ESNI: initial build/setup

Closes #4011

3 files changed, 141 insertions, 0 deletions

diff --git a/docs/ESNI.md b/docs/ESNI.md
new file mode 100644
index 000000000..eefb6662b
--- /dev/null
+++ b/docs/ESNI.md
@@ -0,0 +1,139 @@
+# TLS: ESNI support in curl and libcurl
+
+## Summary
+
+**ESNI** means **Encrypted Server Name Indication**, a TLS 1.3
+extension which is currently the subject of an
+[IETF Draft][tlsesni].
+
+This file is intended to show the latest current state of ESNI support
+in **curl** and **libcurl**.
+
+At end of August 2019, an [experimental fork of curl][niallorcurl],
+built using an [experimental fork of OpenSSL][sftcdopenssl], which in
+turn provided an implementation of ESNI, was demonstrated
+interoperating with a server belonging to the [DEfO
+Project][defoproj].
+
+Further sections here describe
+
+-   resources needed for building and demonstrating **curl** support
+    for ESNI,
+
+-   progress to date,
+
+-   TODO items, and
+
+-   additional details of specific stages of the progress.
+
+## Resources needed
+
+To build and demonstrate ESNI support in **curl** and/or **libcurl**,
+you will need
+
+-   a TLS library, supported by **libcurl**, which implements ESNI;
+
+-   an edition of **curl** and/or **libcurl** which supports the ESNI
+    implementation of the chosen TLS library;
+
+-   an environment for building and running **curl**, and at least
+    building **OpenSSL**;
+
+-   a server, supporting ESNI, against which to run a demonstration
+    and perhaps a specific target URL;
+
+-   some instructions.
+
+The following set of resources is currently known to be available.
+
+| Set  | Component    | Location                      | Remarks                                    |
+|:-----|:-------------|:------------------------------|:-------------------------------------------|
+| DEfO | TLS library  | [sftcd/openssl][sftcdopenssl] | Tag *esni-2019-08-30* avoids bleeding edge |
+|      | curl fork    | [niallor/curl][niallorcurl]   | Tag *esni-2019-08-30* likewise             |
+|      | instructions | [ESNI-README][niallorreadme]  |                                            |
+
+## Progress
+
+### PR 4011 (Jun 2019) expected in curl release 7.67.0 (Oct 2019)
+
+-   Details [below](#pr4011);
+
+-   New **curl** feature: `CURL_VERSION_ESNI`;
+
+-   New configuration option: `--enable-esni`;
+
+-   Build-time check for availability of resources needed for ESNI
+    support;
+
+-   Pre-processor symbol `USE_ESNI` for conditional compilation of
+    ESNI support code, subject to configuration option and
+    availability of needed resources.
+
+## TODO
+
+-   (next PR) Add libcurl options to set ESNI parameters.
+
+-   (next PR) Add curl tool command line options to set ESNI parameters.
+
+-   (WIP) Extend DoH functions so that published ESNI parameters can be
+    retrieved from DNS instead of being required as options.
+
+-   (WIP) Work with OpenSSL community to finalize ESNI API.
+
+-   Track OpenSSL ESNI API in libcurl
+
+-   Identify and implement any changes needed for CMake.
+
+-   Optimize build-time checking of available resources.
+
+-   Encourage ESNI support work on other TLS/SSL backends.
+
+## Additional detail
+
+### PR 4011
+
+**TLS: Provide ESNI support framework for curl and libcurl**
+
+The proposed change provides a framework to facilitate work to
+implement ESNI support in curl and libcurl. It is not intended
+either to provide ESNI functionality or to favour any particular
+TLS-providing backend. Specifically, the change reserves a
+feature bit for ESNI support (symbol `CURL_VERSION_ESNI`),
+implements setting and reporting of this bit, includes dummy
+book-keeping for the symbol, adds a build-time configuration
+option (`--enable-esni`), provides an extensible check for
+resources available to provide ESNI support, and defines a
+compiler pre-processor symbol (`USE_ESNI`) accordingly.
+
+Proposed-by: @niallor (Niall O'Reilly)\
+Encouraged-by: @sftcd (Stephen Farrell)\
+See-also: [this message](https://curl.haxx.se/mail/lib-2019-05/0108.html)
+
+Limitations:
+-   Book-keeping (symbols-in-versions) needs real release number, not 'DUMMY'.
+
+-   Framework is incomplete, as it covers autoconf, but not CMake.
+
+-   Check for available resources, although extensible, refers only to
+    specific work in progress ([described
+    here](https://github.com/sftcd/openssl/tree/master/esnistuff)) to
+    implement ESNI for OpenSSL, as this is the immediate motivation
+    for the proposed change.
+
+## References
+
+CloudFlare blog: [Encrypting SNI: Fixing One of the Core Internet Bugs][corebug]
+
+Cloudflare blog: [Encrypt it or lose it: how encrypted SNI works][esniworks]
+
+IETF Draft: [Encrypted Server Name Indication for TLS 1.3][tlsesni]
+
+---
+
+[tlsesni]:		https://datatracker.ietf.org/doc/draft-ietf-tls-esni/
+[esniworks]:	https://blog.cloudflare.com/encrypted-sni/
+[corebug]:		https://blog.cloudflare.com/esni/
+[defoproj]:		https://defo.ie/
+[sftcdopenssl]: https://github.com/sftcd/openssl/
+[niallorcurl]:	https://github.com/niallor/curl/
+[niallorreadme]: https://github.com/niallor/curl/blob/master/ESNI-README.md
diff --git a/docs/Makefile.am b/docs/Makefile.am
index 2bde6bcb1..b26967024 100644
--- a/docs/Makefile.am
+++ b/docs/Makefile.am
@@ -53,6 +53,7 @@ EXTRA_DIST =                                    \
  CODE_STYLE.md                                  \
  CONTRIBUTE.md                                  \
  DEPRECATE.md                                   \
+ ESNI.md                                        \
  EXPERIMENTAL.md                                \
  FAQ                                            \
  FEATURES                                       \
diff --git a/docs/libcurl/symbols-in-versions b/docs/libcurl/symbols-in-versions
index 3e144a719..bf23b4488 100644
--- a/docs/libcurl/symbols-in-versions
+++ b/docs/libcurl/symbols-in-versions
@@ -927,6 +927,7 @@ CURL_VERSION_BROTLI             7.57.0
 CURL_VERSION_CONV               7.15.4
 CURL_VERSION_CURLDEBUG          7.19.6
 CURL_VERSION_DEBUG              7.10.6
+CURL_VERSION_ESNI               7.67.0
 CURL_VERSION_GSSAPI             7.38.0
 CURL_VERSION_GSSNEGOTIATE       7.10.6        7.38.0
 CURL_VERSION_HTTP2              7.33.0