Pull c= value parsing out into Canonicalizationpolicy.from_c_value.

dkim/canonicalization.py

@@ -90,6 +90,35 @@ class CanonicalizationPolicy:
         self.header_algorithm = header_algorithm
         self.body_algorithm = body_algorithm
 
+    @classmethod
+    def from_c_value(cls, c, logger=None):
+        """Construct the canonicalization policy described by a c= value.
+
+        @param c: c= value from a DKIM-Signature header field
+        @return: a C{CanonicalizationPolicy}, or C{None} if the value is
+            invalid
+        """
+        if c is None:
+            c = b'simple/simple'
+        m = c.split(b'/')
+        if len(m) not in (1, 2):
+            if logger:
+                logger.error(
+                    "c= value is not in format method/method: %s" % c)
+            return None
+        if len(m) == 1:
+            m.append(b'simple')
+        can_headers, can_body = m
+        try:
+            header_algorithm = algorithms[can_headers]
+            body_algorithm = algorithms[can_body]
+        except KeyError as e:
+            if logger:
+                logger.error(
+                    "unknown canonicalization algorithm: %s" % e.message)
+            return None
+        return cls(header_algorithm, body_algorithm)
+
     def canonicalize_headers(self, headers):
         return self.header_algorithm.canonicalize_headers(headers)