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Fast Plaintext-Ciphertext Matrix Multiplication from Additively Homomorphic Encryption

T0 review · 3 major / 3 minor · reviewed 2026-08-16 · deepseek-v4-flash

Pith's one-line read Moving Cussen's compression-reconstruction algorithm into the encrypted domain gives plaintext-ciphertext matrix multiplication up to an order-of-magnitude speedup over Strassen for large low-bit-width matrices.

desk verdict A solid, scoped systems result: Cussen's compression trick moves cleanly into unpacked-AHE PC-MM and delivers real speedups on a Raspberry Pi 5, with the open caveat that the gains depend on plaintext column structure. read the letter →

arxiv 2504.14497 v1 pith:SVQM6375 submitted 2025-04-20 cs.CR

classification cs.CR
keywords plaintext-ciphertextmatrixmultiplicationadditivelyhomomorphicencryptionellipticcurveElGamalCussencompression-reconstructionprivacy-preservinginferenceedgecomputingStrassen'salgorithm
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Plaintext-ciphertext matrix multiplication multiplies an unencrypted matrix $A$ by an encrypted matrix $B$ and returns an encrypted $A\times B$, the operation behind private inference where model weights are public and user data is encrypted. With additively homomorphic encryption that stores one message per ciphertext, the schoolbook method costs $O(n^3)$ scalar-ciphertext multiplications, each far more expensive than a ciphertext addition; Strassen lowers the exponent but keeps the same imbalance. The paper's claim is that Cussen's compression-reconstruction algorithm, applied to the columns of $A$ before any encrypted arithmetic, converts most of those expensive scalar multiplications into cheap additions in the encrypted domain, and that the conversion is exact. In a software implementation on a Raspberry Pi 5 with elliptic-curve ElGamal encryption, the proposed method runs up to about 12 times faster than Strassen for large matrices with small element bit-widths, cutting the $(n=512,t=8)$ case from about 2.73 hours to about 15.6 minutes. This matters because it lowers the computational barrier for privacy-preserving machine learning and signal processing on resource-constrained edge devices.

What carries the argument

The load-bearing mechanism is Cussen's compression-reconstruction algorithm, a plaintext method that rewrites an $n$-entry vector-scalar product as $m'$ scalar multiplications plus reconstruction additions: sort the vector, delete duplicate values, replace consecutive entries by their differences, and repeat; multiply the short compressed vector by the scalar; then undo the differences by prefix additions, un-sort, and re-insert duplicates. In the encrypted setting the compression phase runs on plaintext columns of $A$ and costs nothing homomorphic, while the reconstruction phase is executed with elliptic-curve point additions, which are cheap relative to the scalar multiplications being saved. The algorithm's work is to reduce the dominant plaintext-ciphertext scalar multiplication count from $n$ to $m'$ per column, with the compressed length $m'$ controlled by bit-width and duplicate structure rather than by $n$.

What would settle it

Count the compressed column length $m'$ for a fixed input matrix before running the encrypted multiplication. If for most columns $m'$ stays close to $m$—as happens for random 16-bit entries or for matrices with no duplicate patterns—the proposed approach's measured wall-clock time will not beat Strassen, and for $(n,t)=(8,16)$ and $(16,16)$ the paper's own measurements already show it is slower.

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Extended reading notes

Core claim

The central discovery is that the trade-off inside Cussen's plaintext algorithm—spend sorting and differencing to shrink a vector, then reconstruct with additions—becomes decisive when the 'multiplications' are scalar-times-ciphertext operations. In elliptic-curve ElGamal, one scalar-ciphertext multiplication costs roughly $2t$ point operations (two elliptic-curve scalar multiplications of bit-width $t$), while one ciphertext-ciphertext addition costs two point additions, so replacing a scalar multiplication by several additions is a net win for moderate $t$. The paper adapts Cussen's compression phase (sort, remove duplicates, take consecutive differences) to each column of the plaintext matrix $A$, performs only $m'$ scalar-ciphertext multiplications per column, and then applies the reconstruction phase as elliptic-curve point additions to recover the exact encrypted outer products. Compression is amortized across all columns of $B$, and the reconstruction is exact, so the result matches plaintext matrix multiplication with no accuracy loss. Measured on random square matrices, the proposed PC-MM beats both schoolbook and Strassen for large $n$ and small $t$, with speedups over Strassen reaching about $12\times$ at $(n,t)=(512,4)$ and $(256,4)$, and about $11\times$ at $(512,8)$ and $(128,4)$.

Load-bearing premise

The speedup rests on the plaintext matrix columns compressing well: after sorting, removing duplicates, and taking differences, the compressed column length $m'$ must be much smaller than the original length $m$, which is true for random low-bit-width vectors but not guaranteed for adversarial or high-bit-width data.

