Proposal 285: Directory documents should be standardized as UTF-8
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.
On 14 Nov 2017, at 05:51, Nick Mathewson <nickm@torproject.org> 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.
We also can't reject bridge descriptors at the authority level. (Bridge clients download bridge descriptors directly from bridges.) Do we need bridge clients to also use this consensus parameter? T
Hi, Teor, and sorry for the long delay! You had a lot of good questions on this proposal, and I didn't know how to answer them all. So in hopes of making progress here, I'm taking wild guesses and asking for help in making the wild guesses better :) On Mon, Nov 13, 2017 at 5:28 PM, teor <teor2345@gmail.com> wrote:
On 14 Nov 2017, at 05:51, Nick Mathewson <nickm@torproject.org> 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?
I think that when last I checked, the number was something like 3.
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?
I tried to do so as follows: We define the allowable set of UTF-8 as: * Encoding the codepoints U+01 through U+10FFFF, * but excluding the codepoints U+D800 through U+DFFF, * each encoded with the shortest possible encoding. * without any BOM Are there other restrictions we should make? If so, how should we phrase them? [...]
How do we carry forward existing ASCII restrictions into UTF-8?
I don't understand this question.
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.
Ack.
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.
Right. I don't see an obvious attack here, but we should keep it in mind. Do you have a different suggestion of what to do here?
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.)
Also let's consider all the nonprinting ASCII: it's already a potential display problem if you're using a bad editor, or whatever.
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?
I think having non-ASCII in extrainfos is a reasonable possibility. I'm not so sure about the others: there could be reasons in the future. My rationale for declaring everything to be UTF-8 was that it seemed more reasonable to have a single set of rules for parsing everything than to have different rules for different documents.
Also, does "non-directory-descriptor directory documents" mean we can reject non-UTF-8 microdescriptors? I think we should.
I think so.
Does the NS consensus contain any lines that are copied verbatim from descriptors?
I don't think so. [...]
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.
We also can't reject bridge descriptors at the authority level. (Bridge clients download bridge descriptors directly from bridges.) Do we need bridge clients to also use this consensus parameter?
I added an extra section for this, basically saying "bridge clients should do that too": 2.2. Bridge descriptors Since clients download bridge descriptors directly from the bridges, they also need a two-phase plan as for hidden service descriptors above. Here we take the same approach as in section 2.1 above, except using the parameter "reject-bridge-descriptor-non-utf-8".
On 10 Jan 2018, at 04:34, Nick Mathewson <nickm@alum.mit.edu> wrote:
On Mon, Nov 13, 2017 at 5:28 PM, teor <teor2345@gmail.com> wrote:
On 14 Nov 2017, at 05:51, Nick Mathewson <nickm@torproject.org> 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?
I think that when last I checked, the number was something like 3.
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?
I tried to do so as follows:
We define the allowable set of UTF-8 as: * Encoding the codepoints U+01 through U+10FFFF, * but excluding the codepoints U+D800 through U+DFFF,
These are called "Unicode Scalar Values". https://www.unicode.org/glossary/#unicode_scalar_value Let's reference that.
* each encoded with the shortest possible encoding. * without any BOM
Are there other restrictions we should make? If so, how should we phrase them?
These seem fine, and not tied to a particular unicode version. But I don't know enough about Unicode to know if there is anything else we should specify. I know how we'd do this in C (raw bytes with a check before parsing), and I think we can do this in Rust using char: https://doc.rust-lang.org/1.0.0/unicode/char/ Here are some other things we might want to document: Unassigned Code Points Accepting arbitrary unassigned unicode code points may cause issues for some parsers, because as far as I am aware, parsers typically only handle a particular unicode version. We should note this in the spec. The potential attack here is that Tor accepts a newly introduced character, and a downstream parser rejects it. But that's not Tor's problem. The right way for parsers to handle this is to replace unknown characters with an appropriate replacement character. (Unicode has rules for this.) Or possibly throw an error. We can't make this decision for them: it depends on the goals of the parser. Equality and Normalisation We should also make sure that equality is specified as byte-for-byte equality. This means that several different byte sequences could be visually similar, and even have identical normalised forms, but we would treat them as different. Unicode has several levels of normalisation: https://en.wikipedia.org/wiki/Unicode_equivalence#Normalization We should not require any of them in our inputs. Again, normalisation may be a potential issue for parsers. Again, we can't decide how they will want to handle it, but we should document it. Also, if we change our minds about this in future, we can make tor relays normalise the contents of their descriptors, and the authority implementation will continue to work. And then we can make authorities reject non-normalised inputs a few releases later.
[...]
How do we carry forward existing ASCII restrictions into UTF-8?
I don't understand this question.
I think it was intended as a general question. Then I wrote some specific questions.
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.
Ack.
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.
Right. I don't see an obvious attack here, but we should keep it in mind.
Do you have a different suggestion of what to do here?
No, I really think this is like the potential parser bugs: not our problem. People should get a better editor. And editors should get better.
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.)
Also let's consider all the nonprinting ASCII: it's already a potential display problem if you're using a bad editor, or whatever.
Yes. Just like we can't decide how editors or parsers handle bad ASCII, we can't decide how they handle bad (or new) Unicode. T -- Tim Wilson-Brown (teor) teor2345 at gmail dot com PGP C855 6CED 5D90 A0C5 29F6 4D43 450C BA7F 968F 094B ricochet:ekmygaiu4rzgsk6n ------------------------------------------------------------------------
Quoting teor (2018-01-10 00:19:54)
These are called "Unicode Scalar Values". https://www.unicode.org/glossary/#unicode_scalar_value
Let's reference that.
"Unicode Scalar Value" includes U+0, which I think we probably want to exclude.
* each encoded with the shortest possible encoding. * without any BOM
Are there other restrictions we should make? If so, how should we phrase them?
These seem fine, and not tied to a particular unicode version.
But I don't know enough about Unicode to know if there is anything else we should specify.
Skimming through https://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt, I think it might be good to additionally forbid the code points listed at the end: U+nFFF{E,F} for n = 0..10, and U+FDD0 through U+FDEF.
It is great that we are identifying places to improve support for Rust in Tor. Along this same line of thinking, are there other places in Tor where we will need to move to supporting UTF-8? For example, should the statefile be UTF-8 also? On 11/13/2017 01:51 PM, Nick Mathewson 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.
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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On Fri, Nov 24, 2017 at 4:05 PM, chelsea komlo <me@chelseakomlo.com> wrote:
It is great that we are identifying places to improve support for Rust in Tor.
Along this same line of thinking, are there other places in Tor where we will need to move to supporting UTF-8? For example, should the statefile be UTF-8 also?
I think we could safely say that the torrc and state files need to be UTF-8, though that wouldn't need a design proposal. We'd want to do a two-phase transition, though: first warning, then disallowing. We could also, at some point, specify that the control protocol needs to be UTF-8 too, I think. That one would need a proposal, and a transition plan. -- Nick
participants (5)
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Alex Xu -
chelsea komlo -
Nick Mathewson -
Nick Mathewson -
teor