Visual FoxPro

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 lnSuccess
LOCAL loDkim
LOCAL loPrivKey
LOCAL loMimeData

lnSuccess = 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.

loDkim = CreateObject('Chilkat.Dkim')

*  Configure the DKIM signature.
loDkim.DkimDomain = "example.com"
loDkim.DkimSelector = "myselector"

*  Signing algorithm written to the a= tag.
loDkim.DkimAlg = "rsa-sha256"

*  Canonicalization for headers and body, written to the c= tag.
loDkim.DkimCanon = "relaxed/relaxed"

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

*  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
loDkim.DkimBodyLengthCount = 0

*  Load the RSA private key and give it to the Dkim object.
loPrivKey = CreateObject('Chilkat.PrivateKey')
lnSuccess = loPrivKey.LoadPemFile("qa_data/dkim_private.pem")
IF (lnSuccess = 0) THEN
    ? loPrivKey.LastErrorText
    RELEASE loDkim
    RELEASE loPrivKey
    CANCEL
ENDIF

lnSuccess = loDkim.SetDkimPrivateKey(loPrivKey)
IF (lnSuccess = 0) THEN
    ? loDkim.LastErrorText
    RELEASE loDkim
    RELEASE loPrivKey
    CANCEL
ENDIF

*  Load the complete MIME message to be signed.  It must already contain all headers, transfer
*  encodings, MIME boundaries, and body bytes.
loMimeData = CreateObject('Chilkat.BinData')
lnSuccess = loMimeData.LoadFile("qa_data/message.eml")
IF (lnSuccess = 0) THEN
    ? loMimeData.LastErrorText
    RELEASE loDkim
    RELEASE loPrivKey
    RELEASE loMimeData
    CANCEL
ENDIF

*  Sign.  The DKIM-Signature header is prepended to the MIME in place.
lnSuccess = loDkim.DkimSign(loMimeData)
IF (lnSuccess = 0) THEN
    ? loDkim.LastErrorText
    RELEASE loDkim
    RELEASE loPrivKey
    RELEASE loMimeData
    CANCEL
ENDIF

*  Save the signed message.
lnSuccess = loMimeData.WriteFile("qa_output/signed.eml")
IF (lnSuccess = 0) THEN
    ? loMimeData.LastErrorText
    RELEASE loDkim
    RELEASE loPrivKey
    RELEASE loMimeData
    CANCEL
ENDIF

? "Message signed."

RELEASE loDkim
RELEASE loPrivKey
RELEASE loMimeData