diff --git a/includes/class-http.php b/includes/class-http.php
index 58551a4..240e5ea 100644
--- a/includes/class-http.php
+++ b/includes/class-http.php
@@ -2,7 +2,7 @@
 namespace Activitypub;
 
 use WP_Error;
-use Activitypub\Model\User;
+use Activitypub\User_Factory;
 
 /**
  * ActivityPub HTTP Class
@@ -63,7 +63,7 @@ class Http {
 	 */
 	public static function get( $url ) {
 		$date = \gmdate( 'D, d M Y H:i:s T' );
-		$signature = Signature::generate_signature( User::APPLICATION_USER_ID, 'get', $url, $date );
+		$signature = Signature::generate_signature( User_Factory::APPLICATION_USER_ID, 'get', $url, $date );
 
 		$wp_version = \get_bloginfo( 'version' );
 		$user_agent = \apply_filters( 'http_headers_useragent', 'WordPress/' . $wp_version . '; ' . \get_bloginfo( 'url' ) );
diff --git a/includes/class-signature.php b/includes/class-signature.php
index a91ea0e..c1ccbdf 100644
--- a/includes/class-signature.php
+++ b/includes/class-signature.php
@@ -136,11 +136,8 @@ class Signature {
 		\openssl_sign( $signed_string, $signature, $key, \OPENSSL_ALGO_SHA256 );
 		$signature = \base64_encode( $signature ); // phpcs:ignore
 
-		if ( User::APPLICATION_USER_ID === $user_id ) {
-			$key_id = \get_rest_url( null, 'activitypub/1.0/application#main-key' );
-		} else {
-			$key_id = \get_author_posts_url( $user_id ) . '#main-key';
-		}
+		$user   = User_Factory::get_by_id( $user_id );
+		$key_id = $user->get_url() . '#main-key';
 
 		if ( ! empty( $digest ) ) {
 			return \sprintf( 'keyId="%s",algorithm="rsa-sha256",headers="(request-target) host date digest",signature="%s"', $key_id, $signature );