Scaffolding for ML-KEM Decaps (FIPS 203 Alg 18):
- op_i widened to 2-bit: 00=KeyGen, 01=Encaps, 10=Decaps (op_r too).
- New ST_DEC_LOAD state (D0: settles to DONE so load/parse is dbg-checkable).
- dk (=sk) streamed via dk_in_*; load logic routes each byte by region:
[0,384K)->dk_pke (dkp_bram), [384K,768K+32)->ek_pke (ek_bram),
[768K+32,+32)->H(ek) (hek_r), [768K+64,+32)->z (z_r). Routing uses the
LIVE k_i input, not start-captured k_r (dk is streamed before start_i).
- c (=ct) streamed via c_in_* into a SEPARATE c_in_bram, so the computed c'
(ct_bram) can later be compared against original c and J(z||c) can read c.
- New dbg taps: dbg_mprime_o/dbg_kbar_o/dbg_decz_o/dbg_dech_o.
TB: tb_mlkem_dec_katK_xsim verifies dk parse (H(ek), z, ek_pke/dk_pke BRAM
round-trip). gen_decaps_vectors.py emits dec_k{K}_c{N}_{dk,ct,ss,ctn,ssn}.hex
from the NIST KAT. run_tb.sh gains a 'dec' module (mirrors 'enc').
Regression fix: old KeyGen/Encaps TBs didn't connect the new input ports,
floating them to X and corrupting the ek/dkp write muxes -> tied off
dk_in_*/c_in_*/new dbg taps in both.
Verified: dec D0 K=2/3/4 PASS; KeyGen K=2 + Encaps K=2 unregressed.
run_tb.sh gains an 'enc' module that shares the 'top' KeyGen tcl compile
list (same RTL datapath) and swaps in tb_mlkem_enc_katK_xsim. Usage mirrors
'top': ./run_tb.sh enc [K] [CASE]; no args -> full sweep K=2/3/4 cases 0..2.
--list shows enc; per-case summary lines parse PASS (E7). run_enc.sh deleted.
tb_mlkem_enc_katK_xsim verify_e7 now dumps the hardware-produced ct (read
from ct_bram via the dbg_ct tap) on one line (byte 0 first), same format as
ml-kem-r's encaps_io example, so ss/ct can be eyeballed and diffed. On
mismatch it re-scans to print the first 8 differing byte positions.
Verified: ./run_tb.sh enc -> 9/9 PASS (E7) (ct==KAT.ct && ss==KAT.ss);
./run_tb.sh top 3 0 KeyGen unregressed.
Reuses the E5 bit-packer FSM for ST_ENC_C2, generalized over region:
- coeff source: cp_coeff_src = bank_t[UPSUM] (v) for C2, bank_se (u) for C1
- bit width: cp_d already = dv_rt for C2 else du_rt
- poly count: cp_poly_max = 1 (single v) for C2, K for C1
- ct write pointer cp_wa CONTINUES from c1_bytes into C2 (not reset), so c2
lands right after c1. c1 ends on a whole-byte/poly boundary (256*du/8
integral), so cp_buf/nbits are empty at the C1->C2 handoff.
FSM tail: ST_ENC_V -> ST_ENC_C2 -> ST_DONE.
TB: verify_e7 compares the full ct (c1||c2, CTB bytes) to KAT.ct byte-exact
via the dbg_ct tap. Combined with the E0 ss==KAT.ss check this is the full
end-to-end Encaps KAT (ct==KAT.ct && ss==KAT.ss).
Verified end-to-end for K=2/3/4, cases 0-2 (K2) / 0-1 (K3,K4):
ct==KAT.ct && ss==KAT.ss. ML-KEM Encaps complete. KeyGen unregressed.
Storage choreography (per plan risk notes):
- TDEC now decodes t_hat[j] into bank_a slot j*K (was bank_t). This makes
V's MAC reuse E4's u_aslot=u_j*K+u_row addressing with u_row=0, no mux
change. bank_t has no room for K=4 (t_hat would fill all 4 slots vs
psum's UPSUM slot), hence bank_a (16 slots, A_hat dead after E4).
- New ST_ENC_E2MV state relocates e2 (bank_t[0]) -> bank_a[1] so V-ADD
reads psum (bank_t) and e2 (bank_a) from different banks (no port
conflict). bank_se (y_hat + u) stays intact -> verify_e2/e3/e4 unaffected.
- V reuses the u_* MAC/INTT/ADD machine with u_row tied to 0 (u_row_max=1).
ADD computes psum + e2 + mu mod Q -> bank_t[UPSUM] in place;
mu[w] = m_r[w] ? 1665 : 0 (Decompress_1). FSM: C1->TDEC->E2MV->V->DONE.
Bug found+fixed during bring-up: e2 relocation was off-by-one (wrote
e2[i+1] into slot i) because em_we/em_widx were registered an extra cycle
past the bram read. Fixed: em_widx==em_ridx, write scheduled for the cycle
bt_rd_data presents e2[em_ridx].
TB: verify_e6 compares v (bank_t dbg slot 9, K=2) to ml-kem-r golden.
verify_e1 dropped (TDEC overwrites bank_a A_hat slots; A_hat transitively
verified by E4). Verified: K=2 E2/E3/E4/E6 == golden, E5 c1 == KAT prefix;
K=3/4 E0+E5 pass; KeyGen K=2 unregressed.
