PHP Extension

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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PHP Extension

<?php

include("chilkat.php");

$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 = new CkDkim();

//  Configure the DKIM signature.
$dkim->put_DkimDomain('example.com');
$dkim->put_DkimSelector('myselector');

//  Signing algorithm written to the a= tag.
$dkim->put_DkimAlg('rsa-sha256');

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

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

//  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
$dkim->put_DkimBodyLengthCount(0);

//  Load the RSA private key and give it to the Dkim object.
$privKey = new CkPrivateKey();
$success = $privKey->LoadPemFile('qa_data/dkim_private.pem');
if ($success == false) {
    print $privKey->lastErrorText() . "\n";
    exit;
}

$success = $dkim->SetDkimPrivateKey($privKey);
if ($success == false) {
    print $dkim->lastErrorText() . "\n";
    exit;
}

//  Load the complete MIME message to be signed.  It must already contain all headers, transfer
//  encodings, MIME boundaries, and body bytes.
$mimeData = new CkBinData();
$success = $mimeData->LoadFile('qa_data/message.eml');
if ($success == false) {
    print $mimeData->lastErrorText() . "\n";
    exit;
}

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
$success = $dkim->DkimSign($mimeData);
if ($success == false) {
    print $dkim->lastErrorText() . "\n";
    exit;
}

//  Save the signed message.
$success = $mimeData->WriteFile('qa_output/signed.eml');
if ($success == false) {
    print $mimeData->lastErrorText() . "\n";
    exit;
}

print 'Message signed.' . "\n";

?>