-- mod_s2s_auth_dane

-- Copyright (C) 2013-2014 Kim Alvefur

--

-- This file is MIT/X11 licensed.

--

-- Implements DANE and Secure Delegation using DNS SRV as described in

-- http://tools.ietf.org/html/draft-miller-xmpp-dnssec-prooftype

--

-- Known issues:

-- Could be done much cleaner if mod_s2s was using util.async

--

-- TODO Things to test/handle:

-- Negative or bogus answers

-- No encryption offered

-- Different hostname before and after STARTTLS - mod_s2s should complain

-- Interaction with Dialback

--

-- luacheck: ignore module

module:set_global();

local noop = function () end

local type = type;

local t_insert = table.insert;

local set = require"util.set";

local dns_lookup = require"net.adns".lookup;

local hashes = require"util.hashes";

local base64 = require"util.encodings".base64;

local idna_to_ascii = require "util.encodings".idna.to_ascii;

local idna_to_unicode = require"util.encodings".idna.to_unicode;

local nameprep = require"util.encodings".stringprep.nameprep;

local cert_verify_identity = require "util.x509".verify_identity;

do

	local net_dns = require"net.dns";

	if not net_dns.types or not net_dns.types[52] then

		module:log("error", "No TLSA support available, DANE will not be supported");

		return

	end

end

local pat = "%-%-%-%-%-BEGIN ([A-Z ]+)%-%-%-%-%-\r?\n"..

"([0-9A-Za-z=+/\r\n]*)\r?\n%-%-%-%-%-END %1%-%-%-%-%-";

local function pem2der(pem)

	local typ, data = pem:match(pat);

	if typ and data then

		return base64.decode(data), typ;

	end

end

local use_map = { ["DANE-EE"] = 3; ["DANE-TA"] = 2; ["PKIX-EE"] = 1; ["PKIX-CA"] = 0 }

local implemented_uses = set.new { "DANE-EE", "PKIX-EE" };

do

	local cert_mt = debug.getregistry()["SSL:Certificate"];

	if cert_mt and cert_mt.__index.issued then

		-- Need cert:issued() for these

		implemented_uses:add("DANE-TA");

		implemented_uses:add("PKIX-CA");

	else

		module:log("warn", "Unable to support DANE-TA and PKIX-CA");

	end

end

local configured_uses = module:get_option_set("dane_uses", { "DANE-EE", "DANE-TA" });

local enabled_uses = set.intersection(implemented_uses, configured_uses) / function(use) return use_map[use] end;

-- Find applicable TLSA records

-- Takes a s2sin/out and a callback

local function dane_lookup(host_session, cb)

	cb = cb or noop;

	if host_session.dane ~= nil then return end -- Has already done a lookup

	if host_session.direction == "incoming" then

		if not host_session.from_host then

			module:log("debug", "Session doesn't have a 'from' host set");

			return;

		end

		-- We don't know what hostname or port to use for Incoming connections

		-- so we do a SRV lookup and then request TLSA records for each SRV

		-- Most servers will probably use the same certificate on outgoing

		-- and incoming connections, so this should work well

		local name = host_session.from_host and idna_to_ascii(host_session.from_host);

		if not name then

			module:log("warn", "Could not convert '%s' to ASCII for DNS lookup", tostring(host_session.from_host));

			return;

		end

		host_session.dane = dns_lookup(function (answer, err)

			host_session.dane = false; -- Mark that we already did the lookup

			if not answer then

				module:log("debug", "Resolver error: %s", tostring(err));

				return cb(host_session);

			end

			if not answer.secure then

				module:log("debug", "Results are not secure");

				return cb(host_session);

			end

			local n = answer.n or #answer;

			if n == 0 then

				-- No SRV records, we could proceed with the domainname and

				-- default port but that will currently not work properly since

				-- mod_s2s doesn't keep the answer around for that

				return cb(host_session);

			end

			if n == 1 and answer[1].srv.target == '.' then

				return cb(host_session); -- No service ... This shouldn't happen?

