feat: sign native artifacts from synth (ELF / MCUboot image format)

[avrabe]

Context

sigil currently signs Wasm modules via wasmsign2 custom sections. when synth transcodes Wasm to native ARM, the output is an ELF binary or raw firmware image — no more Wasm custom sections to embed signatures in.

the attestation chain needs to extend across the Wasm → native boundary:

.wasm → meld (wasm sig) → loom (wasm sig) → synth → ELF/firmware (???)

Approach

sigil needs format-aware signing backends beyond Wasm:

MCUboot TLV (primary — embedded Cortex-M targets)

MCUboot is the de facto secure boot standard for 32-bit MCUs (Zephyr, TF-M, most RTOS ecosystems). it uses a header + TLV (Type-Length-Value) structure appended to the image containing:

• SHA256 hash
• signature (ECDSA-P256, Ed25519, or RSA)
• key hash
• protected TLVs (included in hash calculation)

sigil producing MCUboot-compatible signed images means the device's existing secure boot chain can verify them without custom tooling.

ELF .signature section (secondary — Linux targets)

for ELF binaries targeting larger ARM platforms: hash the PT_LOAD segments, sign the result, embed in a .signature or .note.sigil section. the kernel binfmt_elf loader can verify at load time.

Detached attestation (already partially supported)

SLSA provenance and Sigstore bundles are format-agnostic — they link an artifact hash to the pipeline that produced it. this layer already works for any output format, but doesn't provide on-device verification at boot.

Scope

• define artifact format detection (Wasm component → Wasm module → ELF → raw binary)
• implement MCUboot TLV signing backend
• implement ELF section signing backend
• carry attestation chain metadata across the Wasm → native boundary in SLSA provenance
• ensure the full pipeline (meld → loom → synth → sigil) produces a verifiable artifact end-to-end

References

• MCUboot design
• MCUboot imgtool
• ARM TF-M secure boot
• signelf — ELF signing
• LWN — Signing ELF binaries
• feat: post-quantum signature support (SLH-DSA / FIPS 205) #46 — post-quantum signatures (SLH-DSA) relevant for both Wasm and native signing