April 2022,
2022
-
[24]
D. Jao, L. De Feo.: Towards quantum-resistant cryptosy stems from supersin- gular elliptic curve isogenies. In: Yang, B.-Y. (ed.) PQCrypto 2011. LNCS, vol. 7071, pp. 19–34. Springer, Heidelberg (2011). https://doi.org/10.1007/ 978-3-642-25405-5_2
2011
-
[25]
Maino, C
L. Maino, C. Martindale. An attack on SIDH with arbitrar y starting curve. Cryptology ePrint Archive, Paper 2022/1026 (2022). https://eprint.iacr. org/2022/1026
2022
-
[26]
J. L. Massey. Minimal codewords and secret sharing. In Proc. 6th Joint Swedish- Russian Workshop on Information Theory, pages 276-279, Molle, Sweden, Au- gust 1993
1993
-
[27]
J. L. Massey. Some applications of coding theory. In P. G. Ferell, editor, Cryptography, Codes and Ciphers: Cryptography and Coding IV, pages 33-47. Formara Ltd, Essex, 1995
1995
-
[28]
R. J. McEliece and D. V. Sarwate. On sharing secrets and R eed-Solomon codes. Commun. ACM, 24:583-584, 1981
1981
-
[29]
MacWilliams and N
F. MacWilliams and N. Sloane. The Theory of Error-Corre cting Codes North- holland Publishing Company, 2nd edition, (1978)
1978
-
[30]
Nakamoto
S. Nakamoto. (2008). Bitcoin: A peer-to-peer electron ic cash system. 2008 https://bitcoin.org/bitcoin.pdf
2008
-
[31]
Peralta, L
R. Peralta, L. T. Brand˜ ao.: Nist first call for multi-pa rty threshold schemes. National Institute of Standards and Technology (2023) . https://doi.org/10. 6028/NIST.IR.8214C.ipd, https://doi.org/10.6028/NIST.IR.8214C.ipd
2023 doi
-
[32]
del Pino, S
R. del Pino, S. Katsumata, M. Maller, F. Mouhartem, T. Pr est, MJ. Saarinen. (2024). Threshold Raccoon: Practical Threshold Signature s from Standard Lattice Assumptions. In: Joye, M., Leander, G. (eds) Advances in Cryptology – EUROCRYPT 2024. EUROCRYPT 2024. Lecture Notes in ...
2024 doi
-
[33]
Renvall and C
A. Renvall and C. Ding. The access structure of some secr et-sharing schemes. In J. Pieprzyk and J. Seberry, editors, Information Security a nd Privacy, vol- ume 1172 of Lecture Notes in Comput. Sci., pages 67-78. Springer, Berlin, Heidelberg, 1996
1996
-
[34]
Breaking SIDH in polynomial time
D. Robert. “Breaking SIDH in polynomial time”. In: Eurocrypt 2023 (Apr. 2023). Ed. by C. Hazay and M. Stam, pp. 472–503
2023
-
[35]
D. Robert. Breaking SIDH in polynomial time, August 202 2
-
[36]
How to Share a Secret
Adi Shamir. How to Share a Secret. Communications of the Association for Computing Machinery 22, 11 (Nov. 1979), 612-613
1979
-
[37]
Tibouchi.: Elligator squared: uniform points on ell iptic curves of prime order as uniform random strings
M. Tibouchi.: Elligator squared: uniform points on ell iptic curves of prime order as uniform random strings. In: Christin, N., Safavi-Naini, R. (eds.) FC 2014. LNCS, vol. 8437, pp. 139–156. Springer, Heidelberg (2014). https: //doi.org/10.1007/978-3-662-45472-5_10 13
2014 doi
-
[38]
Tibouchi, T
M. Tibouchi, T. Kim.: Improved elliptic curve hashing a nd point representa- tion. Des. Codes Cryptogr. 82(1-2), 161-177 (2017)
2017
-
[39]
Efficient secret sharing schemes achieving optimal information rate,
Y. Wang and Y. Desmedt, “Efficient secret sharing schemes achieving optimal information rate,” in Proc. IEEE Inf. Theory Workshop ITW, Hobart, TAS, Australia, Nov. 2014, pp. 516-520. 14
2014
-
[2019]
Springer, Cham
Lecture Notes in Computer Science() , vol 11929. Springer, Cham. https: //doi.org/10.1007/978-3-030-35199-1_7
-
[2022]
Springer , Cham
Lecture Notes in Computer Science, vol 13791. Springer , Cham. https: //doi.org/10.1007/978-3-031-22963-3_3
-
[2024]
Lecture Notes in Computer Science, vol 1 4657
EUROCRYPT 2024. Lecture Notes in Computer Science, vol 1 4657. Springer, Cham. https://doi.org/10.1007/978-3-031-58754-2_7
2024 doi
Reviewed August 12, 2026 · model on record in the stance chip above.
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