Xbase++

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 nSuccess
LOCAL oDkim
LOCAL oPrivKey
LOCAL oMimeData

nSuccess := 0

//  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 := CreateObject("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 := CreateObject("Chilkat.PrivateKey")
nSuccess := oPrivKey:LoadPemFile("qa_data/dkim_private.pem")
IF (nSuccess == 0)
    ? oPrivKey:LastErrorText
    oDkim:destroy()
    oPrivKey:destroy()
    RETURN
ENDIF

nSuccess := oDkim:SetDkimPrivateKey(oPrivKey)
IF (nSuccess == 0)
    ? oDkim:LastErrorText
    oDkim:destroy()
    oPrivKey:destroy()
    RETURN
ENDIF

//  Load the complete MIME message to be signed.  It must already contain all headers, transfer
//  encodings, MIME boundaries, and body bytes.
oMimeData := CreateObject("Chilkat.BinData")
nSuccess := oMimeData:LoadFile("qa_data/message.eml")
IF (nSuccess == 0)
    ? oMimeData:LastErrorText
    oDkim:destroy()
    oPrivKey:destroy()
    oMimeData:destroy()
    RETURN
ENDIF

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
nSuccess := oDkim:DkimSign(oMimeData)
IF (nSuccess == 0)
    ? oDkim:LastErrorText
    oDkim:destroy()
    oPrivKey:destroy()
    oMimeData:destroy()
    RETURN
ENDIF

//  Save the signed message.
nSuccess := oMimeData:WriteFile("qa_output/signed.eml")
IF (nSuccess == 0)
    ? oMimeData:LastErrorText
    oDkim:destroy()
    oPrivKey:destroy()
    oMimeData:destroy()
    RETURN
ENDIF

? "Message signed."

oDkim:destroy()
oPrivKey:destroy()
oMimeData:destroy()