Dart

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 Dart Downloads

Download Chilkat for Dart

Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // 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.

  final dkim = CkDkim();

  // 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.
  final privKey = CkPrivateKey();
  try {
    privKey.loadPemFile('qa_data/dkim_private.pem');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  try {
    dkim.setDkimPrivateKey(privKey);
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Load the complete MIME message to be signed.  It must already contain all headers, transfer
  // encodings, MIME boundaries, and body bytes.
  final mimeData = CkBinData();
  try {
    mimeData.loadFile('qa_data/message.eml');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Sign.  The DKIM-Signature header is prepended to the MIME in place.
  try {
    dkim.dkimSign(mimeData);
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Save the signed message.
  try {
    mimeData.writeFile('qa_output/signed.eml');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  print('Message signed.');
}