php-dkim

Finally, a PHP5 class for not just signing, but verifying DKIM signatures.

Requirements

PHP 5.3 or greater (namespaces), with the openssl and hash extensions. CI covers PHP 7.4 through 8.4.

A legacy code path will use phpseclib 1.x if its global Crypt_RSA/Crypt_Hash classes happen to be loaded. That path is untested and not recommended; phpseclib 2.x and later are namespaced and never trigger it.

Usage

Verifying

$verifier = new angrychimp\DKIM\Verify($rawMessage);
foreach ($verifier->validate() as $num => $results) {
    foreach ($results as $result) {
        echo $result['status'], ': ', $result['reason'], "\n";   // pass | permfail
    }
}

Each DKIM-Signature header in the message gets its own result set. The public key is fetched from DNS; a message is only verified if both the body hash and the header hash pass.

Signing

$signer = new angrychimp\DKIM\Sign($rawMessage, array(
    'domain'      => 'example.com',
    'selector'    => 'mail',
    'private_key' => file_get_contents('/path/to/private.pem'),  // or 'file:///path/to/private.pem'
));
echo $signer->sign();                   // the message with DKIM-Signature prepended
echo $signer->getSignatureHeader();     // just the header

Optional parameters:

Parameter	Default	Notes
passphrase	none	for an encrypted private key
hash	sha256	sha1 or sha256
canonicalization	relaxed/relaxed	simple survives transit far less often
headers_to_sign	a recommended set	array of header names; From is mandatory

Signatures produced here are checked against dkimpy in CI, in both directions, so they interoperate rather than merely round-tripping.

Compatibility

The pre-0.4.0 global class names (DKIM_Sign, DKIM_Verify, DKIM_Exception) still work as aliases when the package is loaded through Composer. They will be removed in a future major release.

Changelog

v0.4.0

Breaking release. Classes are namespaced and the signing half of the library finally exists.

• Breaking: moved to PSR-4 under angrychimp\DKIM\. DKIM_Sign → Sign, DKIM_Verify → Verify, DKIM_Exception → Exception. The old global names remain as aliases via Composer and will be dropped in a future major release.
• Signing implemented, open since 2012. Supports relaxed/simple canonicalization, sha1/sha256, configurable signed-header sets, and RFC 6376 5.4.2 bottom-to-top ordering for repeated headers.
• Security: openssl_verify() returns -1 on internal error, which is truthy — an errored verification was being reported as a pass. It now requires an explicit success.
• Security: a= was never validated, so a signature naming any algorithm (e.g. ed25519-sha256) was still verified as RSA. Restricted to rsa-sha1/rsa-sha256, and checked before the DNS lookup so a bogus a= costs no network traffic.
• Fixed header lookup matching on a bare prefix: Message-ID also matched Message-ID-Hash (which Mailman 3 adds), silently producing signatures no other implementation accepts.
• Fixed relaxed canonicalization of an empty body, which emitted CRLF instead of a null input. This affected verification too, rejecting valid signatures on empty-bodied mail.
• Fixed h= handling for a header name listed more than once; it was deduplicated, so messages with repeated headers could never verify.
• Fixed signature expiration: x= was never enforced, and reading t= unguarded errored when x= appeared alone.
• c= now defaults to simple/simple and tolerates a lone algorithm, per RFC 6376 3.5.
• Hardened parsing against malformed input: signature tags and key records with no = or a trailing ;, q= with no /, headers with no colon, failed DNS lookups, and messages with no blank line (whose body was previously read from the middle of a header).
• Added test suites covering signing, verification, and tag handling, plus GitHub Actions running them on PHP 7.4–8.4 and cross-checking signatures against dkimpy in both directions.

v0.2.1 11:28 AM 3/3/2016

• Fixed index variable issue (#7)
• Addressed validation issue when public key record did not have public-key data (#7)
• Minor version numbering corrections
• Dropped old copyright info for as-yet-still-empty Sign code
• Fixed new-line trimming issue (potentially causing verification problems?) (#7)

v0.2 5:36 PM 1/2/2013

• Splitting TODOs into separate file.
• Finally got the header hash to match my expected value, based on debugging output from Mail::DKIM::Validate.
• Removed var_dump() calls
• Still doesn't verify signatures properly - not sure where to go from here.

v0.1 10:55 AM 12/31/2012 Initial commit. Most of the structure is in place, and the body hashes are validating, but I haven't been able to get the signature validation correct just yet. I must have some whitespace issue or some random public key problem.