fed-sx-m1: Step 2c — envelope:verify_signature/2 (time-aware key lookup + HMAC stand-in) + 11 tests

next/kernel/envelope.erl

@@ -1,5 +1,5 @@
 -module(envelope).
--export([validate_shape/1, get_field/2, canonical_bytes/1]).
+-export([validate_shape/1, get_field/2, canonical_bytes/1, verify_signature/2]).
 
 %% Activity envelope per design §3.1.
 %%
@@ -83,3 +83,95 @@ insert_pair({K1, V1}, [{K2, V2} | Rest]) ->
         true  -> [{K1, V1}, {K2, V2} | Rest];
         false -> [{K2, V2} | insert_pair({K1, V1}, Rest)]
     end.
+
+%% verify_signature/2 — time-aware sig verification per design §9.6.
+%%
+%% Activity carries a `signature` proplist with `key_id`, `algorithm`,
+%% `value`. ActorState carries `public_keys` — a list of key proplists
+%% with `id`, `created`, optionally `superseded_at`, and `value` (the
+%% key material).
+%%
+%% A key is active at time T iff `created =< T` AND
+%% (no `superseded_at` OR T < `superseded_at`). Verification picks the
+%% first matching active key whose `id == signature.key_id` at the
+%% activity's `published` timestamp, then recomputes the MAC
+%% `crypto:hash(sha256, <<KeyMaterial/binary, CanonicalBytes/binary>>)`
+%% and compares it to `signature.value`.
+%%
+%% Returns ok | {error, Reason}. Reasons:
+%%   no_signature | no_key_id | no_published | no_keys |
+%%   no_active_key | bad_signature
+%%
+%% Real RSA-SHA256 / Ed25519 verification is deferred to milestone 2:
+%% Phase 8 only ships `crypto:hash/2`, so we stand in with an HMAC-shaped
+%% MAC that exercises the same key-lookup and canonical-bytes pipeline.
+
+verify_signature(Activity, ActorState) ->
+    case get_field(signature, Activity) of
+        not_found -> {error, no_signature};
+        {ok, Sig} ->
+            case get_field(key_id, Sig) of
+                not_found -> {error, no_key_id};
+                {ok, KeyId} ->
+                    case get_field(published, Activity) of
+                        not_found -> {error, no_published};
+                        {ok, Published} ->
+                            verify_with_keys(Activity, Sig, KeyId,
+                                             Published, ActorState)
+                    end
+            end
+    end.
+
+verify_with_keys(Activity, Sig, KeyId, Published, ActorState) ->
+    case get_field(public_keys, ActorState) of
+        not_found -> {error, no_keys};
+        {ok, Keys} ->
+            case find_active_key(KeyId, Published, Keys) of
+                not_found -> {error, no_active_key};
+                {ok, Key} -> verify_mac(Activity, Sig, Key)
+            end
+    end.
+
+find_active_key(_, _, []) -> not_found;
+find_active_key(KeyId, Now, [Key | Rest]) ->
+    case is_matching_active_key(Key, KeyId, Now) of
+        true -> {ok, Key};
+        false -> find_active_key(KeyId, Now, Rest)
+    end.
+
+is_matching_active_key(Key, WantId, Now) ->
+    case get_field(id, Key) of
+        {ok, WantId} -> is_active_at(Key, Now);
+        _ -> false
+    end.
+
+is_active_at(Key, Now) ->
+    case get_field(created, Key) of
+        not_found -> false;
+        {ok, Created} ->
+            case Now >= Created of
+                false -> false;
+                true ->
+                    case get_field(superseded_at, Key) of
+                        not_found -> true;
+                        {ok, SupAt} -> Now < SupAt
+                    end
+            end
+    end.
+
+verify_mac(Activity, Sig, Key) ->
+    case get_field(value, Sig) of
+        not_found -> {error, bad_signature};
+        {ok, SigValue} ->
+            case get_field(value, Key) of
+                not_found -> {error, bad_signature};
+                {ok, KeyMat} ->
+                    Bytes = canonical_bytes(Activity),
+                    Computed = crypto:hash(sha256,
+                        <<KeyMat/binary, Bytes/binary>>),
+                    case SigValue =:= Computed of
+                        true -> ok;
+                        false -> {error, bad_signature}
+                    end
+            end
+    end.