Delphi DLL

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: Boolean;
dkim: HCkDkim;
privKey: HCkPrivateKey;
mimeData: HCkBinData;

begin
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.

dkim := CkDkim_Create();

//  Configure the DKIM signature.
CkDkim_putDkimDomain(dkim,'example.com');
CkDkim_putDkimSelector(dkim,'myselector');

//  Signing algorithm written to the a= tag.
CkDkim_putDkimAlg(dkim,'rsa-sha256');

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

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

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

//  Load the RSA private key and give it to the Dkim object.
privKey := CkPrivateKey_Create();
success := CkPrivateKey_LoadPemFile(privKey,'qa_data/dkim_private.pem');
if (success = False) then
  begin
    Memo1.Lines.Add(CkPrivateKey__lastErrorText(privKey));
    Exit;
  end;
success := CkDkim_SetDkimPrivateKey(dkim,privKey);
if (success = False) then
  begin
    Memo1.Lines.Add(CkDkim__lastErrorText(dkim));
    Exit;
  end;

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

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
success := CkDkim_DkimSign(dkim,mimeData);
if (success = False) then
  begin
    Memo1.Lines.Add(CkDkim__lastErrorText(dkim));
    Exit;
  end;

//  Save the signed message.
success := CkBinData_WriteFile(mimeData,'qa_output/signed.eml');
if (success = False) then
  begin
    Memo1.Lines.Add(CkBinData__lastErrorText(mimeData));
    Exit;
  end;
Memo1.Lines.Add('Message signed.');

CkDkim_Dispose(dkim);
CkPrivateKey_Dispose(privKey);
CkBinData_Dispose(mimeData);