The client auth protocol allows attacker-controlled x25519 private keys being
passed around, which allows an attacker to potentially trigger the all-zeroes
assert for client_auth_sk in hs_descriptor.c:decrypt_descriptor_cookie().
We fixed that by making sure that an all-zeroes client auth key will not be
used.
There are no guidelines for validating x25519 private keys, and the assert was
there as a sanity check for code flow issues (we don't want to enter that
function with an unitialized key if client auth is being used). To avoid such
crashes in the future, we also changed the assert to a BUG-and-err.
Because the function that parses client auth credentials saved on
disk (parse_auth_file_content()) is not future compatible, there is no way to
add support for storing the nickname on the disk. Hence, nicknames cannot
persist after Tor restart making them pretty much useless.
In the future we can introduce nicknames by adding a new file format for client
auth credentials, but this was not deemed worth doing at this stage.
- See hs_client_register_auth_credentials() for the entry point.
- Also set the permanent flag for credentials we read from the filesystem.
- Also add some missing documentation.
George Kadianakis
Nick Mathewson