fed-sx-m1: Step 2b — envelope:canonical_bytes/1 + 8 determinism tests

next/kernel/envelope.erl

@@ -1,5 +1,5 @@
 -module(envelope).
--export([validate_shape/1, get_field/2]).
+-export([validate_shape/1, get_field/2, canonical_bytes/1]).
 
 %% Activity envelope per design §3.1.
 %%
@@ -51,3 +51,35 @@ validate_signature_shape(Env) ->
         false ->
             {error, {bad_signature, not_a_proplist}}
     end.
+
+%% canonical_bytes/1 — the byte string the signature covers.
+%%
+%% Real fed-sx will use dag-cbor over a JSON-LD-canonicalised form
+%% (design §3.2). For milestone 1 we stand in for that with the host
+%% BIF `cid:to_string/1`, which produces a CIDv1 over the deterministic
+%% textual form of the term. Two prior steps make this work:
+%%   1. The signature pair is stripped (sig covers everything except
+%%      itself).
+%%   2. The top-level property list is sorted by key so field order in
+%%      the source envelope is not load-bearing.
+%%
+%% The result is an Erlang binary suitable as the sig-cover input.
+
+canonical_bytes(Env) when is_list(Env) ->
+    Stripped = strip_signature(Env),
+    Sorted = sort_pairs(Stripped),
+    cid:to_string(Sorted).
+
+strip_signature([]) -> [];
+strip_signature([{signature, _} | Rest]) -> strip_signature(Rest);
+strip_signature([P | Rest]) -> [P | strip_signature(Rest)].
+
+sort_pairs([]) -> [];
+sort_pairs([H | T]) -> insert_pair(H, sort_pairs(T)).
+
+insert_pair(P, []) -> [P];
+insert_pair({K1, V1}, [{K2, V2} | Rest]) ->
+    case K1 < K2 of
+        true  -> [{K1, V1}, {K2, V2} | Rest];
+        false -> [{K2, V2} | insert_pair({K1, V1}, Rest)]
+    end.