key: authenticated mode = not passphrase protected

docs/changes.rst

@@ -133,13 +133,21 @@ Version 1.1.0b6 (unreleased)
 
 Compatibility notes:
 
-- Repositories in a repokey mode with a blank passphrase are now treated
-  as unencrypted repositories for security checks
+- Repositories in a repokey mode (including "authenticated" mode) with a
+  blank passphrase are now treated as unencrypted repositories for security checks
   (e.g. BORG_UNKNOWN_UNENCRYPTED_REPO_ACCESS_IS_OK).
 - Running "borg init" via a "borg serve --append-only" server will *not* create
   an append-only repository anymore. Use "borg init --append-only" to initialize
   an append-only repository.
 
+  Previously there would be no prompts nor messages if an unknown repository
+  in one of these modes with a blank passphrase was encountered. This would
+  allow an attacker to swap a repository, if one assumed that the lack of
+  password prompts was due to a set BORG_PASSPHRASE.
+
+  Since the "trick" does not work if BORG_PASSPHRASE is set, this does generally
+  not affect scripts.
+
 Version 1.1.0b5 (2017-04-30)
 ----------------------------
 

src/borg/crypto/key.py

@@ -749,6 +749,18 @@ class AuthenticatedKey(ID_BLAKE2b_256, RepoKey):
     ARG_NAME = 'authenticated'
     STORAGE = KeyBlobStorage.REPO
 
+    # It's only authenticated, not encrypted.
+    passphrase_protected = False
+
+    def load(self, target, passphrase):
+        success = super().load(target, passphrase)
+        self.passphrase_protected = False
+        return success
+
+    def save(self, target, passphrase):
+        super().save(target, passphrase)
+        self.passphrase_protected = False
+
     def encrypt(self, chunk):
         data = self.compressor.compress(chunk)
         return b''.join([self.TYPE_STR, data])