COSE_Mac

The COSE_Mac structure carries the MAC key to its recipients the way COSE_Encrypt carries a content encryption key: one COSE_recipient per recipient, filled by a key management algorithm.

use CBOR\ListObject;
use CBOR\Tag\CoseMacTag;
use Cose\Mac\MacStructure;
use Cose\Structure\CoseRecipient;

// Context "MAC": the same header and payload give a different tag than under "MAC0"
$toBeMaced = MacStructure::create($protectedHeader, $payload);
$tag = ByteStringObject::create($algorithm->hash((string) $toBeMaced, $macKey));

$recipients = ListObject::create([
    ListObject::create([/* recipient 1: protected, unprotected, wrapped MAC key */]),
]);

$coseMac = CoseMacTag::create(ListObject::create([
    $protectedHeader,
    $unprotectedHeader,
    $payload,
    $tag,
    $recipients,
]));

// Reading the recipients back as checked views rather than raw lists
foreach (CoseRecipient::all($coseMac->getRecipients()) as $recipient) {
    $kid = $recipient->getUnprotectedHeaderParameter(4);
}

The Enc_structure a recipient of a COSE_Mac is computed over is RecipientStructure::forMacRecipient() ("Mac_Recipient"); RecipientLayer::fromRecipient() takes the MAC algorithm as the algorithm of the layer below, so that the COSE_KDF_Context binds the derived key to it. RFC 9052 §6.1 writes the list as [+COSE_recipient]: at least one entry, which CoseRecipient::all() enforces, nested levels included.