[kwsantiago]

The cryptography the device needs to take part in keep's KFP v2 signing protocol itself, with its transport key derived inside frost_tr from the stored key package so it never leaves the device and no caller supplies it. This PR adds the primitives and their C interface; the protocol handling that calls them (announce, sign request, commitments and shares as device-built events) follows in the next PR, so the firmware does not call them yet.

Primitives (components/frost_tr/rust/src/kfp.rs, declared in frost_tr.h)

• Transport key: the first SHA256("keep-frost-transport-v2" || share || group_xonly || index_be || counter) that is a valid secret key, as keep derives it. ftr_kfp_transport_pubkey returns the public half only.
• Announce proof: BIP-340 by the signing share over SHA256 of keep's proof digest (domain, group, index, verifying share, transport key, timestamp), which keep's verify_proof checks. keep signs with zero aux; the device draws aux from its RNG, so a fault injected into one of two runs over the same timestamp never meets a repeated nonce, and verifies the signature before releasing it. The transport key it binds is derived inside, not passed in.
• Rendezvous address: keep's routing address for each member (public).
• NIP-44 v2 seal and open between the transport key and a peer's x-only key (ChaCha20, HKDF and HMAC-SHA256, base64), with a nonce from the device RNG. Each refuses before allocating when the caller's buffer cannot hold the result and works in a single buffer, so an oversized payload cannot exhaust the internal heap. Open checks the MAC before decrypting and the length prefix against the padding, and writes nothing on failure. The header marks seal and open, like event signing, as for firmware-built plaintext only, and notes that a payload this device sealed opens too.
• Event signing: the NIP-01 id of a serialization the firmware builds and its BIP-340 signature by the transport key (aux from the device RNG, verified before release). Documented as never to be reached with host-chosen bytes; the next PR keeps it behind device-built events.
• keep's salted session id (BLAKE2b-512, first 32 bytes) and keep's salt binding check for a request's path and taproot tweak (a hardened index is refused, as it could read as the tweak marker).
• New dependencies, pinned exactly and passing cargo-deny: chacha20 0.9.1 (the version the nostr crate keep uses, with its zeroize feature), hkdf 0.12.4, hmac 0.12.1, blake2 0.10.6, base64ct 1.8.3, and k256's schnorr feature. No RNG crate.

Tests

• Vectors generated by keep's own code (vectors/kfp.json, written by test/vectors/gen-kfp, which depends on keep-core and keep-frost-net pinned to a keep commit and on nostr 0.44.7; each run writes a fresh set from keep's dealer) for three groups (2-of-3 twice, 3-of-5): each member's transport key matches, announce proofs match byte for byte under keep's zero aux and the device's own proofs verify; the announce keep's node actually publishes carries the device's proof for its timestamp, is authored by the transport key, and tags every other member's rendezvous address; 165 NIP-44 payloads (1 to 65 408 bytes) match keep's for the same nonce and open both ways; keep's own sealed ping and pong events open to the plaintext keep wrote; NIP-01 ids match keep's, including every escaped character, and the device's signatures verify; session ids match keep's for empty, path and tweak salts.
• Refusals: a payload too large for the output buffer (before decoding), a correctly padded message longer than NIP-44 allows, a flipped byte anywhere, another sender, a non-point key, bad base64, a short or oversized payload, a length prefix that disagrees with the padding or overruns the buffer (refused, not read past), an empty message, and output buffers too small.
• Disabling or altering any derivation step, the MAC check, the padding and length rules, the capacity check, the device's aux, the participant sort or the salt rules fails a test.

[coderabbitai]

Walkthrough

The change adds KFP v2 cryptographic operations to the Rust library and exposes them through C APIs. It includes transport-key derivation, proofs, encryption and decryption, event signing, session IDs, salt-binding checks, and related tests.

Changes

KFP v2 operations

Layer / File(s)	Summary
Public API and cryptographic dependencies components/frost_tr/include/frost_tr.h, components/frost_tr/rust/Cargo.toml, components/frost_tr/rust/src/lib.rs	The C header adds KFP constants and API declarations. The Rust manifest enables Schnorr support and adds cryptographic dependencies. The Rust library declares the KFP module.
KFP cryptographic operations components/frost_tr/rust/src/kfp.rs	Implements transport-key derivation, announce proofs, rendezvous addresses, NIP-44 v2 sealing and opening, event signing, session IDs, salt-binding checks, and plaintext-padding validation.
C API integration and vector tests components/frost_tr/rust/src/lib.rs	Adds C entry points with input and output-capacity checks. Tests cover KFP vectors, NIP-44 interoperability and rejection cases, nonce freshness, event signatures, session IDs, and salt binding.

Priority: ⬇️ Low

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Firmware
  participant CAPI as ftr_kfp_seal
  participant KFP as kfp::seal
  participant NIP44 as NIP-44 v2 routines
  Firmware->>CAPI: Submit key package, recipient, and plaintext
  CAPI->>KFP: Pass decoded key package and inputs
  KFP->>NIP44: Seal plaintext with generated nonce
  NIP44-->>KFP: Return encoded authenticated payload
  KFP-->>CAPI: Return payload
  CAPI-->>Firmware: Copy payload and output length

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Merge Risk: 🔵 Low · up to 59de9

Enable cipher-state wiping before merging if practical. The remaining concern is limited to residual key material in memory; the nonce-generation concern was refuted.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 48 functions across 3 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main KFP v2 additions: transport functionality, announce proofs, and NIP-44 primitives for frost_tr.

Full details: Docstring Coverage

Explanation

Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 48 functions across 3 files. (1 skipped: 1 unsupported.)

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_{A rabbit checks the keys at dawn
Then sends a proof across the lawn
A padded message hops away
A signed event joins the display
The salt binds paths just as they should
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Actionable comments posted: 1

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Inline comments:
Review comments at @components/frost_tr/rust/Cargo.toml:
- Line 21: Update the chacha20 dependency declaration to enable its zeroize
feature while preserving the pinned version and disabled default features, so
ChaCha20 state is cleared on drop.

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Reviewing files that changed from the base of the PR and between 2d2f133 and 59de994.

⛔ Files ignored due to path filters (1)

• components/frost_tr/rust/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (5)

• components/frost_tr/include/frost_tr.h
• components/frost_tr/rust/Cargo.toml
• components/frost_tr/rust/src/kfp.rs
• components/frost_tr/rust/src/lib.rs
• components/frost_tr/rust/vectors/kfp.json

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