Merge lp:~mbp/dkimpy/587783-relaxed into lp:dkimpy

[Martin Pool]

* when verifying Relaxed mode signatures, the partial DKIM-Signature
   header must be canonicalized before hashing (Martin Pool)
   <https://launchpad.net/bugs/587783>

This is a forward-port and update of the fix originally done in <https://bugs.launchpad.net/pydkim/+bug/587783>:

When generating a signature dkim specifies that a placeholder DKIM-Signature should be hashed. Without this patch, pydkim doesn't apply the right canonicalization rules and therefore can't it can't verify Relaxed-mode signatures it generates itself.

[Scott Kitterman (kitterman)]

Ship it.

review: Approve

[lp:~mbp/dkimpy/587783-relaxed]

11. By Martin Pool on 2011-03-15

ChangeLog correction

=== modified file 'ChangeLog'
--- ChangeLog	2011-03-08 04:25:49 +0000
+++ ChangeLog	2011-03-15 05:46:12 +0000
@@ -1,3 +1,8 @@
+Not yet released
+    - when producing Relaxed mode signatures, the partial DKIM-Signature
+      header must be canonicalized before hashing (Martin Pool)
+      <https://launchpad.net/bugs/587783>
+
 2008-06-25 Version 0.3
 
     - length parameter to sign() is now a boolean
 
=== modified file 'dkim/__init__.py'
--- dkim/__init__.py	2011-03-15 04:10:03 +0000
+++ dkim/__init__.py	2011-03-15 05:46:12 +0000
@@ -313,18 +313,23 @@
         ('bh', bodyhash),
         ('b', ""),
     ] if x]
-    sig = "DKIM-Signature: " + "; ".join("%s=%s" % x for x in sigfields)
 
-    sig = fold(sig)
+    sig_value = fold("; ".join("%s=%s" % x for x in sigfields))
+    dkim_header = canonicalize[0].canonicalize_headers([
+        ['DKIM-Signature', ' ' + sig_value]])[0]
+    # the dkim sig is hashed with no trailing crlf, even if the
+    # canonicalization algorithm would add one.
+    if dkim_header[1][-2:] == '\r\n':
+        dkim_header = (dkim_header[0], dkim_header[1][:-2])
+    sign_headers.append(dkim_header)
 
     if debuglog is not None:
-        print >>debuglog, "sign headers:", sign_headers + [("DKIM-Signature", " "+"; ".join("%s=%s" % x for x in sigfields))]
+        print >>debuglog, "sign headers:", sign_headers
     h = hashlib.sha256()
     for x in sign_headers:
         h.update(x[0])
         h.update(":")
         h.update(x[1])
-    h.update(sig)
     d = h.digest()
     if debuglog is not None:
         print >>debuglog, "sign digest:", " ".join("%02x" % ord(x) for x in d)
@@ -334,9 +339,9 @@
             d, HASHID_SHA256, pk['privateExponent'], pk['modulus'])
     except DigestTooLargeError:
         raise ParameterError("digest too large for modulus")
-    sig += base64.b64encode(sig2)
+    sig_value += base64.b64encode(sig2)
 
-    return sig + "\r\n"
+    return 'DKIM-Signature: ' + sig_value + "\r\n"
 
 def verify(message, debuglog=None, dnsfunc=dnstxt):
     """Verify a DKIM signature on an RFC822 formatted message.