[tor-dev] Proposal 285: Directory documents should be standardized as UTF-8

[Nick Mathewson]

Filename: 285-utf-8.txt
Title: Directory documents should be standardized as UTF-8
Author: Nick Mathewson
Created: 13 November 2017
Status: Open

1. Summary and motivation

   People frequently want to include non-ASCII text in their router
   descriptors.  The Contact line is a favorite place to do this, but in
   principle the platform line would also be pretty logical.

   Unfortunately, there's no specified way to encode non-ASCII in our
   directory documents.

   Fortunately, almost everybody who does it, uses UTF-8 anyway.

   As we move towards Rust support in Tor, we gain another motivation
   for standarding on UTF-8, since Rust's native strings strongly prefer
   UTF-8.

   So, in this proposal, we describe a migration path to having all
   directory documents be fully UTF-8.

2. Proposal

   First, we should have Tor relays reject ContactInfo lines (and any
   other lines copied directly into router descriptors) that are not
   UTF-8.

   At the same time, we should have authorities reject any router
   descriptors or extrainfo documents that are not valid UTF-8.
   Simultaneously, we can have all Tor instances reject all
   non-directory-descriptor directory documents that are not UTF-8,
   since none should exist today.

   Finally, once the authorities have updated, we should have all Tor
   instances reject all directory documents that are not UTF-8.  (We
   should not take this step until the authorities have upgraded, or
   else the behavior of updated and non-updated clients could be
   distinguished.)

2.1. Hidden service descriptors' encrypted bodies

   For the encrypted bodies of hidden service descriptors, we cannot
   reject them at the authority level, and so we need to take a slightly
   different approach to prevent client fingerprinting attacks.

   First, we should make Tor instances start warning about any hidden
   service descriptors whose bodies, post-decryption, contain non-utf-8
   plaintext.  At the same time, we add a consensus parameter to
   indicate that hidden service descriptors with non-utf-8 plantexts
   should be rejected entirely: "reject-encrypted-non-utf-8".  If that
   parameter is set to 1, then hidden service clients will not only
   warn, but reject the descriptors.

   Once the vast majority of clients are running versions that support
   the "reject-encrypted-non-utf-8" parameter, that parameter can be set
   to 1.
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