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I recently noticed that opendkim on my mail server is objecting to DKIM signatures from a client, saying their key is insecure. It may be that that's due to lack of secure DNS (confirmation?) but I also noticed that the signing algorithm is shown as a=rsa-sha1, and with the move to Deprecate SHA-1 in other contexts, I'm wondering how urgent this is for DKIM?

It is now feasible to brute-force SHA-1 hashes. Am I correct in thinking that this would need to be done for each message an attacker wished to forge the signature for, without being able to re-use that work for the next message?

  • There is an answer in mc0e's crosspost on crypto.stackexchange.com/questions/19278/… – Mitja Jun 20 '15 at 11:10
  • @Mitja It's interesting that that identical cross-post doesn't get picked up and displayed in the 'related' posts down the side of this page (at least not for me). I wonder how that algorithm works. – mc0e Jun 21 '15 at 19:08
  • The problem here is that 'Related' only shows questions from serverfault. Sadly, there is no 'Related Network Questions' category on the side at this time. – Mitja Jun 22 '15 at 9:22

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