[ietf-dkim] z= question with X headers

[Eric Allman]

>     // Hash Headers
>
>     hash = empty;
>     for each hdr in (dkim_h_list) do
>
>       s  = mail_headers[hdr];
>       sz = dkim_z_list[hdr];  // see is copy is available
>
>       if (s != sz) {
>          WHAT? INVALID?  Should they be the same?
>          What can cause this? Mailing list?
>       }
>
>       if (s == "") s = sz;  // correction
>
>       if (s != "")
>          hash += hash_string(s)
>       else
>          WHAT? INVALID?
>
>     end for

This makes me nervous (assuming I'm understanding the pseudo-code). 
It seems to say that if a header doesn't exist you use the header 
from the z= tag.  But since MUAs aren't going to see that, I think 
it's problematic.  Also, it doesn't address the "mailing list adds a 
prefix" problem.

The z= tag is only supposed to be used for "diagnostic purposes", not 
for computing the hash.  Changing that would have major implications 
that we would have to examine very carefully.

eric