libopenarc: remove "c" from the list of required tags

c= Message canonicalization (plain-text; OPTIONAL, default is
    "simple/simple").

CHANGELOG.md

@@ -12,6 +12,7 @@ All notable changes to this project will be documented in this file.
 
 ### Fixed
 - libopenarc - `arc_free()` accepts NULL.
+- libopenarc - `c` is not a required tag in `ARC-Message-Signature`.
 
 ## [1.1.0](https://github.com/flowerysong/OpenARC/releases/tag/v1.1.0) - 2024-11-05
 

libopenarc/arc.c

@@ -404,9 +404,10 @@ arc_genamshdr(ARC_MESSAGE        *msg,
         arc_dstring_printf(dstr, ";%scv=%s", delim,
                            arc_code_to_name(chainstatus, msg->arc_cstate));
     }
-    else
+    else if (msg->arc_canonhdr != ARC_CANON_SIMPLE ||
+             msg->arc_canonbody != ARC_CANON_SIMPLE)
     {
-        /* c= */
+        /* Add c= for the AMS if it's not the RFC default simple/simple */
         arc_dstring_printf(
             dstr, ";%sc=%s/%s", delim,
             arc_code_to_name(canonicalizations, msg->arc_canonhdr),
@@ -1686,8 +1687,7 @@ arc_process_set(ARC_MESSAGE    *msg,
             arc_param_get(set, "d") == NULL ||
             arc_param_get(set, "b") == NULL ||
             arc_param_get(set, "bh") == NULL ||
-            arc_param_get(set, "i") == NULL ||
-            arc_param_get(set, "c") == NULL || arc_param_get(set, "a") == NULL)
+            arc_param_get(set, "i") == NULL || arc_param_get(set, "a") == NULL)
         {
             arc_error(msg, "missing parameter(s) in %s data", settype);
             set->set_bad = true;

test/test_milter.py

@@ -135,7 +135,7 @@ def test_milter_staticmsg(run_miltertest):
 def test_milter_canon_simple(run_miltertest):
     """Sign a message with simple canonicalization and then verify it"""
     res = run_miltertest()
-    assert 'c=simple/simple' in res['headers'][1][1]
+    assert 'c=relaxed' not in res['headers'][1][1]
 
     res = run_miltertest(res['headers'])
     assert 'cv=pass' in res['headers'][0][1]