DKIM signing is broken

[Kirby]

Affected version

2.3.7

XenForo currently generates the DKIM signature in \XF\Mail\Mail::setFinalHeaders().

This was fine in XenForo 2.2, but in XenForo 2.3 the email is basically rebuild in \XF\Mail\Mailer::send():

$email = MessageConverter::toEmail($email);

This modifies the body as new MIME boundaries are generated which obviosuly invalidates the previously generated body hash.

Suggested Fix
Generate the DKIM signature immediately before actually sending the email, this ensures that DKIM signing is done as the last step before sending.
The attached patch implements this and seems to work fine.

 

[Kirby]

I've revised the patch, so here comes v2.

v1 basically just moved the signing code from \XF\Mail\Mail::setFinalHeaders() to \XF\Mail\Mailer::send() and left an unused use statement in XF\Mail\Mail.

While this seems to work fine, it's kinda inefficient:
The original code created a signer instance for every single mail.

v2 changes this by creating the signer just once in the constructor, according to my completely unscientific tests this seems to be significantly faster when sending a bunch of emails and shouldn't have a negative effect on single emails.