Re: [courier-users] Using courierfilter to modify outbound mail

[Sam V. <mr...@co...>]

MK via courier-users writes:

> However, doing a minimal DKIM hash of the From and body is dependent on  
> those fields
> not subsequently changing.  I have read in some posts that courier may do  
> some re-writing
> or re-structuring.  Is there a potential problem here, or am I good to go?

Yes, this is a very remote possibility but this can happen.

If Courier tries to send a message with non-7bit content to a mail server  
that does not claim 8BITMIME support Courier will re-encode the message into  
safe 7-bit quoted-printable encoding.

I always took a position that cryptographic signatures of encoded MIME  
content was a fundamental design flaw, right from their inception.  
Cryptographic hashes should be calculated on the content before it gets MIME- 
encoded, not after, so that the signatures remain valid if the mail content  
get re-encoded.

But, that's already a done deal, a lot of water under the bridge. Also, by  
now the chances of encountering a 7-bit dinosaur on the Internet are very  
rare. But, technically this can happen and everything will break.

If you have control over the mail then if you make sure that it does not  
have 8-bit content, or if it's already quoted-printable-encoded then you're  
out of the woods.

And it would've been real nice if DKIM used MIME and a multipart/signed  
content type, to indicate signed content. Courier is smart enough not to  
reencode something that's explicitly identified as signed content. But that  
was another missed opportunity, and more water under the bridge.