Sync protocol implemented incorrectly when deciding if the update contains the next sync committee

[srdtrk]

Summary of Bug

Internal audit item: The sync-protocol implementation when determining whether an update contains a next sync committee is incorrect.

Expected Behaviour

Here is our code snippet:

solidity-ibc-eureka/packages/ethereum/ethereum-light-client/src/verify.rs

Lines 253 to 275 in 08c77b9

	if let (Some(next_sync_committee), Some(stored_next_sync_committee)) = (
	&update.next_sync_committee,
	trusted_consensus_state.next_sync_committee(),
	) {
	if update_attested_period == stored_period {
	ensure!(
	next_sync_committee == stored_next_sync_committee,
	EthereumIBCError::NextSyncCommitteeMismatch {
	expected: stored_next_sync_committee.aggregate_pubkey,
	found: next_sync_committee.aggregate_pubkey,
	}
	);
	}
	// This validates the given next sync committee against the attested header's state root.
	validate_merkle_branch(
	next_sync_committee.tree_hash_root(),
	update.next_sync_committee_branch.unwrap_or_default().into(),
	NEXT_SYNC_COMMITTEE_BRANCH_DEPTH,
	get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
	update.attested_header.beacon.state_root,
	)
	.map_err(|e| EthereumIBCError::ValidateNextSyncCommitteeFailed(Box::new(e)))?;
	}

And here is the sync protocol specs:

https://github.com/ethereum/consensus-specs/blob/36d80adb44c21c66379c6207a9578f9b1dcc8a2d/specs/altair/light-client/sync-protocol.md?plain=1#L419-L429

Our implementation misses the case where:

• the update contains a next_sync_committee
• the next_sync_committee in our store is unknown

Version

main

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned
• Estimate provided