Swift

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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Swift


func chilkatTest() {
    var success: Bool = 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.

    let dkim = CkoDkim()!

    //  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.
    let privKey = CkoPrivateKey()!
    success = privKey.loadPemFile(path: "qa_data/dkim_private.pem")
    if success == false {
        print("\(privKey.lastErrorText!)")
        return
    }

    success = dkim.setPrivateKey(privateKey: privKey)
    if success == false {
        print("\(dkim.lastErrorText!)")
        return
    }

    //  Load the complete MIME message to be signed.  It must already contain all headers, transfer
    //  encodings, MIME boundaries, and body bytes.
    let mimeData = CkoBinData()!
    success = mimeData.loadFile(path: "qa_data/message.eml")
    if success == false {
        print("\(mimeData.lastErrorText!)")
        return
    }

    //  Sign.  The DKIM-Signature header is prepended to the MIME in place.
    success = dkim.dkimSign(mimeData: mimeData)
    if success == false {
        print("\(dkim.lastErrorText!)")
        return
    }

    //  Save the signed message.
    success = mimeData.writeFile(path: "qa_output/signed.eml")
    if success == false {
        print("\(mimeData.lastErrorText!)")
        return
    }

    print("Message signed.")

}