Add pepper to password hashing

synapse/config/password.py

@@ -23,10 +23,14 @@ class PasswordConfig(Config):
     def read_config(self, config):
         password_config = config.get("password_config", {})
         self.password_enabled = password_config.get("enabled", True)
+        self.password_pepper = password_config.get("pepper", "")
 
     def default_config(self, config_dir_path, server_name, **kwargs):
         return """
         # Enable password for login.
         password_config:
            enabled: true
+           # Change to a secret random string.
+           # DO NOT CHANGE THIS AFTER INITIAL SETUP!
+           #pepper: ""
         """

synapse/handlers/auth.py

@@ -750,7 +750,8 @@ def hash(self, password):
         Returns:
             Hashed password (str).
         """
-        return bcrypt.hashpw(password, bcrypt.gensalt(self.bcrypt_rounds))
+        return bcrypt.hashpw(password + self.hs.config.password_pepper,
+                             bcrypt.gensalt(self.bcrypt_rounds))
 
     def validate_hash(self, password, stored_hash):
         """Validates that self.hash(password) == stored_hash.
@@ -763,6 +764,7 @@ def validate_hash(self, password, stored_hash):
             Whether self.hash(password) == stored_hash (bool).
         """
         if stored_hash:
-            return bcrypt.hashpw(password, stored_hash.encode('utf-8')) == stored_hash
+            return bcrypt.hashpw(password + self.hs.config.password_pepper,
+                                 stored_hash.encode('utf-8')) == stored_hash
         else:
             return False