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

On 14 Nov 2017, at 05:51, Nick Mathewson <nickm@xxxxxxxxxxxxxx> wrote:

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.

How many current descriptors will be rejected as non-UTF-8?

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.

How do we define UTF-8?

Do we exclude all invalid byte sequences?

Do we exclude all invalid code points (some libraries don't)?

https://en.m.wikipedia.org/wiki/UTF-8#Invalid_byte_sequences

Do we reject unassigned or reserved code points?

Do we reject private use code points?

https://en.m.wikipedia.org/wiki/Unicode#General_Category_property

How do we avoid tying ourselves to a particular version of Unicode?

(By accepting reserved code points? Some libraries don't do this.)

Will we allow a byte order mark?

(We can't during the transition, it doesn't parse as ASCII.

And we probably shouldn't for any verbatim lines, because they

are copied into the middle of the descriptor.)

How do we carry forward existing ASCII restrictions into UTF-8?

We will need to update the directory spec to acknowledge that

contact and platform lines may be parsed as UTF-8 or

ASCII-including-arbitrary-bytes-except-NUL, and that they are

terminated by single-byte newlines regardless.

How do we deal with format confusion attacks?

UTF-8 has a few alternative whitespace characters. These could

be used in an attack that confuses either humans viewing the file,

or automated software:

If a human uses a UTF-8 compatible viewer or editor, it likely shows

Unicode newlines and ASCII newlines in an identical way. Similarly,

it may show Unicode spaces and ASCII spaces in the same way.

This may confuse the human reader.

Similarly, if automated software parses using a Unicode whitespace

or newline character class, it will mis-parse directory documents.

(Our Rust protover code looks for ASCII spaces, so it appears to

be fine.)

Note that we already have this issue with line feeds and carriage

returns, which I thought we had solved by banning carriage returns

in directory documents. But it appears we allow "any printing ASCII

character". (We will have to edit this to include Unicode.)

https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n218

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.

If we apply the existing restrictions in dir-spec, which require

non-directory-descriptor directory documents to be ASCII, they will

also be UTF-8.

Isn't it confusing to say "UTF-8", when what we really mean is "ASCII"?

Do we expect to migrate these to non-ASCII UTF-8 at some point?

Also, does "non-directory-descriptor directory documents" mean we

can reject non-UTF-8 microdescriptors? I think we should.

Does the NS consensus contain any lines that are copied verbatim from

descriptors?

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

typo: plaintexts

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.