keep-nip46: fail closed instead of panicking on unrouted encryption method

[kwsantiago]

Hardening found by an audit of panic sites on untrusted-input decode paths.

The encryption dispatch in the NIP-46 server re-matches the client-supplied method string inside the arm that the outer dispatch routes nip44_encrypt / nip44_decrypt / nip04_encrypt / nip04_decrypt into, with _ => unreachable!(). The invariant holds today, but it couples two match arms in different places on an attacker-controlled string: if the outer list ever gains a method the inner match does not handle, an unsupported method string panics the signer process (denial of service) instead of returning an error.

Changes

• Replace the unreachable!() arm with a logged, fail-closed Unsupported method error response.

Audit context

A sweep of unwrap() / expect() / panic! / unreachable! in non-test code across the wire and IPC decode surfaces (keep-frost-net protocol/event/node, keep-nip46 request handling, keep-mobile FFI) found no attacker-reachable panic besides this fragile spot. The remaining sites are locally-verified invariants (expect("just inserted") immediately after a map insert), startup-time initialization, or environmental conditions, none reachable from untrusted input.

Verification

• cargo test -p keep-nip46: 189 passed.
• cargo clippy -p keep-nip46 --all-targets: clean.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 525e9624-4f1d-4fcd-93fb-d1dc31cc7fee

📥 Commits

Reviewing files that changed from the base of the PR and between f468f81 and 4c143f0.

📒 Files selected for processing (1)

• keep-nip46/src/server.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/nip46-unreachable-method-arm

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