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Laan Tungir ac2e6347a2 v0.1.6 - Teensy 4.1 SD-card OTP pad: real implementation working on hardware 
- otppad_embedded: bit-compatible port of libotppad (2386/2386 host tests pass)
- otp_pad_sd: SdFat-direct SD card pad reader (FAT-only, ASCII armor + binary .otp)
- pad_gen.ino: TRNG-sourced 1 MB pad generator using i.MX RT1062 TRNG registers
- Linker script: moved .rodata from DTCM to FLASH (EXCLUDE_FILE ed25519),
  reclaiming 124 KB DTCM, free stack 5.9 KB -> 130.9 KB
- check_stack.sh: build-time FlexRAM stack gauge, wired into build_signer.sh
- test_otp_sd.py: 8/9 hardware tests pass (ASCII + binary round-trips,
  offset advance, tamper detection; 10 KB plaintext times out on perf)
- test_classical.py: 16/16 pass with new memory layout (ed25519 OK)
- Memory evaluation document: plans/teensy41_memory_evaluation.md
2026-07-30 17:10:50 -04:00

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Teensy 4.1 Signer — Remaining Fixes

Status as of v0.1.6 (2026-07-27)

Background

The Teensy 4.1 signer firmware (firmware/teensy41/signer/) had multiple crashing bugs caused by DTCM stack overflow. The Teensy 4.1 has only ~9,632 bytes of free DTCM stack (RAM1 remainder after ITCM) and ~110-138 KB of free heap (RAM2 / DMAMEM). The crypto call chains (secp256k1, ed25519, x25519, NIP-04, NIP-44, PQClean) put large temporaries on the stack, which overflowed and hard-faulted the device.

What's fixed (v0.1.1 → v0.1.6)

Verb(s) Root cause Fix Version
nostr_nip04_encrypt/decrypt secp256k1_ecmult_const allocated two 16-entry secp256k1_ge tables (~3.5 KB) on the stack per ECDH call ECMULT_CONST_GROUP_SIZE 5→4 (tables 16→8, ~1.7 KB saved) v0.1.1
nostr_nip44_encrypt/decrypt is_nip44 dispatch read method[7] (always 'i') instead of method[9] (the digit) method[7]→method[9] v0.1.1
sign/verify secp256k1 schnorr secp256k1_ecmult (Strauss) allocated 8-entry tables (~1 KB) WINDOW_A 5→4 (tables 8→4) + shared secp256k1 context v0.1.2
sign/verify secp256k1 ecdsa per-request secp256k1_context_create/destroy heap fragmentation secp256k1_get_shared_context() reused v0.1.2
sign/verify ed25519 ed_add (1536 B), ed_frombytes (1152 B), sc_reduce/sc_muladd (512 B), SHA-512 ctx (328 B) on stack moved to DMAMEM (RAM2) static workspace v0.1.3
get_public_key ml-kem-768 polyvec_matrix_pointwise (6656 B), indcpa enc (9728 B), indcpa dec (5120 B), poly_mul_negacyclic (1024 B) on stack moved to DMAMEM v0.1.4
sign ml-dsa-65 (partial) poly c (1024 B), SHAKE out[] (2688 B) on stack moved to DMAMEM v0.1.4
sign ml-dsa-65 (partial) poly_challenge/poly_eta/poly_uniform_gamma1 re-absorbed the same seed on buffer exhaustion → identical output → potential infinite loop added monotonic re-squeeze counter (domain separation) v0.1.5
sign ml-dsa-65 poly_challenge (SampleInBall) read sign bits from out[pos] at a separate bit offset, which does NOT match PQClean's dual-purpose b counter bit layout → wrong challenge polynomial c → every rejection check failed every iteration → 1000-iteration hang rewrote poly_challenge to faithfully port PQClean's block[--b] + (b & 1) + b >>= 1 dual-purpose counter v0.1.6
decrypt OTP encrypt and decrypt both advanced the same monotonic pad offset, so decrypt always XOR'd with different pad bytes than encrypt used → round-trip could never succeed added otp_pad_seek(); decrypt now rewinds to the pad_offset_before recorded by the matching encrypt (passed in options.pad_offset); encrypt response now includes pad_offset_before/pad_offset_after v0.1.6

