<?php
/**
 * Signed-request scheme shared with the CommercePing backend.
 *
 * signature = hex( HMAC-SHA256( secret,
 *   "v1\n<direction>\n<METHOD>\n<path>\n<unix ts>\n<nonce>\n<sha256hex(body)>" ) )
 *
 * Directions: "site-to-cp" (this plugin to the backend) and "cp-to-site"
 * (backend to this plugin). The path is the logical path, not the site URL.
 *
 * @package OrderPing
 */

defined( 'ABSPATH' ) || exit;

const ORDERPING_MAX_SKEW_SECONDS = 300;
const ORDERPING_NONCE_TTL        = 600;

/**
 * Computes the request signature.
 *
 * @param string $secret    Per-store secret.
 * @param string $direction "site-to-cp" or "cp-to-site".
 * @param string $method    HTTP method.
 * @param string $path      Logical request path.
 * @param int    $timestamp Unix seconds.
 * @param string $nonce     Unique per request.
 * @param string $body      Raw request body.
 * @return string Lowercase hex HMAC.
 */
function orderping_sign( $secret, $direction, $method, $path, $timestamp, $nonce, $body ) {
	$canonical = implode(
		"\n",
		array(
			'v1',
			$direction,
			strtoupper( $method ),
			$path,
			(string) $timestamp,
			$nonce,
			hash( 'sha256', $body ),
		)
	);
	return hash_hmac( 'sha256', $canonical, $secret );
}

/**
 * Verifies a request from the backend. Returns true, or a short failure code.
 *
 * @param string $secret    Per-store secret.
 * @param string $direction Direction the request must have been signed for.
 * @param string $method    HTTP method.
 * @param string $path      Logical request path.
 * @param string $timestamp Timestamp header value.
 * @param string $nonce     Nonce header value.
 * @param string $signature Signature header value.
 * @param string $body      Raw request body.
 * @param int    $now       Current Unix time.
 * @return true|string
 */
function orderping_verify( $secret, $direction, $method, $path, $timestamp, $nonce, $signature, $body, $now ) {
	if ( ! preg_match( '/^\d{1,12}$/', $timestamp ) || ! preg_match( '/^[A-Za-z0-9_-]{16,64}$/', $nonce ) || ! preg_match( '/^[0-9a-fA-F]{64}$/', $signature ) ) {
		return 'malformed';
	}
	if ( abs( $now - (int) $timestamp ) > ORDERPING_MAX_SKEW_SECONDS ) {
		return 'stale_timestamp';
	}
	$expected = orderping_sign( $secret, $direction, $method, $path, (int) $timestamp, $nonce, $body );
	if ( ! hash_equals( $expected, strtolower( $signature ) ) ) {
		return 'bad_signature';
	}
	return true;
}