Side-Channel › kem-35 · Report

Constant-time review — kem-35 Scloud+

Scope: reference and optimized implementations, supplemented by the measured trace in kem-35-1.

Secret-bearing values: private decapsulation key, recovered message/error state, rejection secret and shared secret. Public values: public key, ciphertext and encoded lengths. A decoder result is not automatically public merely because its ciphertext input is public.

• Source trace: Implementations and Test_Vectors/Implementations/_shared/scloudplus_core/common/kem.c:108 — After private decryption at line 103, fixed-length verification produces fail_mask; scloudplus_cmov at lines 110–112 selects the rejection input. This FO selection is masked, not a secret-value branch in the inspected core. Barnes–Wall decoding remains a deeper audit target.
• Deeper trace: re-encryption derives its sampler seed from the decrypted message. The BD6/BD12 rejection samplers in common/sample.c and avx2/sample_avx2.c consume a data-dependent number of batches and XOF squeezes and compact accepted bits through a secret-derived index. This affects the 256-, 384-, and 512-bit sets; the non-rejection 128- and 192-bit samplers are controls.
• Branches, indexed accesses and division/remainder: public-seed rejection and fixed-divisor arithmetic are not treated as leaks; absence of unrelated leaks has not been proved.

Assessment: the decrypted-message timing channel is filed as kem-35-1. A practical follow-up recovers 12,743 of 13,024 secret coefficients from physical timings, but supplies neither a method for correcting the remaining 281 coefficients nor a working recovered decapsulation key. Lower-level decoding and unrelated compiler output remain open audit work.

Reproduction is the source/dataflow inspection above plus the deterministic squeeze-count and timing witness in reproduce_reencryption_timing.sh.