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136 lines
4.6 KiB
Markdown
136 lines
4.6 KiB
Markdown
# TLS: ECH support in curl and libcurl
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## Summary
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**ECH** means **Encrypted Client Hello**, a TLS 1.3 extension which is
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currently the subject of an [IETF Draft][tlsesni]. (ECH was formerly known as
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ESNI).
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This file is intended to show the latest current state of ECH support
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in **curl** and **libcurl**.
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At end of August 2019, an [experimental fork of curl][niallorcurl], built
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using an [experimental fork of OpenSSL][sftcdopenssl], which in turn provided
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an implementation of ECH, was demonstrated interoperating with a server
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belonging to the [DEfO Project][defoproj].
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Further sections here describe
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- resources needed for building and demonstrating **curl** support
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for ECH,
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- progress to date,
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- TODO items, and
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- additional details of specific stages of the progress.
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## Resources needed
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To build and demonstrate ECH support in **curl** and/or **libcurl**,
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you will need
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- a TLS library, supported by **libcurl**, which implements ECH;
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- an edition of **curl** and/or **libcurl** which supports the ECH
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implementation of the chosen TLS library;
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- an environment for building and running **curl**, and at least
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building **OpenSSL**;
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- a server, supporting ECH, against which to run a demonstration
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and perhaps a specific target URL;
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- some instructions.
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The following set of resources is currently known to be available.
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| Set | Component | Location | Remarks |
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|:-----|:-------------|:------------------------------|:-------------------------------------------|
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| DEfO | TLS library | [sftcd/openssl][sftcdopenssl] | Tag *esni-2019-08-30* avoids bleeding edge |
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| | curl fork | [niallor/curl][niallorcurl] | Tag *esni-2019-08-30* likewise |
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| | instructions | [ESNI-README][niallorreadme] | |
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## Progress
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### PR 4011 (Jun 2019) expected in curl release 7.67.0 (Oct 2019)
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- Details [below](#pr4011);
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- New configuration option: `--enable-ech`;
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- Build-time check for availability of resources needed for ECH
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support;
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- Pre-processor symbol `USE_ECH` for conditional compilation of
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ECH support code, subject to configuration option and
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availability of needed resources.
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## TODO
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- (next PR) Add libcurl options to set ECH parameters.
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- (next PR) Add curl tool command line options to set ECH parameters.
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- (WIP) Extend DoH functions so that published ECH parameters can be
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retrieved from DNS instead of being required as options.
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- (WIP) Work with OpenSSL community to finalize ECH API.
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- Track OpenSSL ECH API in libcurl
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- Identify and implement any changes needed for CMake.
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- Optimize build-time checking of available resources.
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- Encourage ECH support work on other TLS/SSL backends.
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## Additional detail
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### PR 4011
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**TLS: Provide ECH support framework for curl and libcurl**
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The proposed change provides a framework to facilitate work to implement ECH
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support in curl and libcurl. It is not intended either to provide ECH
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functionality or to favour any particular TLS-providing backend. Specifically,
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the change reserves a feature bit for ECH support (symbol
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`CURL_VERSION_ECH`), implements setting and reporting of this bit, includes
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dummy book-keeping for the symbol, adds a build-time configuration option
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(`--enable-ech`), provides an extensible check for resources available to
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provide ECH support, and defines a compiler pre-processor symbol (`USE_ECH`)
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accordingly.
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Proposed-by: @niallor (Niall O'Reilly)\
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Encouraged-by: @sftcd (Stephen Farrell)\
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See-also: [this message](https://curl.se/mail/lib-2019-05/0108.html)
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Limitations:
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- Book-keeping (symbols-in-versions) needs real release number, not 'DUMMY'.
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- Framework is incomplete, as it covers autoconf, but not CMake.
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- Check for available resources, although extensible, refers only to
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specific work in progress ([described
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here](https://github.com/sftcd/openssl/tree/master/esnistuff)) to
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implement ECH for OpenSSL, as this is the immediate motivation
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for the proposed change.
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## References
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Cloudflare blog: [Encrypting SNI: Fixing One of the Core Internet Bugs][corebug]
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Cloudflare blog: [Encrypt it or lose it: how encrypted SNI works][esniworks]
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IETF Draft: [Encrypted Server Name Indication for TLS 1.3][tlsesni]
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---
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[tlsesni]: https://datatracker.ietf.org/doc/draft-ietf-tls-esni/
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[esniworks]: https://blog.cloudflare.com/encrypted-sni/
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[corebug]: https://blog.cloudflare.com/esni/
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[defoproj]: https://defo.ie/
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[sftcdopenssl]: https://github.com/sftcd/openssl/
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[niallorcurl]: https://github.com/niallor/curl/
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[niallorreadme]: https://github.com/niallor/curl/blob/master/ESNI-README.md
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