diff --git a/activitypub.php b/activitypub.php
index 049d7d7..4292684 100644
--- a/activitypub.php
+++ b/activitypub.php
@@ -41,6 +41,7 @@ function init() {
 
 	require_once \dirname( __FILE__ ) . '/includes/model/class-activity.php';
 	require_once \dirname( __FILE__ ) . '/includes/model/class-post.php';
+	require_once \dirname( __FILE__ ) . '/includes/model/class-user.php';
 	require_once \dirname( __FILE__ ) . '/includes/model/class-follower.php';
 
 	require_once \dirname( __FILE__ ) . '/includes/class-migration.php';
diff --git a/includes/class-http.php b/includes/class-http.php
index 247d87e..58cace9 100644
--- a/includes/class-http.php
+++ b/includes/class-http.php
@@ -2,6 +2,7 @@
 namespace Activitypub;
 
 use WP_Error;
+use Activitypub\Model\User;
 
 /**
  * ActivityPub HTTP Class
@@ -62,7 +63,7 @@ class Http {
 	 */
 	public static function get( $url ) {
 		$date = \gmdate( 'D, d M Y H:i:s T' );
-		$signature = Signature::generate_signature( -1, 'get', $url, $date );
+		$signature = Signature::generate_signature( User::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 f82806a..9472128 100644
--- a/includes/class-signature.php
+++ b/includes/class-signature.php
@@ -4,6 +4,7 @@ namespace Activitypub;
 use WP_Error;
 use DateTime;
 use DateTimeZone;
+use Activitypub\Model\User;
 
 /**
  * ActivityPub Signature Class
@@ -26,7 +27,7 @@ class Signature {
 			self::generate_key_pair( $user_id );
 		}
 
-		if ( -1 === $user_id ) {
+		if ( User::APPLICATION_USER_ID === $user_id ) {
 			$key = \get_option( 'activitypub_magic_sig_public_key' );
 		} else {
 			$key = \get_user_meta( $user_id, 'magic_sig_public_key', true );
@@ -52,7 +53,7 @@ class Signature {
 			self::generate_key_pair( $user_id );
 		}
 
-		if ( -1 === $user_id ) {
+		if ( User::APPLICATION_USER_ID === $user_id ) {
 			$key = \get_option( 'activitypub_magic_sig_private_key' );
 		} else {
 			$key = \get_user_meta( $user_id, 'magic_sig_private_key', true );
@@ -85,7 +86,7 @@ class Signature {
 		\openssl_pkey_export( $key, $priv_key );
 		$detail = \openssl_pkey_get_details( $key );
 
-		if ( -1 === $user_id ) {
+		if ( User::APPLICATION_USER_ID === $user_id ) {
 			// private key
 			\update_option( 'activitypub_magic_sig_private_key', $priv_key );
 
@@ -140,7 +141,7 @@ class Signature {
 		\openssl_sign( $signed_string, $signature, $key, \OPENSSL_ALGO_SHA256 );
 		$signature = \base64_encode( $signature ); // phpcs:ignore
 
-		if ( -1 === $user_id ) {
+		if ( User::APPLICATION_USER_ID === $user_id ) {
 			$key_id = \get_rest_url( null, 'activitypub/1.0/service#main-key' );
 		} else {
 			$key_id = \get_author_posts_url( $user_id ) . '#main-key';
diff --git a/includes/model/class-user.php b/includes/model/class-user.php
new file mode 100644
index 0000000..5c62473
--- /dev/null
+++ b/includes/model/class-user.php
@@ -0,0 +1,23 @@
+<?php
+namespace Activitypub\Model;
+
+/**
+ * ActivityPub User Class
+ *
+ * @author Matthias Pfefferle
+ */
+class User {
+	/**
+	 * The ID of the Blog User
+	 *
+	 * @var int
+	 */
+	const BLOG_USER_ID = 0;
+
+	/**
+	 * The ID of the Application User
+	 *
+	 * @var int
+	 */
+	const APPLICATION_USER_ID = -1;
+}
diff --git a/includes/rest/class-server.php b/includes/rest/class-server.php
index 7e8f720..a2d837a 100644
--- a/includes/rest/class-server.php
+++ b/includes/rest/class-server.php
@@ -3,6 +3,7 @@ namespace Activitypub\Rest;
 
 use WP_REST_Response;
 use Activitypub\Signature;
+use Activitypub\Model\User;
 
 /**
  * ActivityPub Server REST-Class
@@ -57,7 +58,7 @@ class Server {
 		$json->publicKey = (object) array( // phpcs:ignore WordPress.NamingConventions
 			'id' => \get_rest_url( null, 'activitypub/1.0/application#main-key' ),
 			'owner' => \get_rest_url( null, 'activitypub/1.0/application' ),
-			'publicKeyPem' => Signature::get_public_key( -1 ), // phpcs:ignore WordPress.NamingConventions
+			'publicKeyPem' => Signature::get_public_key( User::APPLICATION_USER_ID ), // phpcs:ignore WordPress.NamingConventions
 		);
 
 		$response = new WP_REST_Response( $json, 200 );