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?