			end

			local srv_hosts = { answer = answer };

			host_session.srv_hosts = srv_hosts;

			local dane;

			for _, record in ipairs(answer) do

				t_insert(srv_hosts, record.srv);

				dns_lookup(function(dane_answer)

					n = n - 1;

					-- There are three kinds of answers

					-- Insecure, Secure and Bogus

					--

					-- We collect Secure answers for later use

					--

					-- Insecure (legacy) answers are simply ignored

					--

					-- If we get a Bogus (dnssec error) reply, keep the

					-- status around.  If there were only bogus replies, the

					-- connection will be aborted.  If there were at least

					-- one non-Bogus reply, we proceed.  If none of the

					-- replies matched, we consider the connection insecure.

					if (dane_answer.bogus or dane_answer.secure) and not dane then

						-- The first answer we care about

						-- For services with only one SRV record, this will be the only one

						dane = dane_answer;

					elseif dane_answer.bogus then

						dane.bogus = dane_answer.bogus;

					elseif dane_answer.secure then

						for _, dane_record in ipairs(dane_answer) do

							t_insert(dane, dane_record);

						end

					end

					if n == 0 then

						if dane then

							host_session.dane = dane;

							if #dane > 0 and dane.bogus then

								-- Got at least one non-bogus reply,

								-- This should trigger a failure if one of them did not match

								host_session.log("warn", "Ignoring bogus replies");

								dane.bogus = nil;

							end

							if #dane == 0 and dane.bogus == nil then

								-- Got no usable data

								host_session.dane = false;

							end

						end

						return cb(host_session);

					end

				end, ("_%d._tcp.%s."):format(record.srv.port, record.srv.target), "TLSA");

			end

		end, "_xmpp-server._tcp."..name..".", "SRV");

		return true;

	elseif host_session.direction == "outgoing" then

		-- Prosody has already done SRV lookups for outgoing session, so check if those are secure

		local srv_hosts = host_session.srv_hosts;

		if not ( srv_hosts and srv_hosts.answer and srv_hosts.answer.secure ) then

			return; -- No secure SRV records, fall back to non-DANE mode

		end

		-- Do TLSA lookup for currently selected SRV record

		local srv_choice = srv_hosts[host_session.srv_choice];

		host_session.dane = dns_lookup(function(answer)

			if answer and ((answer.secure and #answer > 0) or answer.bogus) then

				srv_choice.dane = answer;

			else

				srv_choice.dane = false;

			end

			host_session.dane = srv_choice.dane;

			return cb(host_session);

		end, ("_%d._tcp.%s."):format(srv_choice.port, srv_choice.target), "TLSA");

		return true;

	end

end

local function resume(host_session)

	host_session.log("debug", "DANE lookup completed, resuming connection");

	host_session.conn:resume()

end

function module.add_host(module)

	local function on_new_s2s(event)

		local host_session = event.origin;

		if host_session.type == "s2sout" or host_session.type == "s2sin" then

			return; -- Already authenticated

		end

		if host_session.dane ~= nil then

			return; -- Already done DANE lookup

		end

		if dane_lookup(host_session, resume) then

			host_session.log("debug", "Pausing connection until DANE lookup is completed");

			host_session.conn:pause()

		end

	end

	-- New outgoing connections

	module:hook("stanza/http://etherx.jabber.org/streams:features", on_new_s2s, 501);

	module:hook("s2sout-authenticate-legacy", on_new_s2s, 200);

	-- New incoming connections

	module:hook("s2s-stream-features", on_new_s2s, 10);

	module:hook("s2s-authenticated", function(event)

		local session = event.session;

		if session.dane and next(session.dane) ~= nil and not session.secure then

			-- TLSA record but no TLS, not ok.

			-- TODO Optional?

