diff --git a/lib/Db/AclMapper.php b/lib/Db/AclMapper.php
index e0ed3c838..732fb54f8 100644
--- a/lib/Db/AclMapper.php
+++ b/lib/Db/AclMapper.php
@@ -35,19 +35,19 @@ class AclMapper extends DeckMapper implements IPermissionMapper {
 		return $this->findEntities($sql, [$boardId], $limit, $offset);
 	}
 
-	public function isOwner($userId, $aclId) {
+	public function isOwner($userId, $aclId): bool {
 		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_board_acl` WHERE id = ?)';
 		$stmt = $this->execute($sql, [$aclId]);
 		$row = $stmt->fetch();
 		return ($row['owner'] === $userId);
 	}
 
-	public function findBoardId($aclId) {
+	public function findBoardId($aclId): ?int {
 		$entity = $this->find($aclId);
 		return $entity->getBoardId();
 	}
 
-	public function findByParticipant($type, $participant) {
+	public function findByParticipant($type, $participant): array {
 		$sql = 'SELECT * from *PREFIX*deck_board_acl WHERE type = ? AND participant = ?';
 		return $this->findEntities($sql, [$type, $participant]);
 	}
diff --git a/lib/Db/AssignmentMapper.php b/lib/Db/AssignmentMapper.php
index 0491cff1e..85a9cf197 100644
--- a/lib/Db/AssignmentMapper.php
+++ b/lib/Db/AssignmentMapper.php
@@ -79,11 +79,11 @@ class AssignmentMapper extends QBMapper implements IPermissionMapper {
 	}
 
 
-	public function isOwner($userId, $cardId) {
+	public function isOwner($userId, $cardId): bool {
 		return $this->cardMapper->isOwner($userId, $cardId);
 	}
 
-	public function findBoardId($cardId) {
+	public function findBoardId($cardId): ?int {
 		return $this->cardMapper->findBoardId($cardId);
 	}
 
diff --git a/lib/Db/AttachmentMapper.php b/lib/Db/AttachmentMapper.php
index 69681b0c1..46e63f7c4 100644
--- a/lib/Db/AttachmentMapper.php
+++ b/lib/Db/AttachmentMapper.php
@@ -156,7 +156,7 @@ class AttachmentMapper extends DeckMapper implements IPermissionMapper {
 	 * @param $id int|string unique entity identifier
 	 * @return boolean
 	 */
-	public function isOwner($userId, $id) {
+	public function isOwner($userId, $id): bool {
 		try {
 			$attachment = $this->find($id);
 			return $this->cardMapper->isOwner($userId, $attachment->getCardId());
@@ -172,7 +172,7 @@ class AttachmentMapper extends DeckMapper implements IPermissionMapper {
 	 * @param $id int|string unique entity identifier
 	 * @return int|null id of Board
 	 */
-	public function findBoardId($id) {
+	public function findBoardId($id): ?int {
 		try {
 			$attachment = $this->find($id);
 		} catch (\Exception $e) {
diff --git a/lib/Db/BoardMapper.php b/lib/Db/BoardMapper.php
index 72ac5b4ec..1ef690b8c 100644
--- a/lib/Db/BoardMapper.php
+++ b/lib/Db/BoardMapper.php
@@ -220,12 +220,12 @@ class BoardMapper extends DeckMapper implements IPermissionMapper {
 		return parent::delete($entity);
 	}
 
-	public function isOwner($userId, $boardId) {
+	public function isOwner($userId, $boardId): bool {
 		$board = $this->find($boardId);
 		return ($board->getOwner() === $userId);
 	}
 
-	public function findBoardId($id) {
+	public function findBoardId($id): ?int {
 		return $id;
 	}
 
diff --git a/lib/Db/CardMapper.php b/lib/Db/CardMapper.php
index c8d8f4d2a..7bcf65741 100644
--- a/lib/Db/CardMapper.php
+++ b/lib/Db/CardMapper.php
@@ -308,7 +308,7 @@ class CardMapper extends QBMapper implements IPermissionMapper {
 		$qb->execute();
 	}
 
-	public function isOwner($userId, $cardId) {
+	public function isOwner($userId, $cardId): bool {
 		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_stacks` WHERE id IN (SELECT stack_id FROM `*PREFIX*deck_cards` WHERE id = ?))';
 		$stmt = $this->db->prepare($sql);
 		$stmt->bindParam(1, $cardId, \PDO::PARAM_INT);
@@ -317,7 +317,7 @@ class CardMapper extends QBMapper implements IPermissionMapper {
 		return ($row['owner'] === $userId);
 	}
 
-	public function findBoardId($cardId) {
+	public function findBoardId($cardId): ?int {
 		$sql = 'SELECT id FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_stacks` WHERE id IN (SELECT stack_id FROM `*PREFIX*deck_cards` WHERE id = ?))';
 		$stmt = $this->db->prepare($sql);
 		$stmt->bindParam(1, $cardId, \PDO::PARAM_INT);
diff --git a/lib/Db/IPermissionMapper.php b/lib/Db/IPermissionMapper.php
index 2b94405b8..39a900253 100644
--- a/lib/Db/IPermissionMapper.php
+++ b/lib/Db/IPermissionMapper.php
@@ -33,7 +33,7 @@ interface IPermissionMapper {
 	 * @param $id int|string unique entity identifier
 	 * @return boolean
 	 */
-	public function isOwner($userId, $id);
+	public function isOwner($userId, $id): bool;
 
 	/**
 	 * Query boardId for Entity of given $id
@@ -41,5 +41,5 @@ interface IPermissionMapper {
 	 * @param $id int|string unique entity identifier
 	 * @return int|null id of Board
 	 */
-	public function findBoardId($id);
+	public function findBoardId($id): ?int;
 }
diff --git a/lib/Db/LabelMapper.php b/lib/Db/LabelMapper.php
index ec0b279a0..26f89bf3a 100644
--- a/lib/Db/LabelMapper.php
+++ b/lib/Db/LabelMapper.php
@@ -92,14 +92,14 @@ class LabelMapper extends DeckMapper implements IPermissionMapper {
 		$stmt->execute();
 	}
 
-	public function isOwner($userId, $labelId) {
+	public function isOwner($userId, $labelId): bool {
 		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_labels` WHERE id = ?)';
 		$stmt = $this->execute($sql, [$labelId]);
 		$row = $stmt->fetch();
 		return ($row['owner'] === $userId);
 	}
 
-	public function findBoardId($labelId) {
+	public function findBoardId($labelId): ?int {
 		$entity = $this->find($labelId);
 		return $entity->getBoardId();
 	}
diff --git a/lib/Db/StackMapper.php b/lib/Db/StackMapper.php
index 58b343929..88cffeeba 100644
--- a/lib/Db/StackMapper.php
+++ b/lib/Db/StackMapper.php
@@ -67,14 +67,14 @@ class StackMapper extends DeckMapper implements IPermissionMapper {
 		return parent::delete($entity);
 	}
 
-	public function isOwner($userId, $stackId) {
+	public function isOwner($userId, $stackId): bool {
 		$sql = 'SELECT owner FROM `*PREFIX*deck_boards` WHERE `id` IN (SELECT board_id FROM `*PREFIX*deck_stacks` WHERE id = ?)';
 		$stmt = $this->execute($sql, [$stackId]);
 		$row = $stmt->fetch();
 		return ($row['owner'] === $userId);
 	}
 
-	public function findBoardId($stackId) {
+	public function findBoardId($stackId): ?int {
 		$entity = $this->find($stackId);
 		return $entity->getBoardId();
 	}