diff --git a/lib/private/Security/Crypto.php b/lib/private/Security/Crypto.php
index 1dd680ea13..d2be148427 100644
--- a/lib/private/Security/Crypto.php
+++ b/lib/private/Security/Crypto.php
@@ -108,7 +108,7 @@ class Crypto implements ICrypto {
 	 * @return string plaintext
 	 * @throws \Exception If the HMAC does not match
 	 */
-	public function decrypt($authenticatedCiphertext, $password = '') {
+	public function decrypt(string $authenticatedCiphertext, string $password = ''): string {
 		if($password === '') {
 			$password = $this->config->getSystemValue('secret');
 		}
diff --git a/lib/public/Security/ICrypto.php b/lib/public/Security/ICrypto.php
index 0de2e47b75..aa2b9eed2c 100644
--- a/lib/public/Security/ICrypto.php
+++ b/lib/public/Security/ICrypto.php
@@ -61,5 +61,5 @@ interface ICrypto {
 	 * @throws \Exception If the HMAC does not match
 	 * @since 8.0.0
 	 */
-	public function decrypt($authenticatedCiphertext, $password = '');
+	public function decrypt(string $authenticatedCiphertext, string $password = ''): string;
 }