--On 16 September 2010 03:37:53 -0400 Hector Santos <[email protected]> wrote:
> Alessandro Vesely wrote: >>> I see these generic possibilities: >>> >>> First party Example: >>> >>> From: [email protected] >>> DKIM-Required: example.com >>> DKIM-Signature: d=example.com h="From:DKIM-Required" >> >> Yes, that's possible, but seems useless. What I've been thinking about >> is >> >> From: [email protected] >> DKIM-Required: list.example >> DKIM-Signature: d=example.com h="From:DKIM-Required" >> >> If it were agreed that a signature may be not valid unless >> countersigned by the recipient, the risk of replay attacks would be >> greatly diminished. > > So in the above, the originating message author is telling the world > that the domain list.example is expected to resign? so it ends up > like this? > > From: [email protected] > DKIM-Required: list.example > X-DKIM-Signature: d=example.com h="From:DKIM-Required" > DKIM-Signature: d=list.example.com h="From:DKIM-Required" > > X-DKIM-Signature means that it was stripped and/or nullified in the > in distribution. Illustrated above to show there was a change. > > Am I reading your proposal correct here? I don't think so. The original signature should only sign the DKIM-required and From headers, and perhaps enough other headers to reduce utility of replay attacks. Importantly, they should only sign parts that are likely to be unbroken by the MLM, thus satisfying ADSP requirements. However, the recipient knows that a valid signature from the MLM is required, too. Thus, the original DKIM signature is only valid for messages going through the list - off list replay isn't possible. On-list replay can be limited by ALSO including a full DKIM signature, for the list to check before redistributing. -- Ian Eiloart IT Services, University of Sussex 01273-873148 x3148 For new support requests, see http://www.sussex.ac.uk/its/help/ _______________________________________________ NOTE WELL: This list operates according to http://mipassoc.org/dkim/ietf-list-rules.html