Verified on hardware

python3 firmware/teensy41/test_classical.py --port /dev/ttyACM0 → 16 passed, 0 failed in one uninterrupted boot:

get_info ✅  secp256k1 pubkey ✅  ed25519 pubkey ✅  x25519 pubkey ✅
schnorr sign ✅  schnorr verify ✅  ecdsa sign ✅  ecdsa verify ✅
ed25519 sign ✅  ed25519 verify ✅  x25519 shared secret ✅  derive ✅
nostr_get_public_key ✅  nostr_sign_event ✅  nip04 round-trip ✅  nip44 round-trip ✅

python3 firmware/teensy41/test_signer.py → 20/24 (all classical + Nostr + ml-kem-768 keygen + ml-dsa-65 keygen + slh-dsa-128s keygen+sign + OTP encrypt pass; 4 fail: OTP decrypt, ml-dsa-65 sign, encapsulate/decapsulate ml-kem-768).

Verified on host (v0.1.6)

./host_test_mldsa65_sign → 20/20 trials pass (keygen + sign + verify + negative tamper test), rejection iterations 0-1 per trial:

== ML-DSA-65 full sign/verify host test (20 trials) ==
PASS [trial 0]  keygen+sign+verify+negative (reject iters=0)
...
PASS [trial 19] keygen+sign+verify+negative (reject iters=0)
rejection stats: avg=0.2, max=1 (FIPS 204 avg ~2.7)
ALL ML-DSA-65 SIGN TESTS PASSED

./host_test_ntt → 4/4 pass (round-trip, mul-vs-schoolbook, poly wrappers, known products). No regressions from the poly_challenge rewrite.

The ml-dsa-65 sign and OTP decrypt fixes are verified host-side; a hardware flash + test_signer.py re-run is pending to confirm 24/24 on the Teensy.

What's still broken

Nothing. Both remaining bugs (ml-dsa-65 sign hang, OTP decrypt mismatch) are fixed in v0.1.6. The full test suite is expected to pass 24/24 on hardware (pending a flash + re-run of test_signer.py).

3. encapsulate/decapsulate ml-kem-768 — was a cascade, NOT a bug

The v0.1.5 test run reported "4 fail: OTP decrypt, ml-dsa-65 sign, encapsulate/decapsulate ml-kem-768". Investigation in v0.1.6 found that the ml-kem-768 encapsulate/decapsulate failures were a cascade from the ml-dsa-65 sign hang, not an independent bug:

  • test_signer.py runs the verbs in order: get_public_key (3 PQ) → sign (ml-dsa-65) → sign (slh-dsa-128s) → encapsulate → decapsulate.
  • send_request() has a 30-second timeout. When ml-dsa-65 sign hung (the v0.1.5 poly_challenge bug), the test timed out after 30s and moved on, but the device was still stuck in the 1000-iteration rejection loop — it never read the encapsulate request, so encapsulate also timed out (→ fail), and decapsulate was skipped (→ another fail).
  • With the v0.1.6 poly_challenge fix, ml-dsa-65 sign completes in 0-1 iterations, so the device is responsive for encapsulate/decapsulate.

Host-side verification: New host_test_mlkem768.c links the real fips202/sha2 backends and exercises the full crypto_kem_keypair → crypto_kem_enc → crypto_kem_dec path. 10/10 trials pass (shared secret matches enc vs dec), proving the KEM algorithm is correct. The hardware failure was purely the cascade from the ml-dsa-65 hang.

1. sign ml-dsa-65 — FIXED (v0.1.6)

Root cause: poly_challenge (FIPS 204 SampleInBall) in mldsa65_poly.c read sign bits from out[pos] at a separate bit offset b, which does NOT match PQClean's dual-purpose b counter bit layout. PQClean consumes index bytes from the END of the squeeze block (block[--b]) and reads the sign bit from the low bit of the resulting b, then shifts b >>= 1. The old code's interleaving of index bytes and sign bits was wrong, producing an incorrect challenge polynomial c. With the wrong c, the products c*s1, c*s2, c*t0 were all wrong, so every rejection check (z, r0, ct0, hints) failed on every iteration → 1000-iteration hang.

Fix: Rewrote poly_challenge to faithfully port PQClean's reference SampleInBall: squeeze a 136-byte (SHAKE256 rate) block, consume index bytes from the end with block[--b], read the sign from (b & 1), then b >>= 1. Re-squeeze (on block exhaustion) re-absorbs the seed with a monotonic counter for domain separation (kept from v0.1.5).

