C++

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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C++

#include <CkDkim.h>
#include <CkPrivateKey.h>
#include <CkBinData.h>

void ChilkatSample(void)
    {
    bool 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.

    CkDkim dkim;

    //  Configure the DKIM signature.
    dkim.put_DkimDomain("example.com");
    dkim.put_DkimSelector("myselector");

    //  Signing algorithm written to the a= tag.
    dkim.put_DkimAlg("rsa-sha256");

    //  Canonicalization for headers and body, written to the c= tag.
    dkim.put_DkimCanon("relaxed/relaxed");

    //  Colon-separated list of header fields to sign, written to the h= tag.
    dkim.put_DkimHeaders("From:To:Subject:Date:Message-ID");

    //  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
    dkim.put_DkimBodyLengthCount(0);

    //  Load the RSA private key and give it to the Dkim object.
    CkPrivateKey privKey;
    success = privKey.LoadPemFile("qa_data/dkim_private.pem");
    if (success == false) {
        std::cout << privKey.lastErrorText() << "\r\n";
        return;
    }

    success = dkim.SetDkimPrivateKey(privKey);
    if (success == false) {
        std::cout << dkim.lastErrorText() << "\r\n";
        return;
    }

    //  Load the complete MIME message to be signed.  It must already contain all headers, transfer
    //  encodings, MIME boundaries, and body bytes.
    CkBinData mimeData;
    success = mimeData.LoadFile("qa_data/message.eml");
    if (success == false) {
        std::cout << mimeData.lastErrorText() << "\r\n";
        return;
    }

    //  Sign.  The DKIM-Signature header is prepended to the MIME in place.
    success = dkim.DkimSign(mimeData);
    if (success == false) {
        std::cout << dkim.lastErrorText() << "\r\n";
        return;
    }

    //  Save the signed message.
    success = mimeData.WriteFile("qa_output/signed.eml");
    if (success == false) {
        std::cout << mimeData.lastErrorText() << "\r\n";
        return;
    }

    std::cout << "Message signed." << "\r\n";
    }