Editorial extensions

If this is right

  • For large matrices with element bit-widths of 4 to 8, PC-MM from unpacked additively homomorphic encryption becomes practical on a low-power edge board: the $n=512$, $t=8$ case runs in about 15.6 minutes instead of 2.73 hours with Strassen.
  • Because reconstruction is exact (the ciphertext decrypts to the true integer product), the speedup carries no accuracy trade-off beyond the integer or fixed-point encoding required by the encryption scheme.
  • The same compression logic transfers to other unpacked additively homomorphic schemes such as Paillier, where scalar-ciphertext 'multiplication' is modular exponentiation and ciphertext addition is modular multiplication.
  • Sparse plaintext matrices compress further, so low-precision and sparse machine-learning weight matrices are natural target inputs; rectangular matrices and convolution-as-matrix-multiplication are handled by the same algorithm.
  • The speedup grows with matrix dimension and shrinks with element bit-width, so the practical operating region is large, low-precision matrices rather than all matrix shapes.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Editorial inference: the speedup is data-dependent, so the natural stress test is to run the same implementation on real quantized neural-network weight matrices; repeated-value columns would compress at least as well as the random vectors measured, while dense high-entropy columns would erode the gain.
  • Editorial inference: the paper's own measurements show a sharp crossover—at bit-width 16 the method only wins from $n=32$ onward—so a practical deployment rule would check the compressed column length $m'$ before choosing between this algorithm and schoolbook.
  • Editorial inference: combining Strassen block decomposition with Cussen compression at the leaves may compound the gains, since the two mechanisms attack different costs (matrix multiplication depth versus scalar-multiplication count).
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 3 minor

Summary. The paper proposes a method for plaintext-ciphertext matrix multiplication (PC-MM) with unpacked additively homomorphic encryption (AHE), based on Cussen's compression-reconstruction algorithm. For each column of the plaintext matrix A, the method repeatedly sorts, deduplicates, and differences the entries to obtain a compressed vector of length m'; the required vector-scalar products with encrypted entries of B are then computed with m' plaintext-ciphertext scalar multiplications (elliptic curve scalar multiplications for EC-ElGamal) and reconstructed using ciphertext additions. The authors instantiate the method with EC-ElGamal, implement it on a Raspberry Pi 5 using the MIRACL library, and report speedups up to about 10.5x over Strassen and about 23x over the schoolbook method for large matrices with small element bit-widths. The concrete example at n=512, t=8 runs in about 15.6 minutes versus about 2.73 hours for Strassen and 5.72 hours for schoolbook. The paper also sketches an extension to the Paillier cryptosystem.

Significance. If the performance claim holds, this is a useful and easy-to-deploy technique for PC-MM from unpacked AHE in resource-constrained settings, where prior practice is essentially limited to schoolbook multiplication and Strassen's algorithm. The paper's strengths include a reproducible open-source implementation, exact reconstruction with no approximation error, a clear cost model based on the ECSM-to-point-addition cost ratio, and measurements across a wide range of matrix sizes and bit-widths. The main limitation is that the claimed speedup is demonstrated only on independent, uniformly random small-bit-width matrices; since the compression mechanism is data-dependent, the practical breadth of the headline claim is not yet established without data on realistic inputs.

major comments (3)
  1. [Section 3.3, Algorithm 3 step 3; Tables 7-10] The headline speedup is driven entirely by the assumption that Cussen compression yields m' much smaller than m for each column of A. The experiments in Section 4.2 use only independent uniform random t-bit entries; the paper provides no measurements of m' or runtime for real quantized machine-learning weight matrices, signal-processing matrices, or structured/adversarial inputs. The paper's own Table 10 shows the advantage is already lost or reversed for (n,t) in {(8,12), (8,16), (16,16)} and is only 0.94 versus Strassen at (32,16), so the beneficial regime is narrower than 'small bit-widths' alone. Since m' is not determined by the bit-width t but by the actual data distribution, the statement in Section 5 that such matrices 'are quite common' in machine learning is unsupported. Please either add measurements on realistic matrices (e.g., quantized transformer or CNN weights) or clearly restrict the claim to uniformly random small-bit-width matrices.
  2. [Section 4.2; Figures 3-12 and Tables 1-10] All experiments fix the number of Cussen iterations at N=4, but the paper gives no justification for this choice and no sensitivity analysis. N is a free parameter that controls the trade-off between stronger compression (fewer ECSMs) and more reconstruction additions in Algorithm 2/3. The reported speedups are therefore conditional on an unexamined parameter setting. Please report how the compressed length, operation counts, and measured runtime vary with N, and justify the chosen value.
  3. [Section 3.3, cost model] The analytical cost comparison in Figures 6/7 and Tables 5/6 assumes that an elliptic curve point doubling costs approximately the same as a point addition. The paper itself notes that the relative costs differ across curves and implementations. Since the measured timings are the primary evidence for the speedup, this assumption is not fatal, but the analytic estimates should either be validated against the measured point-doubling/point-addition cost ratio or de-emphasized in favor of the direct timing measurements.
minor comments (3)
  1. [Section 4.2, Tables 7-10] The timing results are reported as averages, but no standard deviations, confidence intervals, or number of repetitions are given for the matrix-level measurements in Table 9. Please report the variance and the number of trials to support reproducibility.
  2. [Abstract and Section 1] The phrase 'state-of-the-art' is broader than the actual comparison, which benchmarks only the schoolbook method and Strassen's algorithm among unpacked-AHE techniques. Please qualify the comparison set explicitly.
  3. [Section 4.3.1] The statement that sparse plaintext matrices can be 'further compressed' is plausible but not measured. If it remains, support it with data or mark it as a speculative advantage to be validated in future work.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the proposed PC-MM speedups are measured against standard baselines, and the underlying Cussen compression is an external algorithm whose behavior is an empirical input, not a fitted consequence of the claimed result.

