Create a DKIM-Signature for a MIME Message

See more DKIM / DomainKey Examples

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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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.

Chilkat.Dkim dkim = new Chilkat.Dkim();

//  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.
Chilkat.PrivateKey privKey = new Chilkat.PrivateKey();
success = privKey.LoadPemFile("qa_data/dkim_private.pem");
if (success == false) {
    Debug.WriteLine(privKey.LastErrorText);
    return;
}

success = dkim.SetDkimPrivateKey(privKey);
if (success == false) {
    Debug.WriteLine(dkim.LastErrorText);
    return;
}

//  Load the complete MIME message to be signed.  It must already contain all headers, transfer
//  encodings, MIME boundaries, and body bytes.
Chilkat.BinData mimeData = new Chilkat.BinData();
success = mimeData.LoadFile("qa_data/message.eml");
if (success == false) {
    Debug.WriteLine(mimeData.LastErrorText);
    return;
}

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
success = dkim.DkimSign(mimeData);
if (success == false) {
    Debug.WriteLine(dkim.LastErrorText);
    return;
}

//  Save the signed message.
success = mimeData.WriteFile("qa_output/signed.eml");
if (success == false) {
    Debug.WriteLine(mimeData.LastErrorText);
    return;
}

Debug.WriteLine("Message signed.");