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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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Add-Type -Path "C:\chilkat\ChilkatDotNet47-x64\ChilkatDotNet47.dll"

$success = $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.

$dkim = New-Object Chilkat.Dkim

#  Configure the DKIM signature.
$dkim.DkimDomain = "example.com"
$dkim.DkimSelector = "myselector"

#  Signing algorithm written to the a= tag.
$dkim.DkimAlg = "rsa-sha256"

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

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

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

#  Load the RSA private key and give it to the Dkim object.
$privKey = New-Object Chilkat.PrivateKey
$success = $privKey.LoadPemFile("qa_data/dkim_private.pem")
if ($success -eq $false) {
    $($privKey.LastErrorText)
    exit
}

$success = $dkim.SetDkimPrivateKey($privKey)
if ($success -eq $false) {
    $($dkim.LastErrorText)
    exit
}

#  Load the complete MIME message to be signed.  It must already contain all headers, transfer
#  encodings, MIME boundaries, and body bytes.
$mimeData = New-Object Chilkat.BinData
$success = $mimeData.LoadFile("qa_data/message.eml")
if ($success -eq $false) {
    $($mimeData.LastErrorText)
    exit
}

#  Sign.  The DKIM-Signature header is prepended to the MIME in place.
$success = $dkim.DkimSign($mimeData)
if ($success -eq $false) {
    $($dkim.LastErrorText)
    exit
}

#  Save the signed message.
$success = $mimeData.WriteFile("qa_output/signed.eml")
if ($success -eq $false) {
    $($mimeData.LastErrorText)
    exit
}

$("Message signed.")