[api] Prevent API from overriding noise encryption keys set in YAML (#10927)

esphome/components/api/__init__.py

@@ -224,6 +224,7 @@ async def to_code(config):
         if key := encryption_config.get(CONF_KEY):
             decoded = base64.b64decode(key)
             cg.add(var.set_noise_psk(list(decoded)))
+            cg.add_define("USE_API_NOISE_PSK_FROM_YAML")
         else:
             # No key provided, but encryption desired
             # This will allow a plaintext client to provide a noise key,

esphome/components/api/api_server.cpp

@@ -37,12 +37,14 @@ void APIServer::setup() {
 
   this->noise_pref_ = global_preferences->make_preference<SavedNoisePsk>(hash, true);
 
+#ifndef USE_API_NOISE_PSK_FROM_YAML
+  // Only load saved PSK if not set from YAML
   SavedNoisePsk noise_pref_saved{};
   if (this->noise_pref_.load(&noise_pref_saved)) {
     ESP_LOGD(TAG, "Loaded saved Noise PSK");
-
     this->set_noise_psk(noise_pref_saved.psk);
   }
+#endif
 #endif
 
   // Schedule reboot if no clients connect within timeout
@@ -419,6 +421,12 @@ void APIServer::set_reboot_timeout(uint32_t reboot_timeout) { this->reboot_timeo
 
 #ifdef USE_API_NOISE
 bool APIServer::save_noise_psk(psk_t psk, bool make_active) {
+#ifdef USE_API_NOISE_PSK_FROM_YAML
+  // When PSK is set from YAML, this function should never be called
+  // but if it is, reject the change
+  ESP_LOGW(TAG, "Key set in YAML");
+  return false;
+#else
   auto &old_psk = this->noise_ctx_->get_psk();
   if (std::equal(old_psk.begin(), old_psk.end(), psk.begin())) {
     ESP_LOGW(TAG, "New PSK matches old");
@@ -447,6 +455,7 @@ bool APIServer::save_noise_psk(psk_t psk, bool make_active) {
     });
   }
   return true;
+#endif
 }
 #endif
 

tests/integration/fixtures/noise_encryption_key_protection.yaml

@@ -0,0 +1,10 @@
+esphome:
+  name: noise-key-test
+
+host:
+
+api:
+  encryption:
+    key: "zX9/JHxMKwpP0jUGsF0iESCm1wRvNgR6NkKVOhn7kSs="
+
+logger:

tests/integration/test_noise_encryption_key_protection.py

@@ -0,0 +1,51 @@
+"""Integration test for noise encryption key protection from YAML."""
+
+from __future__ import annotations
+
+import base64
+
+from aioesphomeapi import InvalidEncryptionKeyAPIError
+import pytest
+
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_noise_encryption_key_protection(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test that noise encryption key set in YAML cannot be changed via API."""
+    # The key that's set in the YAML fixture
+    noise_psk = "zX9/JHxMKwpP0jUGsF0iESCm1wRvNgR6NkKVOhn7kSs="
+
+    # Keep ESPHome process running throughout all tests
+    async with run_compiled(yaml_config):
+        # First connection - test key change attempt
+        async with api_client_connected(noise_psk=noise_psk) as client:
+            # Verify connection is established
+            device_info = await client.device_info()
+            assert device_info is not None
+
+            # Try to set a new encryption key via API
+            new_key = base64.b64encode(
+                b"x" * 32
+            )  # Valid 32-byte key in base64 as bytes
+
+            # This should fail since key was set in YAML
+            success = await client.noise_encryption_set_key(new_key)
+            assert success is False
+
+        # Reconnect with the original key to verify it still works
+        async with api_client_connected(noise_psk=noise_psk) as client:
+            # Verify connection is still successful with original key
+            device_info = await client.device_info()
+            assert device_info is not None
+            assert device_info.name == "noise-key-test"
+
+        # Verify that connecting with a wrong key fails
+        wrong_key = base64.b64encode(b"y" * 32).decode()  # Different key
+        with pytest.raises(InvalidEncryptionKeyAPIError):
+            async with api_client_connected(noise_psk=wrong_key) as client:
+                await client.device_info()