Java

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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import com.chilkatsoft.chilkat2.BinData;
import com.chilkatsoft.chilkat2.ChilkatException;
import com.chilkatsoft.chilkat2.Dkim;
import com.chilkatsoft.chilkat2.PrivateKey;

public class ChilkatExample {

    public static void main(String[] args) {
        // 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.

        Dkim dkim = new Dkim();

        // Configure the DKIM signature.
        dkim.setDkimDomain("example.com");
        dkim.setDkimSelector("myselector");

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

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

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

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

        // Load the RSA private key and give it to the Dkim object.
        PrivateKey privKey = new PrivateKey();
        try {
            privKey.loadPemFile("qa_data/dkim_private.pem");

            dkim.setDkimPrivateKey(privKey);
        } catch (ChilkatException e) {
            System.out.println(e.getLastErrorText());
            return;
        }

        // Load the complete MIME message to be signed.  It must already contain all headers, transfer
        // encodings, MIME boundaries, and body bytes.
        BinData mimeData = new BinData();
        try {
            mimeData.loadFile("qa_data/message.eml");

            // Sign.  The DKIM-Signature header is prepended to the MIME in place.
            dkim.dkimSign(mimeData);

            // Save the signed message.
            mimeData.writeFile("qa_output/signed.eml");
        } catch (ChilkatException e) {
            System.out.println(e.getLastErrorText());
            return;
        }

        System.out.println("Message signed.");
    }
}