Lianja

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.

Chilkat Lianja Downloads

Download Chilkat for Lianja

Lianja

llSuccess = .F.

//  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("CkDkim")

//  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("CkPrivateKey")
llSuccess = loPrivKey.LoadPemFile("qa_data/dkim_private.pem")
if (llSuccess = .F.) then
    ? loPrivKey.LastErrorText
    release loDkim
    release loPrivKey
    return
endif

llSuccess = loDkim.SetDkimPrivateKey(loPrivKey)
if (llSuccess = .F.) then
    ? loDkim.LastErrorText
    release loDkim
    release loPrivKey
    return
endif

//  Load the complete MIME message to be signed.  It must already contain all headers, transfer
//  encodings, MIME boundaries, and body bytes.
loMimeData = createobject("CkBinData")
llSuccess = loMimeData.LoadFile("qa_data/message.eml")
if (llSuccess = .F.) then
    ? loMimeData.LastErrorText
    release loDkim
    release loPrivKey
    release loMimeData
    return
endif

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
llSuccess = loDkim.DkimSign(loMimeData)
if (llSuccess = .F.) then
    ? loDkim.LastErrorText
    release loDkim
    release loPrivKey
    release loMimeData
    return
endif

//  Save the signed message.
llSuccess = loMimeData.WriteFile("qa_output/signed.eml")
if (llSuccess = .F.) then
    ? loMimeData.LastErrorText
    release loDkim
    release loPrivKey
    release loMimeData
    return
endif

? "Message signed."


release loDkim
release loPrivKey
release loMimeData