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Create a DKIM-Signature for a MIME Message

Demonstrates the Chilkat Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to a complete MIME message held in a BinData, modifying it in place. The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount properties configure the generated signature.

Note: The file paths are relative to the application's current working directory. Absolute paths may also be used. Supply the paths appropriate to your own environment.

Background: The signature is computed over a hash of the body plus a chosen set of headers, so the message must be completely built — every header, encoding, and boundary — before signing; any later change invalidates it. The d= (domain) and s= (selector) tags tell a verifier where to find the public key: at selector._domainkey.domain in DNS. relaxed/relaxed canonicalization tolerates the minor whitespace changes mail systems make in transit, which is why it is the common choice.

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Local $bSuccess = False

;  Demonstrates the Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to
;  a complete MIME message.  The only argument is a BinData holding the MIME, which is modified in
;  place.
;  
;  The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount
;  properties configure the signature that is generated.

$oDkim = ObjCreate("Chilkat.Dkim")

;  Configure the DKIM signature.
$oDkim.DkimDomain = "example.com"
$oDkim.DkimSelector = "myselector"

;  Signing algorithm written to the a= tag.
$oDkim.DkimAlg = "rsa-sha256"

;  Canonicalization for headers and body, written to the c= tag.
$oDkim.DkimCanon = "relaxed/relaxed"

;  Colon-separated list of header fields to sign, written to the h= tag.
$oDkim.DkimHeaders = "From:To:Subject:Date:Message-ID"

;  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
$oDkim.DkimBodyLengthCount = 0

;  Load the RSA private key and give it to the Dkim object.
$oPrivKey = ObjCreate("Chilkat.PrivateKey")
$bSuccess = $oPrivKey.LoadPemFile("qa_data/dkim_private.pem")
If ($bSuccess = False) Then
    ConsoleWrite($oPrivKey.LastErrorText & @CRLF)
    Exit
EndIf

$bSuccess = $oDkim.SetDkimPrivateKey($oPrivKey)
If ($bSuccess = False) Then
    ConsoleWrite($oDkim.LastErrorText & @CRLF)
    Exit
EndIf

;  Load the complete MIME message to be signed.  It must already contain all headers, transfer
;  encodings, MIME boundaries, and body bytes.
$oMimeData = ObjCreate("Chilkat.BinData")
$bSuccess = $oMimeData.LoadFile("qa_data/message.eml")
If ($bSuccess = False) Then
    ConsoleWrite($oMimeData.LastErrorText & @CRLF)
    Exit
EndIf

;  Sign.  The DKIM-Signature header is prepended to the MIME in place.
$bSuccess = $oDkim.DkimSign($oMimeData)
If ($bSuccess = False) Then
    ConsoleWrite($oDkim.LastErrorText & @CRLF)
    Exit
EndIf

;  Save the signed message.
$bSuccess = $oMimeData.WriteFile("qa_output/signed.eml")
If ($bSuccess = False) Then
    ConsoleWrite($oMimeData.LastErrorText & @CRLF)
    Exit
EndIf

ConsoleWrite("Message signed." & @CRLF)