Module masked 

First-order arithmetic masking for DPA/template attack protection. First-order arithmetic masking for ML-KEM polynomials (countermeasure: DPA / DEMA / CPA on the K-PKE secret s).

Principle

Every secret polynomial is kept as two additive shares modulo q: s = s_0 + s_1 (mod q). Shares are drawn uniformly at random and each operation consuming the secret (NTT, pointwise multiplication with a public polynomial, additions / subtractions) is rewritten to operate on the shares without ever materialising the unmasked s. Fresh-randomness refreshes (MaskedPoly::refresh) break cross-operation correlations that a high-order DPA could otherwise exploit.

Because the unmasked s never exists as a single value in memory, the Hamming-weight / Hamming-distance hypothesis at the core of CPA becomes non-identifiable: correlating the power trace with a guess of any byte of s produces no peak since each share alone is uniform.

Available operations

Function	Description
MaskedPoly::mask	Split a plaintext polynomial into two shares
MaskedPoly::unmask	Reconstruct the polynomial from shares
MaskedPoly::refresh	Re-randomize shares (prevents correlation buildup)
masked_ntt / masked_ntt_inv	NTT on each share independently
masked_multiply_public	Multiply masked poly by a public poly
masked_add / masked_sub	Add/subtract two masked polys
masked_multiply_accumulate	Fused multiply-add with a public poly

References

• Side-channel analysis of the ML-KEM pointwise multiplication (IACR ePrint 2025, doc/papers/eprint2025_sca_mlkem_pointwise.pdf) — identifies the pointwise multiplication as the key DPA target; masking the shares defeats the published attack.
• ML-KEM and ML-DSA on OpenTitan: side-channel evaluation (cryptojedi, 2024) — independent evaluation of the trace-count blow-up induced by masking.

Where to look next

• Countermeasure description and threat analysis: doc/sca/countermeasures/ml_kem.rst, section DPA / DEMA / CPA — first-order masking of the KPKE secret.
• Call sites: keygen_internal_sca, decaps_internal_sca, crate::ml_kem::kpke::decrypt_sca.

Scope and residual risk

Masking is first-order. A higher-order DPA that combines two independent time samples can still recover the secret with substantially more traces. Tier-4 item K-SCA3 (not yet scheduled) would extend this to a 3-share scheme (CC EAL4+-grade).

Structs

MaskedPoly

A polynomial split into two additive shares modulo q.

Functions

masked_add

Add two masked polynomials share-wise.

masked_add_public

Add a public polynomial to a masked polynomial.

masked_multiply_accumulate

Fused multiply-accumulate: out += masked * public (NTT domain).

masked_multiply_public

Multiply a masked polynomial by a public polynomial in NTT domain.

masked_ntt

Apply the forward NTT to each share of a MaskedPoly independently.

masked_ntt_inv

Apply the inverse NTT to each share of a MaskedPoly independently.

masked_sub

Subtract two masked polynomials share-wise.