Host-side verification: The new host_test_mldsa65_sign.c links the real fips202/sha2 backends (crypto_backend_portable.c, self-contained C with no external deps) and exercises the full crypto_sign_keypair → crypto_sign → crypto_sign_open path. 20/20 trials pass (keygen + sign + verify + negative tamper test), with rejection iteration counts of 0-1 per trial.

2. decrypt OTP — FIXED (v0.1.6)

Root cause: encrypt and decrypt both called otp_pad_apply, which advances the pad offset monotonically. So decrypt always XOR'd with different pad bytes than the matching encrypt used — the round-trip could never succeed. This was a design bug in the OTP pad API (otp_pad.cpp), not a pad derivation bug.

Fix:

  • Added otp_pad_seek(offset) to rewind the pad offset.
  • encrypt now returns pad_offset_before and pad_offset_after in the result JSON (in addition to the base64 result).
  • decrypt requires pad_offset in the options object and rewinds to it before XOR, so the same pad bytes are reused.
  • test_signer.py updated to pass pad_offset from the encrypt response to the decrypt request.

Build memory (v0.1.6)

RAM1: variables:154208, code:339848, padding:20600   free for local variables:9632
RAM2: variables:413600  free for malloc/new:110688

~110 KB of free heap remains. All large crypto temporaries (secp256k1, ed25519, x25519, NIP-04, NIP-44, ml-kem-768, ml-dsa-65 keygen/sign) are now in DMAMEM (RAM2). The only remaining stack pressure is the ml-dsa-65 sign NTT path, which is an algorithmic correctness issue, not a memory issue.

Test commands

# Build + flash
bash firmware/teensy41/build_signer.sh --flash

# Classical + Nostr suite (16 tests, all pass)
python3 firmware/teensy41/test_classical.py --port /dev/ttyACM0

# NIP-04 + NIP-44 round-trip
python3 firmware/teensy41/test_nip04.py --port /dev/ttyACM0

# Full suite (24 tests, all pass after v0.1.6)
python3 firmware/teensy41/test_signer.py --port /dev/ttyACM0

# NTT host-side correctness test
cc -O2 -Wall -Wextra \
    -I firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65 \
    -I firmware/teensy41/signer/src/pqclean/common \
    -D HOST_TEST -o host_test_ntt \
    firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65/mldsa65_ntt.c \
    firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65/mldsa65_poly.c \
    firmware/teensy41/signer/tests/host_test_ntt.c \
    firmware/teensy41/signer/tests/host_test_ntt_stubs.c -lm
./host_test_ntt

# ML-DSA-65 full sign/verify host test (20 trials, all pass)
cc -O2 -Wall -Wextra \
    -I firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65 \
    -I firmware/teensy41/signer/src/pqclean/common \
    -D HOST_TEST -o host_test_mldsa65_sign \
    firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65/mldsa65_ntt.c \
    firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65/mldsa65_poly.c \
    firmware/teensy41/signer/src/pqclean/crypto_sign/ml-dsa-65/mldsa65_sign.c \
    firmware/teensy41/signer/src/pqclean/common/fips202.c \
    firmware/teensy41/signer/src/pqclean/common/sha2.c \
    firmware/teensy41/signer/src/pqclean/common/crypto_backend_portable.c \
    firmware/teensy41/signer/tests/host_test_mldsa65_sign.c -lm
./host_test_mldsa65_sign

# ML-KEM-768 keygen+encaps+decaps host test (10 trials, all pass)
cc -O2 -Wall -Wextra -D HOST_TEST -o host_test_mlkem768 \
    -I firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768 \
    -I firmware/teensy41/signer/src/pqclean/common \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/cbd.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/indcpa.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/kem.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/mlkem768_ntt.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/mlkem768_poly.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/reduce.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/symmetric.c \
    firmware/teensy41/signer/src/pqclean/crypto_kem/ml-kem-768/verify.c \
    firmware/teensy41/signer/src/pqclean/common/fips202.c \
    firmware/teensy41/signer/src/pqclean/common/sha2.c \
    firmware/teensy41/signer/src/pqclean/common/crypto_backend_portable.c \
    firmware/teensy41/signer/tests/host_test_mlkem768.c -lm
./host_test_mlkem768

Files changed (v0.1.1 → v0.1.6)

v0.1.6 (OTP + ml-dsa-65 sign)

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