REVIEW 4 major objections 5 minor 22 references
When and Where Faults Matter: A Study of Transient Errors in CKKS Multiplication
T0 review · 4 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read In CKKS homomorphic multiplication, a single-bit flip is benign or corrupting depending on whether it enters both uses of a ciphertext component or only one partial product.
desk verdict The timing/location distinction is a real algebraic observation, but the 'masked' conclusion is wrong and the promised empirical evaluation is absent. 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 double-use symmetry of the ciphertext components inside the three partial products of CKKS multiplication, together with the error polynomial $e_{i,j}=2^j X^i$ that models a single-bit flip. Writing the faulted multiplication as Eq. (5) (fault inside both uses of $c_0$) versus Eq. (6) (fault only inside $d_1$) is what carries the argument: the first expansion remains in ciphertext space because the extra term is a product of a scalar error and a valid ciphertext, while the second does not. This distinction is then tied to the empirical masking-versus-SDC pattern through error-propagation traces and maximum relative error measurements.
What would settle it
Run pairwise fault injections on identical CKKS multiplications: one bit flip in $c_0$ that enters both $d_0$ and $d_1$, and the same flip restricted to $d_1$ only. If the decrypted maximum relative error in the first case is statistically indistinguishable from the second, the paper's algebraic masking claim is refuted; the distinction in Eqs. (5) and (6) predicts a clear gap.
Extended reading notes
Core claim
On the paper's own terms, the central claim is that in the CKKS scheme, ciphertext–ciphertext multiplication has a built-in symmetry that decides the fate of a transient single-bit error. The multiplication forms three partial products $(d_0,d_1,d_2)$; $c_0$ appears in $d_0$ and $d_1$, and $c_1$ appears in $d_1$ and $d_2$. If a bit flip in $c_0$ is present in both of its appearances, the faulted result is algebraically the correct product plus an error term $e_{i,j}(c_0'',c_1'')$, which is itself a valid ciphertext; decryption therefore performs its usual cancellation and leaves only a small extra noise term. If the same flip appears only in $d_1$, the result is the correct product plus $e_{i,j}(0,c_1')$, which is not a ciphertext, so the secret-key cancellation fails and the decrypted output is badly corrupted. The paper concludes that homomorphic multiplication is highly susceptible to faults, and that the timing and location of an error in $c_0$ or $c_1$ determine whether the error is masked or silent.
Load-bearing premise
The masked conclusion rests on the assumption that after relinearization a fault hitting both uses of a component still leaves a valid ciphertext whose decryption error stays small; if relinearization mixes the injected error into both components in a way that breaks the cancellation, the masking result would not follow.
Editorial extensions
If this is right
- A fault-injection study that records only the bit position, without the component and timing, will misestimate silent data corruption rates for CKKS multiplication.
- Protection can focus on single-use paths such as a fault landing only in $d_1$, where decryption cannot cancel the injected error.
- A fault that is replicated across both uses of a component can be left unprotected, since decryption still returns the product plus a bounded noise term.
- Because $c_0$ and $c_1$ feed different partial products, their resilience profiles differ, so $c_0$ faults and $c_1$ faults follow distinct propagation paths.
Reading between the lines
- The paper does not test it, but Eq. (5) implies masked faults still add noise, so many such faults could accumulate and erode the CKKS noise budget over a long computation.
- Because the same double-use partial-product structure appears in other RLWE-based multiplication schemes, the masked-versus-SDC distinction may transfer, although the paper does not claim this.
- A natural extension would inject faults into the relinearization-key path, since Eqs. (5) and (6) treat the $d^{evk}$ terms as fault-free.
- Read as a design pointer, the result suggests protecting the $d_1$ computation rather than triplicating the whole multiplication, which the authors do not state explicitly.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies transient single-bit errors in server-side CKKS homomorphic multiplication, using an unoptimized implementation (no NTT/RNS). It claims that the timing and location of bit flips in the ciphertext components c0 and c1 determine whether the error is masked, silently corrupts the output, or is detected. The central theoretical argument, formalized in Eqs. (5) and (6), distinguishes a fault that enters both partial products d0 and d1 (claimed to yield a valid ciphertext plus a low-magnitude error) from a fault entering only d1 (claimed to produce a non-ciphertext term and hence SDC). The paper also asserts empirical support via an injection tool and MREP evaluation, but no empirical data or figures are included.
Significance. If correct, the paper would provide a useful fault-propagation model for CKKS multiplication and guide fault-tolerant HE accelerator design. The algebraic distinction between faults entering one versus both partial products is a conceptually clear starting point. However, the central claim is not established: the key assertion that the error term in Eq. (5) is low-magnitude is incorrect, and the manuscript contains no empirical results to back the claimed observations. The paper is therefore of limited value in its current form.
major comments (4)
- [Section III-C, Eq. (5)] The assertion that the error term e_{i,j}*(c0'', c1'') decrypts to an 'additional low-magnitude term' is incorrect. Decrypting that term yields e_{i,j}*(m'' + e''), where m'' is the plaintext encoded in the second ciphertext and e'' is its inherent noise. In CKKS, plaintext coefficients are scaled by the scaling factor Δ and encode arbitrary real or complex data, so e_{i,j}*m'' is not small: for a bit flip at position j it contributes 2^j times a shifted coefficient of m''. For high-order bit flips this error is comparable to or larger than the message itself. The paper provides no bound on ||e_{i,j}*m''||, so the 'masked' outcome is in fact a data-dependent corruption, not a benign result.
