fix: a temporary master password is shown once

src/Domain/Core/Context/SessionContext.php

@@ -75,7 +75,7 @@ public function getAuthCompleted();
      *
      * @return ?string
      */
-    public function getTemporaryMasterPass(): ?string;
+    public function takeTemporaryMasterPass(): ?string;
 
     /**
      * Return the public key

src/Infrastructure/Adapter/In/Web/Controllers/ConfigManager/IndexController.php

@@ -442,7 +442,7 @@ protected function getEncryptionConfig(): DataTab
         );
 
         $template->assign('tempMasterAttempts', $tempMasterAttempts);
-        $template->assign('tempMasterPass', $this->session->getTemporaryMasterPass());
+        $template->assign('tempMasterPass', $this->session->takeTemporaryMasterPass());
 
         $template->assign(
             'userGroups',

src/Infrastructure/Context/Session.php

@@ -210,11 +210,27 @@ public function getAuthCompleted(): bool
     }
 
     /**
-     * Returns the temporary master password
+     * Takes the temporary master password, and forgets it
+     *
+     * It is put here by the administrator who generated it, so that the page rendered after the
+     * generating request can show it once — there is nowhere else to keep it for that one hop, and
+     * it is deliberately not persisted anywhere in plaintext.
+     *
+     * Reading it clears it. It used to stay for the life of that administrator's session, and the
+     * Configuration page reads it on every load, so a value meant to be shown once was rendered
+     * into the HTML again on every later visit — after it had expired, after its recipients had
+     * used it, and after the master password had been rotated. Nothing anywhere removed it: there
+     * was no clear for it to call.
      */
-    public function getTemporaryMasterPass(): ?string
+    public function takeTemporaryMasterPass(): ?string
     {
-        return $this->getContextKey('tempmasterpass');
+        $tempMasterPass = $this->getContextKey('tempmasterpass');
+
+        if ($tempMasterPass !== null) {
+            $this->setContextKey('tempmasterpass', null);
+        }
+
+        return $tempMasterPass;
     }
 
     /**

tests/Unit/Infrastructure/Context/SessionTest.php

@@ -397,11 +397,36 @@ public function theTemporaryMasterPasswordIsCarried()
     {
         $session = $this->givenAStartedSession();
 
-        self::assertNull($session->getTemporaryMasterPass());
+        self::assertNull($session->takeTemporaryMasterPass());
 
         $session->setTemporaryMasterPass('a-temporary-password');
 
-        self::assertSame('a-temporary-password', $session->getTemporaryMasterPass());
+        self::assertSame('a-temporary-password', $session->takeTemporaryMasterPass());
+    }
+
+    /**
+     * And carried exactly once.
+     *
+     * It lives in the session only so that the page rendered after the generating request can show
+     * it — there is nowhere else to keep it for that one hop, and it is deliberately not persisted
+     * in plaintext anywhere. It used to stay for the life of the administrator's session, and
+     * ConfigManager's index reads it on every load, so a value meant to be shown once was rendered
+     * into the HTML again on every later visit to the Configuration page: after it had expired,
+     * after its recipients had used it, and after the master password had been rotated. Nothing
+     * removed it, because there was nothing that could — no clear existed.
+     *
+     * @throws ContextException
+     * @throws SPException
+     */
+    #[Test]
+    public function theTemporaryMasterPasswordIsCarriedOnlyOnce()
+    {
+        $session = $this->givenAStartedSession();
+
+        $session->setTemporaryMasterPass('a-temporary-password');
+
+        self::assertSame('a-temporary-password', $session->takeTemporaryMasterPass());
+        self::assertNull($session->takeTemporaryMasterPass(), 'it must not survive being read');
     }
 
     /**