Create the master key file readable by its owner only

.github/workflows/python-app.yml

@@ -45,6 +45,7 @@ jobs:
     - name: Test with filesystem backend
       run: |
         cp test_keys/* ../
+        chmod 600 ../server_private_key.key
         cd ..
         python -m unittest -v
       env:
@@ -54,6 +55,7 @@ jobs:
     - name: Test with Redis backend
       run: |
         cp test_keys/* ../
+        chmod 600 ../server_private_key.key
         cd ..
         python -m unittest -v
       env:

README.md

@@ -18,6 +18,19 @@ Green.
 
 `PYTHONPATH=.. python -m blind_pin_server.generateserverkey`
 
+## Securing the server key
+
+Anyone who can read `server_private_key.key` can impersonate this server, so it is created
+mode `0600` and the server refuses to start if it is readable by anyone else.
+
+In the container it is read by `www-data`:
+
+`sudo chown www-data:www-data server_private_key.key`
+
+`sudo chmod 600 server_private_key.key`
+
+Mount it read-only (`:ro`, as below) so the container can't alter it.
+
 ## Build the docker image
 
 `docker build -f Dockerfile . -t dockerized_pinserver`
@@ -28,4 +41,4 @@ Green.
 
 ## Run the docker image (requires the previous steps)
 
-`docker run -v $PWD/server_private_key.key:/server_private_key.key -v $PWD/pinsdir:/pins -p 8096:8096 dockerized_pinserver`
+`docker run -v $PWD/server_private_key.key:/server_private_key.key:ro -v $PWD/pinsdir:/pins -p 8096:8096 dockerized_pinserver`

server.py

@@ -1,6 +1,7 @@
 import time
 from hmac import compare_digest
 import os
+import stat
 from .lib import decrypt, encrypt, E_ECDH
 from wallycore import ec_private_key_verify, ec_sig_from_bytes, sha256, \
     hmac_sha256, EC_FLAG_ECDSA, ec_private_key_bip341_tweak, ec_public_key_from_private_key
@@ -20,7 +21,11 @@ def generate_server_key_pair(cls):
 
         private_key, public_key = cls.generate_ec_key_pair()
 
-        with open(cls.STATIC_SERVER_PRIVATE_KEY_FILE, 'wb') as f:
+        # O_EXCL: fail if it exists, S_IRUSR|S_IWUSR: chmod 600
+        fd = os.open(cls.STATIC_SERVER_PRIVATE_KEY_FILE,
+                     os.O_WRONLY | os.O_CREAT | os.O_EXCL,
+                     stat.S_IRUSR | stat.S_IWUSR)
+        with os.fdopen(fd, 'wb') as f:
             f.write(private_key)
 
         with open(cls.STATIC_SERVER_PUBLIC_KEY_FILE, 'wb') as f:
@@ -32,6 +37,11 @@ def generate_server_key_pair(cls):
     @classmethod
     def load_private_key(cls):
         if not cls.STATIC_SERVER_PRIVATE_KEY:
+            mode = stat.S_IMODE(os.stat(cls.STATIC_SERVER_PRIVATE_KEY_FILE).st_mode)
+            if mode & (stat.S_IRWXG | stat.S_IRWXO):
+                raise Exception(f'{cls.STATIC_SERVER_PRIVATE_KEY_FILE} is mode {mode:04o}, '
+                                'must not be readable by other users')
+
             with open(cls.STATIC_SERVER_PRIVATE_KEY_FILE, 'rb') as f:
                 cls.STATIC_SERVER_PRIVATE_KEY = f.read()
                 ec_private_key_verify(cls.STATIC_SERVER_PRIVATE_KEY)