- [Section III-C, Eq. (5)] The derivation assumes that after relinearization the faulted ciphertext is exactly the correct multiplication result plus e_{i,j}*(c0'', c1'') with (c0'', c1'') a valid ciphertext. The paper does not prove that the relinearization step, which involves key-switching and rounding, preserves this additive structure. If relinearization mixes the error into the ciphertext components differently, the algebraic form of Eq. (5) may not hold. This is a load-bearing gap because Eq. (5) is the entire basis for the claimed masking behavior.
- [Sections I and II] The manuscript explicitly claims empirical support ('These observations are supported by both theoretical analysis and empirical evaluation'; 'we injected single bit transient errors ... using MREP'), but no MREP results, error distributions, or baseline comparisons appear anywhere in the paper. The only referenced figure (Fig. 2) is not included. This absence means the central claim cannot be verified empirically, despite the paper's framing.
- [Sections I and III-B] The proposed dichotomy between 'masked' and SDC collapses once the error term in Eq. (5) is recognized as a data-dependent corruption proportional to e_{i,j}*m''. Both the 'both partial products' and 'one partial product' fault cases then produce silent corruption, differing only in whether an additional secret-key-dependent term appears. The abstract's claim that timing and location determine whether the output is correct is therefore unsupported; the two cases differ in error magnitude, not in masked versus SDC behavior.
minor comments (5)
- [Section III-A] Figure 2 is referenced but not included in the manuscript; add the figure with the error characterization data it is meant to show.
- [Section III-A] The term 'gap' is used without definition; clarify whether it refers to the number of unused slots between FFT slots or to another structural parameter.
- [Section I] The phrase 'the timing of errors' is not operationalized; the analysis concerns which partial product (d0 or d1) receives the fault, not a time axis. Explain what 'timing' means in this context.
- [Section II] The C-CKKS implementation is described as based on OpenFHE, HEaaN, SEAL, and PyFHE, but the manuscript does not specify which components are taken from each; add an implementation note or a repository link for reproducibility.
- [Section III-C] The statement that the sign of e_{i,j} does not affect the analysis should be justified, since a 1-to-0 flip produces a negative error; the magnitude analysis is sign-independent, but this is not stated.
Circularity Check
No significant circularity: the CKKS fault-propagation analysis is self-contained polynomial algebra, and the cited prior works are contextual rather than load-bearing for the central claim.
full rationale
The paper's central derivation, Eqs. (4)-(6), is a direct expansion of CKKS ciphertext multiplication under an injected bit-flip error polynomial. The distinction between a fault entering both partial products d0 and d1 versus only d1 follows algebraically from the structure of the multiplication formula; no parameter is fitted to data and then renamed as a prediction, and no uniqueness theorem or ansatz is imported from the authors' prior work. The self-citations [21] and [22] are used only to refer to prior analyses of the resilience of the scaling factor, modulus, and gap/slot structure (footnote 3), not to establish the paper's new c0/c1 timing and location claims. The empirical evaluation uses the authors' own C-CKKS implementation and the external LLTFI tool, but this does not make the derivation circular. A possible weakness is that the assertion that the error term in Eq. (5) is of 'low-magnitude' is not proven and may be incorrect for high-order bit flips; however, that is a correctness or soundness concern, not a circularity concern, because the claim is not true by construction or by definition of the inputs. The derivation is therefore self-contained and receives a circularity score of 0.
Assumptions & free parameters
assumptions (5)
- domain assumption CKKS decryption cancellation: c0 + c1*sk ≈ message + small error
- domain assumption Relinearization preserves the ciphertext shape: (d0,d1) + d2*evk ≈ (c0,c1)
- domain assumption Fault model: a transient bit flip adds an error polynomial 2^j X^i to a coefficient
- ad hoc to paper Unoptimized CKKS (no NTT/RNS) is representative of the scheme
- domain assumption A fault can affect both uses of c0 or c1 simultaneously
Cite this review
Pith. "Pith review of When and Where Faults Matter: A Study of Transient Errors in CKKS Multiplication." pith.science (2026). https://pith.science/paper/BWYBTB4L
@misc{pith2026260811147,
author = {Pith},
title = {Pith review of: When and Where Faults Matter: A Study of Transient Errors in CKKS Multiplication},
year = {2026},
howpublished = {\url{https://pith.science/paper/BWYBTB4L}},
note = {Machine review of arXiv:2608.11147}
}
abstract
Homomorphic Encryption (HE) is a privacy-preserving encryption paradigm that enables computation directly on encrypted data without requiring decryption. In this paper, we study errors in fully homomorphic encryption (FHE) computations, with a particular focus on server-side homomorphic multiplication in the unoptimized CKKS (Cheon--Kim--Kim--Song) scheme. We show that both the timing and the location of errors in the ciphertext components \(c_0\) and \(c_1\) have a significant impact on the correctness of the final FHE output.
Figures
Reference graph
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Reviewed August 12, 2026 · model on record in the stance chip above.
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