full rationale

The paper's central claim is an experimentally measured speedup: 'Our measurement results indicate up to an order of magnitude speedup with our proposed PC-MM compared to Strassen's algorithm in case of large matrices with relatively small element bit-widths.' This is supported by direct timing comparisons (Tables 7-10) between Algorithm 3, schoolbook PC-MM, and Strassen PC-MM, all implemented with the same EC-ElGamal AHE on a Raspberry Pi 5. No parameter is fitted to the target speedup, and no quantity used in the comparison is defined in terms of the measured outcome. The compression phase of Algorithm 3 is inherited from Cussen's plaintext matrix-multiplication algorithm [CU23], which is an external citation rather than the authors' own prior work, and the compressed column length m' comes from measured random-vector statistics, not from an equation that presupposes the final speedup. The paper's caveat that the advantage disappears for (n,t) in {(8,12),(8,16),(16,16)} and that real or adversarial matrices may not compress as well is a limitation on the breadth of the claim, not evidence that the derivation is circular. The unquantified assertion that constrained large matrices 'are quite common' in machine learning is an empirical generalization that could be challenged as unsupported, but it is not a self-referential or definitional reduction. The measured 10.5x speedup versus Strassen at (512,8) stands as an independent benchmark result, so no circularity score is warranted.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central claim relies on the Cussen algorithm's correctness, the cost asymmetry between scalar multiplications and additions, and the compressibility of small-bit-width plaintext matrices. No new entities are introduced.

free parameters (1)
  • Number of Cussen iterations N = 4
    Fixed at 4 compression-reconstruction iterations for all experiments; no sensitivity analysis or optimization reported, so it is a hand-chosen constant.
assumptions (4)
  • standard math The elliptic curve ElGamal scheme is additively homomorphic for bounded message spaces (Equation 4).
    Used in Section 3.1 to define PC-MM; requires message space bound n*Bs*Bm < q.
  • domain assumption Cussen's compression-reconstruction algorithm correctly computes vector-scalar products exactly (from [CU23]).
    The paper relies on the correctness of Algorithm 2 from [CU23] and provides only a toy example, not a full proof.
  • domain assumption Cost of ECSM with t-bit scalar is approximately 2t point additions, with point doubling and point addition having equal cost.
    Used in Section 3.3 to justify the trade-off; the authors state 'we assume they are approximately the same for simplicity'. Actual measurements support the overall conclusion.
  • domain assumption Random matrices used for profiling are representative of practical ML weight matrices with small bit-widths.
    The speedup is data-dependent; the paper justifies this by noting ML weights are often small integers, but does not evaluate on real ML workloads.

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Pith. "Pith review of Fast Plaintext-Ciphertext Matrix Multiplication from Additively Homomorphic Encryption." pith.science (2026). https://pith.science/paper/SVQM6375

@misc{pith2026250414497,
  author       = {Pith},
  title        = {Pith review of: Fast Plaintext-Ciphertext Matrix Multiplication from Additively Homomorphic Encryption},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SVQM6375}},
  note         = {Machine review of arXiv:2504.14497}
}
read the original abstract

