[teddyh]

There’s a minor technical error in the description of DKIM “happy path”. Here’s what step 4-6 should be:

> 3. The Fastmail SMTP server generates a signature using the secret key, and attaches it to the email, along with a ‘selector’ (a string specifying which which of many possible secret keys key it used), then sends the email to example.com's receiving server.

> 4. Google email server receives this email. It's from sadl.io and has a fastmail DKIM ‘selector”, so it gets the DNS records for that selector in that domain.

> 5. The email has a signature and selector embedded, and the DNS records for the selector in the sadl.io domain declares a public DKIM keys which can be used to verify that signature.

> 6. That DKIM keys matches the one used to make this signature. So the DKIM test passes.

Crucially, there is no way to list all existing DKIM keys for a domain without knowing all the selector strings. Of course you could do a brute-force search and find a lot of them, but you could never know you found them all without doing a full zone transfer (or if DNSSEC is used with the old NSEC records, which allows enumeration).