ST_ENC_C1: per-coeff Compress_du via comp_decomp_sync (mode 0) then
LSB-first byte packing into ct_bram. 5-phase micro-seq reads u[cp_poly]
from bank_se (rel K+poly), feeds the compressor (1-cyc pipe), appends du
bits to cp_buf, and drains whole bytes. Each poly = 256*du bits (whole
bytes) so the bit buffer empties at every poly boundary.
ST_ENC_U now advances to ST_ENC_C1 (was ST_DONE).
TB: verify_e5 compares ct_bram[0..c1_bytes-1] to the KAT.ct prefix via
the dbg_ct tap. run_enc.sh: encaps TB runner (compiles comp_decomp_sync
which the KeyGen tcl omits).
Verified K=2/3/4 c1 == KAT.ct prefix (640/960/1408 B; K=4 du=11
cross-byte path), K=2 cases 0-2.
ST_ENC_U computes u[i] per row in 3 sub-phases reusing shared u_pmul + u_ntt:
sub0 MAC : sum_j A_hat[j][i] o y_hat[j] (TRANSPOSE: slot=j*K+i) -> NTT-domain
psum in bank_t rel slot UPSUM=1 (e2 in slot 0), init 0 at j==0
sub1 INTT: INTT(psum) mode=1 (built-in x3303) in place in bank_t[UPSUM]
sub2 ADD : u[i][w] = psum[w] + e1[i][w] mod Q -> bank_se rel (K+i), over e1
y_hat (bank_se 0..K-1) preserved for V. ntt_core mode + input muxed for the
INTT sub-phase; bank_a/se/t read+write ports extended for all 3 sub-phases.
Fixed a duplicate 'assign bse_we' (stale + new both present -> ADD writes
X-dropped); collapsed to one. Verified (K=2 c0) u[0..1] == ml-kem-r golden
(transpose + INTT + e1 all correct); E0/E1/E3 pass, E2 trimmed to e2 (e1
consumed into u, transitively checked by E4); K=3/4 no timeout.
ST_ENC_N reuses the ST_N forward-NTT datapath (mode=0, read-ahead load,
in-place writeback to bank_se). Slot count parameterized: KeyGen 2K (s,e),
Encaps K (y only; e1/e2 stay time-domain) via n_slot_max. bse rd/wr muxes
and the C->N arming extended for ST_ENC_N.
Verified (K=2 c0) y_hat[0..1] == ml-kem-r golden; E0/E1/E2 still pass
(E2 y check dropped since NTT overwrites y in place -> covered by E3);
K=3/4 run through E3 without timeout.
ST_ENC_C reuses the ST_C CBD datapath with Encaps muxes: seed=r (not sigma),
eta=eta1 for y[0..K-1] then eta2 for e1[0..K-1]/e2, nonce 0..2K. 2K+1 polys
(vs KeyGen 2K). y/e1 -> bank_se rel slots 0..2K-1; e2 -> bank_t rel slot 0
(free during C/N/U since TDEC is deferred to V-prep so the 28-slot banks hold
all of A+y_hat+e1+e2 at K=4 without resizing).
Bring-up golden via ml-kem-r dump_encaps_full (working-tree example):
vectors/encgold/ec_k2_c0_{y,e1,e2}.hex. Verified (K=2 c0) y[0..1],e1[0..1],e2
== ml-kem-r; A_hat (E1) and ss (E0) still pass; K=3/4 no timeout.
ST_ENC_LOAD: stream rho (32B) from ek_bram[384k..] into rho_r (read-ahead,
1-cyc bram latency). ST_ENC_A: regenerate A_hat via SampleNTT into bank_a
(reuses ST_A datapath, gated on st==ST_A||ST_ENC_A). ST_ENC_TDEC:
byteDecode12 ek[i*384..] -> t_hat[i] into bank_t (5-cycle micro-phase per
3-byte/2-coeff triple; bt write port muxed with ST_M).
Verified (K=2 c0) A_hat (1024 coeffs) + t_hat (512) == KeyGen golden via
dbg_coeff_o; E0 ss==KAT.ss still passes all K/cases (no timeout).
Extend mlkem_top with a runtime op_i select (0=KeyGen, 1=Encaps) and the
first Encaps stages, reusing the shared keccak_core and the ST_H multi-block
SHA3-256 machinery:
ST_ENC_H: H(ek) over preloaded ek_bram (same FSM as KeyGen ST_H)
ST_ENC_G: (K,r) = G(m||H(ek)) via new 64-byte single-block SHA3-512
- sha3_top_shared: add mode=2'b11 = SHA3-512 over a full 512-bit message
(g512_pad). Standalone tb_sha3_g512 confirms it byte-exact.
- mlkem_top: new ports op_i, msg_i, ek_in_{we,addr,byte} (ek preload), ss_o,
dbg_ct_*, dbg_r_o/dbg_hek_o. st widened 4->5 bits; ST_ENC_* states added.
Renamed message port to msg_i to avoid collision with ST_M counter m_i.
- TB tb_mlkem_enc_katK + gen_encaps_vectors.py (per-byte ek/m/ct/ss vectors).
Verified ss==KAT.ss for K=2/3/4, cases 0-2 (all PASS). KeyGen unaffected
(K=2 c0 still ek==pk, dk==sk byte-exact).
