From 8db7cdc71321f4a367338df87a7e90629e741ff9 Mon Sep 17 00:00:00 2001
From: Kim Alvefur <zash@zash.se>
Date: Wed, 1 Jun 2022 15:06:59 +0200
Subject: mod_saslauth: Implement RFC 9266 'tls-exporter' channel binding
 (#1760)

Brings back SCRAM-SHA-*-PLUS from its hiatus brought on by the earlier
channel binding method being undefined for TLS 1.3, and the increasing
deployment of TLS 1.3.

See 1bfd238e05ad and #1542

Requires future version of LuaSec, once support for this key material
export method is merged.

See https://github.com/brunoos/luasec/pull/187
---
 CHANGES                  |  1 +
 doc/doap.xml             |  1 +
 net/server_epoll.lua     |  8 ++++++++
 plugins/mod_saslauth.lua | 15 +++++++++++++++
 4 files changed, 25 insertions(+)

diff --git a/CHANGES b/CHANGES
index e1f0854a..48fe37d5 100644
--- a/CHANGES
+++ b/CHANGES
@@ -17,6 +17,7 @@ TRUNK
 ### Security and authentication
 
 - Advertise supported SASL Channel-Binding types (XEP-0440)
+- Implement RFC 9266 'tls-exporter' channel binding with TLS 1.3
 
 ## Removed
 
diff --git a/doc/doap.xml b/doc/doap.xml
index e767115b..530b9c2c 100644
--- a/doc/doap.xml
+++ b/doc/doap.xml
@@ -60,6 +60,7 @@
     <implements rdf:resource="https://www.rfc-editor.org/info/rfc7395"/>
     <implements rdf:resource="https://www.rfc-editor.org/info/rfc7590"/>
     <implements rdf:resource="https://www.rfc-editor.org/info/rfc7673"/>
+    <implements rdf:resource="https://www.rfc-editor.org/info/rfc9266"/>
     <implements rdf:resource="https://datatracker.ietf.org/doc/draft-cridland-xmpp-session/">
       <!-- since=0.6.0 note=Added in hg:0bbbc9042361 -->
     </implements>
diff --git a/net/server_epoll.lua b/net/server_epoll.lua
index a546e1e3..b269bd9c 100644
--- a/net/server_epoll.lua
+++ b/net/server_epoll.lua
@@ -649,6 +649,14 @@ function interface:ssl_peerfinished()
 	return sock:getpeerfinished();
 end
 
+function interface:ssl_exportkeyingmaterial(label, len, context)
+	local sock = self.conn;
+	if sock.exportkeyingmaterial then
+		return sock:exportkeyingmaterial(label, len, context);
+	end
+end
+
+
 function interface:starttls(tls_ctx)
 	if tls_ctx then self.tls_ctx = tls_ctx; end
 	self.starttls = false;
diff --git a/plugins/mod_saslauth.lua b/plugins/mod_saslauth.lua
index 0b350c74..e94b2d78 100644
--- a/plugins/mod_saslauth.lua
+++ b/plugins/mod_saslauth.lua
@@ -245,6 +245,15 @@ local function tls_unique(self)
 	return self.userdata["tls-unique"]:ssl_peerfinished();
 end
 
+local function tls_exporter(conn)
+	if not conn.ssl_exportkeyingmaterial then return end
+	return conn:ssl_exportkeyingmaterial("EXPORTER-Channel-Binding", 32, "");
+end
+
+local function sasl_tls_exporter(self)
+	return tls_exporter(self.userdata["tls-exporter"]);
+end
+
 local mechanisms_attr = { xmlns='urn:ietf:params:xml:ns:xmpp-sasl' };
 local bind_attr = { xmlns='urn:ietf:params:xml:ns:xmpp-bind' };
 local xmpp_session_attr = { xmlns='urn:ietf:params:xml:ns:xmpp-session' };
@@ -266,6 +275,11 @@ module:hook("stream-features", function(event)
 				local info = origin.conn:ssl_info();
 				if info and info.protocol == "TLSv1.3" then
 					log("debug", "Channel binding 'tls-unique' undefined in context of TLS 1.3");
+					if tls_exporter(origin.conn) then
+						log("debug", "Channel binding 'tls-exporter' supported");
+						sasl_handler:add_cb_handler("tls-exporter", sasl_tls_exporter);
+						channel_bindings:add("tls-exporter");
+					end
 				elseif origin.conn.ssl_peerfinished and origin.conn:ssl_peerfinished() then
 					log("debug", "Channel binding 'tls-unique' supported");
 					sasl_handler:add_cb_handler("tls-unique", tls_unique);
@@ -275,6 +289,7 @@ module:hook("stream-features", function(event)
 				end
 				sasl_handler["userdata"] = {
 					["tls-unique"] = origin.conn;
+					["tls-exporter"] = origin.conn;
 				};
 			else
 				log("debug", "Channel binding not supported by SASL handler");
-- 
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