Add config for how many bcrypt rounds to use for password hashes

By default we leave it at the default value of 12. But now we can reduce
it for preparing users for loadtests or running integration tests.

synapse/config/registration.py

@@ -33,6 +33,7 @@ class RegistrationConfig(Config):
 
         self.registration_shared_secret = config.get("registration_shared_secret")
         self.macaroon_secret_key = config.get("macaroon_secret_key")
+        self.bcrypt_rounds = config.get("bcrypt_rounds", 12)
 
     def default_config(self, **kwargs):
         registration_shared_secret = random_string_with_symbols(50)
@@ -48,6 +49,11 @@ class RegistrationConfig(Config):
         registration_shared_secret: "%(registration_shared_secret)s"
 
         macaroon_secret_key: "%(macaroon_secret_key)s"
+
+        # Set the number of bcrypt rounds used to generate password hash.
+        # Larger numbers increase the work factor needed to generate the hash.
+        # The default number of rounds is 12.
+        bcrypt_rounds: 12
         """ % locals()
 
     def add_arguments(self, parser):

synapse/handlers/auth.py

@@ -44,6 +44,7 @@ class AuthHandler(BaseHandler):
             LoginType.EMAIL_IDENTITY: self._check_email_identity,
             LoginType.DUMMY: self._check_dummy_auth,
         }
+        self.bcrypt_rounds = hs.config.bcrypt_rounds
         self.sessions = {}
 
     @defer.inlineCallbacks
@@ -432,7 +433,7 @@ class AuthHandler(BaseHandler):
         Returns:
             Hashed password (str).
         """
-        return bcrypt.hashpw(password, bcrypt.gensalt())
+        return bcrypt.hashpw(password, bcrypt.gensalt(self.bcrypt_rounds))
 
     def validate_hash(self, password, stored_hash):
         """Validates that self.hash(password) == stored_hash.