#202 Signature caching
It would be nice if there was an option to have OpenDKIM cache signatures it applies such that if after canonicalization it has recently signed the same message it will apply the same signature from cache instead of recomputing it. This in RAM cache does not need to be very big, perhaps expiring a couple seconds after a cache entry was last used or maybe just storing the last few entries. This way while a message is being sent to a list of people OpenDKIM only has to compute the signature for the first one (assuming the To header is not being signed) and can then apply the same signature from cache to the rest.
I finally figured out why my mailing list was so slow. OpenDKIM is the bottleneck, but doesn't show up as such in top due to hardware cryptographic acceleration.