Plaintext-ciphertext matrix multiplication (PC-MM) is an indispensable tool in privacy-preserving computations such as secure machine learning and encrypted signal processing. While there are many established algorithms for plaintext-plaintext matrix multiplication, efficiently computing plaintext-ciphertext (and ciphertext-ciphertext) matrix multiplication is an active area of research which has received a lot of attention. Recent literature have explored various techniques for privacy-preserving matrix multiplication using fully homomorphic encryption (FHE) schemes with ciphertext packing and Single Instruction Multiple Data (SIMD) processing. On the other hand, there hasn't been any attempt to speed up PC-MM using unpacked additively homomorphic encryption (AHE) schemes beyond the schoolbook method and Strassen's algorithm for matrix multiplication. In this work, we propose an efficient PC-MM from unpacked AHE, which applies Cussen's compression-reconstruction algorithm for plaintext-plaintext matrix multiplication in the encrypted setting. We experimentally validate our proposed technique using a concrete instantiation with the additively homomorphic elliptic curve ElGamal encryption scheme and its software implementation on a Raspberry Pi 5 edge computing platform. Our proposed approach achieves up to an order of magnitude speedup compared to state-of-the-art for large matrices with relatively small element bit-widths. Extensive measurement results demonstrate that our fast PC-MM is an excellent candidate for efficient privacy-preserving computation even in resource-constrained environments.

Figures

Figures reproduced from arXiv: 2504.14497 by the authors.

Figure 1
Figure 1. Matrix multiplication Am×n × Bn×l = Cm×l using (a) row-and-column inner products and (b) column-and-row outer products (diagram inspired by [Gri17]) [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Toy example showing four iterations each of the Compression Phase and the Reconstruction Phase of plaintext vector-scalar multiplication using Cussen’s algorithm [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
Figure 3
Figure 3. Number of multiplications and additions required for plaintext vector-scalar multiplication using schoolbook approach and Cussen’s algorithm for random vectors of length n ∈ {2 3 , 2 4 , · · · , 2 9} with element bit-widths t ∈ {4, 8, 12, 16} [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figures from the paper (9 more)
Figure 4
Figure 4. Figure 4: Number of multiplications and additions required for plaintext matrix multipli￾cation using schoolbook approach, Strassen’s algorithm and Cussen’s algorithm for random square matrices of dimension n ∈ {2 3 , 2 4 , · · · , 2 9} with element bit-widths t ∈ {4, 8, 12, 16}…
Figure 5
Figure 5. Figure 5: Toy example showing efficient multiplication of plaintext vector and EC-ElGamal ciphertext corresponding to encrypted scalar using proposed approach with four iterations of Cussen’s compression-reconstruction algorithm. with 4 iterations of compression and reconstructi…
Figure 6
Figure 6. Figure 6: Equivalent number of elliptic curve point additions required for plaintext vector and ciphertext scalar multiplication using schoolbook approach and proposed approach based on Cussen’s algorithm for random vectors of length n ∈ {2 3 , 2 4 , · · · , 2 9} with element bi…
Figure 7
Figure 7. Figure 7: Equivalent number of elliptic curve point additions required for plaintext￾ciphertext matrix multiplication PC-MM using schoolbook approach, Strassen’s algorithm and proposed approach based on Cussen’s algorithm for random square matrices of dimension n ∈ {2 3 , 2 4 , …
Figure 8
Figure 8. Figure 8: Experimental setup used for evaluating and profiling proposed fast PC-MM. FCE+23, LZ23], while few have presented implementations on embedded micro-controllers and IoT platforms [RPCS22, AB22, Ban23]. It is generally believed that homomorphic computations on encrypted …
Figure 9
Figure 9. Figure 9: Time taken for plaintext vector and ciphertext scalar multiplication using schoolbook approach and proposed approach based on Cussen’s algorithm for random vectors of length n ∈ {2 3 , 2 4 , · · · , 2 9} with element bit-widths t ∈ {4, 8, 12, 16} [PITH_FULL_IMAGE:figu…
Figure 10
Figure 10. Figure 10: Speedup observed for plaintext vector and ciphertext scalar multiplication using proposed approach based on Cussen’s algorithm compared to schoolbook approach for random vectors of length n ∈ {2 3 , 2 4 , · · · , 2 9} with element bit-widths t ∈ {4, 8, 12, 16}. elemen…
Figure 11
Figure 11. Figure 11: Time taken for plaintext-ciphertext matrix multiplication PC-MM using school￾book approach, Strassen’s algorithm and proposed approach based on Cussen’s algorithm for random square matrices of dimension n ∈ {2 3 , 2 4 , · · · , 2 9} with element bit-widths t ∈ {4, 8, …
Figure 12
Figure 12. Figure 12: Speedups observed for plaintext-ciphertext matrix multiplication PC-MM using proposed approach based on Cussen’s algorithm compared to schoolbook approach and Strassen’s algorithm for random square matrices of dimension n ∈ {2 3 , 2 4 , · · · , 2 9} with element bit-w…

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Reviewed August 16, 2026 · model on record in the stance chip above.