			-- Bogus replies should trigger this path

			-- How does this interact with Dialback?

			session:close({

				condition = "policy-violation",

				text = "Encrypted server-to-server communication is required but was not "

					..((session.direction == "outgoing" and "offered") or "used")

			});

			return false;

		end

		-- Cleanup

		session.dane = nil;

		session.srv_hosts = nil;

	end);

end

-- Compare one TLSA record against a certificate

local function one_dane_check(tlsa, cert)

	local select, match, certdata = tlsa.select, tlsa.match;

	if select == 0 then

		certdata = pem2der(cert:pem());

	elseif select == 1 and cert.pubkey then

		certdata = pem2der(cert:pubkey());

	else

		module:log("warn", "DANE selector %s is unsupported", tlsa:getSelector() or select);

		return;

	end

	if match == 1 then

		certdata = hashes.sha256(certdata);

	elseif match == 2 then

		certdata = hashes.sha512(certdata);

	elseif match ~= 0 then

		module:log("warn", "DANE match rule %s is unsupported", tlsa:getMatchType() or match);

		return;

	end

	if #certdata ~= #tlsa.data then

		module:log("warn", "Length mismatch: Cert: %d, TLSA: %d", #certdata, #tlsa.data);

	end

	return certdata == tlsa.data;

end

module:hook("s2s-check-certificate", function(event)

	local session, cert, host = event.session, event.cert, event.host;

	if not cert then return end

	local log = session.log or module._log;

	local dane = session.dane;

	if type(dane) == "table" then

		local match_found, supported_found;

		for i = 1, #dane do

			local tlsa = dane[i].tlsa;

			module:log("debug", "TLSA #%d: %s", i, tostring(tlsa))

			local use = tlsa.use;

			if enabled_uses:contains(use) then

				-- PKIX-EE or DANE-EE

				if use == 1 or use == 3 then

					-- Should we check if the cert subject matches?

					local is_match = one_dane_check(tlsa, cert);

					if is_match ~= nil then

						supported_found = true;

					end

					if is_match then

						log("info", "DANE validated ok for %s using %s", host, tlsa:getUsage());

						session.cert_identity_status = "valid";

						if use == 3 then -- DANE-EE, chain status equals DNSSEC chain status

							session.cert_chain_status = "valid";

							-- for usage 1, PKIX-EE, the chain has to be valid already

						end

						match_found = true;

						break;

					end

				elseif use == 0 or use == 2 then

					supported_found = true;

					local chain = session.conn:socket():getpeerchain();

					for c = 1, #chain do

						local cacert = chain[c];

						local is_match = one_dane_check(tlsa, cacert);

						if is_match ~= nil then

							supported_found = true;

						end

						if is_match and cacert:issued(cert, unpack(chain)) then

							log("info", "DANE validated ok for %s using %s", host, tlsa:getUsage());

							if use == 2 then -- DANE-TA

								session.cert_identity_status = "valid";

								if cert_verify_identity(host, "xmpp-server", cert) then

									session.cert_chain_status = "valid";

									-- else -- TODO Check against SRV target?

								end

								-- for usage 0, PKIX-CA, identity and chain has to be valid already

							end

							match_found = true;

							break;

						end

					end

					if match_found then break end

				end

			end

		end

		if supported_found and not match_found or dane.bogus then

			-- No TLSA matched or response was bogus

			local why = "No TLSA matched certificate";

			if dane.bogus then

				why = "Bogus: "..tostring(dane.bogus);

			end

			log("warn", "DANE validation failed for %s: %s", host, why);

			session.cert_identity_status = "invalid";

			session.cert_chain_status = "invalid";

		end

	else

		if session.cert_chain_status == "valid" and session.cert_identity_status ~= "valid"

		and session.srv_hosts and session.srv_hosts.answer and session.srv_hosts.answer.secure then

			local srv_hosts, srv_choice, srv_target = session.srv_hosts, session.srv_choice;

			for i = srv_choice or 1, srv_choice or #srv_hosts do

				srv_target = session.srv_hosts[i].target:gsub("%.?$","");

				log("debug", "Comparing certificate with Secure SRV target %s", srv_target);

				srv_target = nameprep(idna_to_unicode(srv_target));

				if srv_target and cert_verify_identity(srv_target, "xmpp-server", cert) then

					log("info", "Certificate for %s matches Secure SRV target %s", host, srv_target);

					session.cert_identity_status = "valid";

					return;

				end

			end

		end

	end

end);