diff --git a/README.md b/README.md
index b407b55..65febe7 100644
--- a/README.md
+++ b/README.md
@@ -21,9 +21,11 @@ To use the Global Site Connector you need to add some config parameters to the c
 Config.php parameters to operate the server in master mode:
 
 ````
-// can be chosen freely, you just have to make sure the master and
-// all slaves have the same key.  Also make sure to choose a strong shared secret.
-'gss.jwt.key' => 'random-key',
+// Shared secret used to sign JWT tokens between master and slave nodes.
+// IMPORTANT: Must be at least 32 characters long (required by HS256 per RFC 7518).
+// Must be identical on master and all slave nodes.
+// Example: use `openssl rand -base64 32` to generate a strong key.
+'gss.jwt.key' => 'random-key-at-least-32-characters',
 
 // operation mode
 'gss.mode' => 'master',
@@ -50,9 +52,10 @@ Config.php parameters to operate the server in master mode:
 Config parameters to operate the server in slave mode:
 
 ````
-// can be chosen freely, you just have to make sure the master and
-// all slaves have the same key. Also make sure to choose a strong shared secret.
-'gss.jwt.key' => 'random-key',
+// Shared secret used to sign JWT tokens between master and slave nodes.
+// IMPORTANT: Must be at least 32 characters long (required by HS256 per RFC 7518).
+// Must be identical on master and all slave nodes.
+'gss.jwt.key' => 'random-key-at-least-32-characters',
 
 // operation mode
 'gss.mode' => 'slave',
diff --git a/lib/GlobalSiteSelector.php b/lib/GlobalSiteSelector.php
index d1af66f..3fada41 100644
--- a/lib/GlobalSiteSelector.php
+++ b/lib/GlobalSiteSelector.php
@@ -23,15 +23,11 @@ class GlobalSiteSelector {
 	public const MASTER = 'master';
 	public const SLAVE = 'slave';
 
-	/** @var IConfig */
-	private $config;
+	public const MIN_JWT_KEY_LENGTH = 32;
 
-	/**
-	 * GlobalSiteSelector constructor.
-	 *
-	 * @param IConfig $config
-	 */
-	public function __construct(IConfig $config) {
+	public function __construct(
+		private IConfig $config,
+	) {
 		$this->config = $config;
 	}
 
@@ -64,10 +60,18 @@ public function isSlave(): bool {
 	 * @return string
 	 */
 	public function getJwtKey(): string {
-		// TODO: returns exception if non-existant
 		return $this->config->getSystemValueString('gss.jwt.key', '');
 	}
 
+	/**
+	 * Validate that the JWT key meets minimum length requirements.
+	 * HS256 requires a key of at least 256 bits (32 bytes) per RFC 7518 §3.2.
+	 */
+	public function isJwtKeyValid(): bool {
+		$key = $this->getJwtKey();
+		return $key !== '' && strlen($key) >= self::MIN_JWT_KEY_LENGTH;
+	}
+
 	/**
 	 * get the URL of the global site selector master
 	 *
diff --git a/lib/Master.php b/lib/Master.php
index 4d65e83..f6f5c38 100644
--- a/lib/Master.php
+++ b/lib/Master.php
@@ -310,6 +310,16 @@ protected function redirectUser($uid, $password, $location, array $options = [])
 	 * @return string
 	 */
 	protected function createJwt($uid, $password, $options) {
+		if (!$this->gss->isJwtKeyValid()) {
+			$this->logger->error(
+				'gss.jwt.key is too short: HS256 requires at least '
+				. GlobalSiteSelector::MIN_JWT_KEY_LENGTH . ' characters (per RFC 7518). '
+				. 'Current key length: ' . strlen($this->gss->getJwtKey()) . '. '
+				. 'Please update gss.jwt.key in config.php on all nodes.',
+				['app' => Application::APP_ID]
+			);
+		}
+
 		$token = [
 			'uid' => $uid,
 			'password' => $this->crypto->encrypt($password, $this->gss->getJwtKey()),
diff --git a/lib/Slave.php b/lib/Slave.php
index 2b06894..4a8aff6 100644
--- a/lib/Slave.php
+++ b/lib/Slave.php
@@ -257,6 +257,16 @@ protected function checkConfiguration(): bool {
 			return false;
 		}
 
+		if (!$this->gss->isJwtKeyValid()) {
+			$this->logger->error(
+				'gss.jwt.key is too short: HS256 requires at least '
+				. GlobalSiteSelector::MIN_JWT_KEY_LENGTH . ' characters (per RFC 7518). '
+				. 'Current key length: ' . strlen($this->authKey) . '. '
+				. 'Please update gss.jwt.key in config.php on all nodes.',
+				['app' => Application::APP_ID]
+			);
+		}
+
 		return true;
 	}
 
diff --git a/tests/unit/lib/GlobalSiteSelectorTest.php b/tests/unit/lib/GlobalSiteSelectorTest.php
index ee75f4a..1d44ccb 100644
--- a/tests/unit/lib/GlobalSiteSelectorTest.php
+++ b/tests/unit/lib/GlobalSiteSelectorTest.php
@@ -60,4 +60,25 @@ public function testGetLookupServerUrl() {
 
 		$this->assertSame('result', $result);
 	}
+
+	public function testIsJwtKeyValidWithShortKey() {
+		$this->config->method('getSystemValueString')
+			->with('gss.jwt.key', '')->willReturn('short-key');
+
+		$this->assertFalse($this->gss->isJwtKeyValid());
+	}
+
+	public function testIsJwtKeyValidWithEmptyKey() {
+		$this->config->method('getSystemValueString')
+			->with('gss.jwt.key', '')->willReturn('');
+
+		$this->assertFalse($this->gss->isJwtKeyValid());
+	}
+
+	public function testIsJwtKeyValidWithValidKey() {
+		$this->config->method('getSystemValueString')
+			->with('gss.jwt.key', '')->willReturn('this-key-is-at-least-32-characters-long!');
+
+		$this->assertTrue($this->gss->isJwtKeyValid());
+	}
 }