Canary runtime_pubkey strict profile rejected at Stage 8 (E_CANARY_INVALID) vs section 05:159 (E_SIG_VERIFICATION)

[samjanny]

Informational: a known, intentional divergence between this crate and the section 05:159 enforcement point for a small-order or non-canonical canary.runtime_pubkey.

Section 05:157/159: the strict profile (canonical encoding, non-small-order) for K_runtime.pub is "enforced when a document is verified under the key", and a failure is reported as E_SIG_VERIFICATION. For K_origin.pub, which verifies no document in v1, enforcement is structural at Stage 9 origin binding with E_BIND_ORIGIN (upstream AMB-17).

This crate additionally rejects a small-order / non-canonical canary.runtime_pubkey early, at Stage 8 canary structural validation, with E_CANARY_INVALID (validate_canary_structure calls validate_runtime_pubkey_strict). The rationale is defense-in-depth: without it, a manifest declaring a bad runtime key would pass Stages 6/8/9 and the violation would surface only on the first content fetch. Failing at canary-structure time aligns the rejection with manifest acceptance.

Net effect, for a manifest carrying a small-order or non-canonical canary.runtime_pubkey:

• this crate rejects the manifest at Stage 8 with E_CANARY_INVALID;
• the section 05:159-favored reading (and the Java reference implementation) defers to content/transaction verification under that key and reports E_SIG_VERIFICATION.

The conformance corpus does not currently exercise this case, so both implementations pass the rc.37 corpus; the divergence is latent.

No fix is proposed. Options for the record:

1. Keep the extra-strict Stage 8 check (defense-in-depth) and document the divergence (this issue).
2. Defer to section 05:159: drop the Stage 8 runtime_pubkey strict check and rely on content-verify E_SIG_VERIFICATION.
3. Raise an upstream ambiguity to bless an early, manifest-acceptance-time rejection point for K_runtime.pub (analogous to the AMB-17 resolution for K_origin.pub) and pin a diagnostic.

Filed for visibility; no action is required for rc.37 conformance.