Split out dkim.canonicalization, and test it. Note that test_wsp_strips_headers is correctly failing, as trailing whitespace is not stripped from header names.

dkim/canonicalization.py

@@ -0,0 +1,66 @@
+# This software is provided 'as-is', without any express or implied
+# warranty.  In no event will the author be held liable for any damages
+# arising from the use of this software.
+#
+# Permission is granted to anyone to use this software for any purpose,
+# including commercial applications, and to alter it and redistribute it
+# freely, subject to the following restrictions:
+#
+# 1. The origin of this software must not be misrepresented; you must not
+#    claim that you wrote the original software. If you use this software
+#    in a product, an acknowledgment in the product documentation would be
+#    appreciated but is not required.
+# 2. Altered source versions must be plainly marked as such, and must not be
+#    misrepresented as being the original software.
+# 3. This notice may not be removed or altered from any source distribution.
+#
+# Copyright (c) 2008 Greg Hewgill http://hewgill.com
+#
+# This has been modified from the original software.
+# Copyright (c) 2011 William Grant <me@williamgrant.id.au>
+
+import re
+
+
+class Simple:
+    """Class that represents the "simple" canonicalization algorithm."""
+
+    name = b"simple"
+
+    @staticmethod
+    def canonicalize_headers(headers):
+        # No changes to headers.
+        return headers
+
+    @staticmethod
+    def canonicalize_body(body):
+        # Ignore all empty lines at the end of the message body.
+        return re.sub(b"(\r\n)*$", b"\r\n", body)
+
+
+class Relaxed:
+    """Class that represents the "relaxed" canonicalization algorithm."""
+
+    name = b"relaxed"
+
+    @staticmethod
+    def canonicalize_headers(headers):
+        # Convert all header field names to lowercase.
+        # Unfold all header lines.
+        # Compress WSP to single space.
+        # Remove all WSP at the start or end of the field value (strip).
+        return [
+            (x[0].lower(),
+             re.sub(br"\s+", b" ", re.sub(b"\r\n", b"", x[1])).strip()
+             + b"\r\n")
+            for x in headers]
+
+    @staticmethod
+    def canonicalize_body(body):
+        # Remove all trailing WSP at end of lines.
+        removed_trailing_wsp = re.sub(b"[\\x09\\x20]+\r\n", b"\r\n", body)
+        # Compress non-line-ending WSP to single space.
+        compressed_wsp = re.sub(br"[\x09\x20]+", b" ", removed_trailing_wsp)
+        # Ignore all empty lines at the end of the message body.
+        removed_trailing_lines = re.sub(b"(\r\n)*$", b"\r\n", compressed_wsp)
+        return removed_trailing_lines