smite: add channel_update and short_channel_id codecs#81
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Adds wire codecs for the BOLT 7 short_channel_id packed type and the
channel_update gossip message (type 258), including sign/verify helpers
backed by secp256k1 ECDSA, and wires ChannelUpdate into the central
Message enum so it can be dispatched off the wire.
short_channel_id:
* Packed u64 representation per BOLT 7 (3 bytes block || 3 bytes tx ||
2 bytes output). new() panics on out-of-range components (24-bit
block / tx index), which would be a programmer error in every
realistic caller. from_u64 is the infallible inverse of as_u64.
channel_update:
* Preserves any trailing unknown bytes via a pub extra: Vec<u8> field
so that re-encoding is byte-identical and the signature still
verifies. Per BOLT 7 the signature covers everything after the
leading signature field, including unknown fields following
fee_proportional_millionths.
* sign(&mut self, sk) writes the body via write_body (which includes
extra) and stores the resulting ECDSA signature.
* verify(&self, pk) -> bool recomputes the digest and returns whether
the stored signature matches the supplied pubkey. Unlike
node_announcement, channel_update does not embed node_id on the
wire, so the receiver must look the key up from the previously-seen
channel_announcement for short_channel_id and pass it explicitly.
* The decoder is intentionally lenient about flag bits (it preserves
message_flags / channel_flags verbatim) and leaves policy decisions
such as enforcement of must_be_one to the caller -- this matches
how we want to fuzz divergent BOLT 7 implementations.
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Adds the wire codec,
Messageenum wiring, and ECDSA sign/verify helpers for BOLT 7channel_update(msg type 258) and the supportingshort_channel_idpacked type.This is the first vertical slice of the BOLT 7 gossip-fuzzing milestone (#71): the codec layer that everything else (IR generators, gossip scenario, harnesses) will sit on top of.
What's in the PR
Commit 1 —
smite: implement short_channel_id and channel_update codecsbolt::short_channel_id::ShortChannelId— packed 8-byte type withnew/from_u64/as_u64/block/tx_index/output_indexaccessors,Displayin BOLT 7<block>x<tx>x<out>form, andWireFormatimpl.bolt::channel_update::ChannelUpdate— full BOLT 7 wire layout (signature, chain_hash, scid, timestamp, flags, cltv_expiry_delta, htlc min/max msat, fees), plussigning_region()exposing the post-signature body bytes.Message::ChannelUpdatevariant + msg_type/encode/decode arms + roundtrip test, so the dispatcher can actually receive/emitchannel_updatefrom the wire (not just dead-code the codec).Commit 2 —
smite: add sign and verify methods to ChannelUpdateChannelUpdate::sign(&mut self, sk)— ECDSA over double-SHA256 of the signing region.ChannelUpdate::verify(&self, pk)— takes the pubkey explicitly, since unlikenode_announcement,channel_updatedoes not embednode_idon the wire (the receiver looks it up from the correspondingchannel_announcementfor the scid).This commit shape mirrors #78.
Notes for review
message_flags/channel_flags: it preserves all bits and leaves policy enforcement (e.g.must_be_one) to the caller. This is what we want when fuzzing implementations that disagree on which bits are reserved.signing_region()is kept as apubmethod (rather than only inlining insidesign/verify) so the gossip generator can reuse it when producing deliberately-malformed signatures.cargo clippy -p smiteis clean on the touched files.Out of scope
channel_announcementcodec — will be a follow-up;channel_updatewas chosen first because it's by far the more frequent gossip message and is the one most useful for fuzzing forwarding-policy logic.ChannelUpdateGenerator) and the gossip scenario — will land in subsequent PRs once Matt'snode_announcementwork (smite: addnode_announcementcodec #78) and this codec are both in.Refs #71.