On 5/11/09 5:09 PM, Steve Atkins wrote: > On May 11, 2009, at 3:55 AM, Charles Lindsey wrote: > > >> On Sat, 09 May 2009 21:08:33 +0100, Steve Atkins<[email protected] >> >>> >> wrote: >> >> >>> l: Body length count >>> >>> This opens up a whole host of security issues, related to being able >>> to change the rendered content of the message entirely after signing >>> without breaking the signature. Removing it would remove a security >>> hole you can drive a bus through. Is it being used? Are there any >>> situations where it has proved useful? >>> >> Signing the body is not essential for the primary purpose of DKIM, >> which >> is to expose phishers and the like. Malicious modification of a >> message >> _after_ is has been posted is relatively rare. >> > If a signer uses l=0 (or, given MIME games that can > be played, any other l= value) then the only thing you can say > about any validly signed message from that sender is that > the subject line of the message is the same as the subject line > of a message that sender signed. I don't think that's a useful > level of protection for any use case. > > It means that I can, for example, take one copy of a service notice > from my bank, leave the headers the same and replace the URLs > in the body of the message to links to my website, then send it > out to a hundred thousand people - and it would be validly signed > by the bank. (The only user-visible content I wouldn't be able to > change is the subject line). > > If there is ever any user-visible sign in an MUA that an email was > DKIM signed, or benefits to delivery for email being DKIM signed by > a trusted signer, *and* some trusted signer actually uses l=0, I'll bet > you'll start to see a lot more malicious modification and retransmission > of messages. > > >> So writing l=0 gives a way >> to sign the headers only (saving quite a bit of overhead if that is >> useful, plus removing all problems arising from changes of encoding >> and >> other mungings during transit. >> > > > I could conceivably see this being an issue on an acm.org style > forwarder, > if there were one that modified the content it forwarded, but not a > normal > mailing list. I don't think that's a iikely use case, though. >
Sorry- I'm not quite sure what you're saying here. That an ACM.ORG-like forwarding address is likely or that they would tag something to the bottom of a message? Eliot _______________________________________________ NOTE WELL: This list operates according to http://mipassoc.org/dkim/ietf-list-rules.html
