Delphi ActiveX

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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var
success: Integer;
dkim: TChilkatDkim;
privKey: TPrivateKey;
mimeData: TChilkatBinData;

begin
success := 0;

//  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 := TChilkatDkim.Create(Self);

//  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.
privKey := TPrivateKey.Create(Self);
success := privKey.LoadPemFile('qa_data/dkim_private.pem');
if (success = 0) then
  begin
    Memo1.Lines.Add(privKey.LastErrorText);
    Exit;
  end;
success := dkim.SetDkimPrivateKey(privKey.ControlInterface);
if (success = 0) then
  begin
    Memo1.Lines.Add(dkim.LastErrorText);
    Exit;
  end;

//  Load the complete MIME message to be signed.  It must already contain all headers, transfer
//  encodings, MIME boundaries, and body bytes.
mimeData := TChilkatBinData.Create(Self);
success := mimeData.LoadFile('qa_data/message.eml');
if (success = 0) then
  begin
    Memo1.Lines.Add(mimeData.LastErrorText);
    Exit;
  end;

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
success := dkim.DkimSign(mimeData.ControlInterface);
if (success = 0) then
  begin
    Memo1.Lines.Add(dkim.LastErrorText);
    Exit;
  end;

//  Save the signed message.
success := mimeData.WriteFile('qa_output/signed.eml');
if (success = 0) then
  begin
    Memo1.Lines.Add(mimeData.LastErrorText);
    Exit;
  end;
Memo1.Lines.Add('Message signed.');