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Prefetch a DKIM Public Key from DNS

Demonstrates the Chilkat Dkim.PrefetchPublicKey method, which retrieves a DKIM public key from DNS and caches it for a specific selector and domain. The arguments are the selector and the domain. The TXT lookup is made at selector._domainkey.domain.

Background: DkimVerify already fetches the key from DNS on demand, so prefetching is about when the lookup happens: doing it up front lets you separate a potentially slow network step from the verification itself, warm the cache before a batch of messages from the same domain, or confirm the key is publishable before relying on it. After a successful prefetch, verification of a matching signature uses the cached key with no further DNS traffic.

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Local $bSuccess = False

;  Demonstrates the Dkim.PrefetchPublicKey method, which retrieves a DKIM public key from DNS and
;  caches it for a specific selector and domain.  The 1st argument is the selector and the 2nd is
;  the domain.

$oDkim = ObjCreate("Chilkat.Dkim")

;  Retrieve the public key from the DNS TXT record at selector._domainkey.domain and cache it.
;  A later DkimVerify for the same selector and domain then uses the cached key.
$bSuccess = $oDkim.PrefetchPublicKey("myselector","example.com")
If ($bSuccess = False) Then
    ConsoleWrite($oDkim.LastErrorText & @CRLF)
    Exit
EndIf

ConsoleWrite("Public key prefetched from DNS and cached." & @CRLF)