auth: add basic authentication filter framework

This leverages the new lua support. See
filters/simple-authentication.lua for explaination of how this works.
There is also additional documentation in cgitrc.5.txt.

Though this is a cookie-based approach, cgit's caching mechanism is
preserved for authenticated pages.

Very plugable and extendable depending on user needs.

The sample script uses an HMAC-SHA1 based cookie to store the
currently logged in user, with an expiration date.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

filters/simple-authentication.lua

@@ -0,0 +1,225 @@
+-- This script may be used with the auth-filter. Be sure to configure it as you wish.
+--
+-- Requirements:
+-- 	luacrypto >= 0.3
+-- 	<http://mkottman.github.io/luacrypto/>
+--
+
+
+--
+--
+-- Configure these variables for your settings.
+--
+--
+
+local protected_repos = {
+	glouglou	= { laurent = true, jason = true },
+	qt		= { jason = true, bob = true }
+}
+
+local users = {
+	jason		= "secretpassword",
+	laurent		= "s3cr3t",
+	bob		= "ilikelua"
+}
+
+local secret = "BE SURE TO CUSTOMIZE THIS STRING TO SOMETHING BIG AND RANDOM"
+
+
+
+--
+--
+-- Authentication functions follow below. Swap these out if you want different authentication semantics.
+--
+--
+
+-- Sets HTTP cookie headers based on post
+function authenticate_post()
+	local password = users[post["username"]]
+	-- TODO: Implement time invariant string comparison function to mitigate against timing attack.
+	if password == nil or password ~= post["password"] then
+		construct_cookie("", "cgitauth")
+	else
+		construct_cookie(post["username"], "cgitauth")
+	end
+	return 0
+end
+
+
+-- Returns 1 if the cookie is valid and 0 if it is not.
+function authenticate_cookie()
+	accepted_users = protected_repos[cgit["repo"]]
+	if accepted_users == nil then
+		-- We return as valid if the repo is not protected.
+		return 1
+	end
+
+	local username = validate_cookie(get_cookie(http["cookie"], "cgitauth"))
+	if username == nil or not accepted_users[username] then
+		return 0
+	else
+		return 1
+	end
+end
+
+-- Prints the html for the login form.
+function body()
+	html("<h2>Authentication Required</h2>")
+	html("<form method='post' action='")
+	html_attr(cgit["login"])
+	html("'>")
+	html("<table>")
+	html("<tr><td><label for='username'>Username:</label></td><td><input id='username' name='username' autofocus /></td></tr>")
+	html("<tr><td><label for='password'>Password:</label></td><td><input id='password' name='password' type='password' /></td></tr>")
+	html("<tr><td colspan='2'><input value='Login' type='submit' /></td></tr>")
+	html("</table></form>")
+
+	return 0
+end
+
+
+--
+--
+-- Cookie construction and validation helpers.
+--
+--
+
+local crypto = require("crypto")
+
+-- Returns username of cookie if cookie is valid. Otherwise returns nil.
+function validate_cookie(cookie)
+	local i = 0
+	local username = ""
+	local expiration = 0
+	local salt = ""
+	local hmac = ""
+
+	if cookie:len() < 3 or cookie:sub(1, 1) == "|" then
+		return nil
+	end
+
+	for component in string.gmatch(cookie, "[^|]+") do
+		if i == 0 then
+			username = component
+		elseif i == 1 then
+			expiration = tonumber(component)
+			if expiration == nil then
+				expiration = 0
+			end
+		elseif i == 2 then
+			salt = component
+		elseif i == 3 then
+			hmac = component
+		else
+			break
+		end
+		i = i + 1
+	end
+
+	if hmac == nil or hmac:len() == 0 then
+		return nil
+	end
+
+	-- TODO: implement time invariant comparison to prevent against timing attack.
+	if hmac ~= crypto.hmac.digest("sha1", username .. "|" .. tostring(expiration) .. "|" .. salt, secret) then
+		return nil
+	end
+
+	if expiration <= os.time() then
+		return nil
+	end
+
+	return username:lower()
+end
+
+function construct_cookie(username, cookie)
+	local authstr = ""
+	if username:len() > 0 then
+		-- One week expiration time
+		local expiration = os.time() + 604800
+		local salt = crypto.hex(crypto.rand.bytes(16))
+
+		authstr = username .. "|" .. tostring(expiration) .. "|" .. salt
+		authstr = authstr .. "|" .. crypto.hmac.digest("sha1", authstr, secret)
+	end
+
+	html("Set-Cookie: " .. cookie .. "=" .. authstr .. "; HttpOnly")
+	if http["https"] == "yes" or http["https"] == "on" or http["https"] == "1" then
+		html("; secure")
+	end
+	html("\n")
+end
+
+--
+--
+-- Wrapper around filter API follows below, exposing the http table, the cgit table, and the post table to the above functions.
+--
+--
+
+local actions = {}
+actions["authenticate-post"] = authenticate_post
+actions["authenticate-cookie"] = authenticate_cookie
+actions["body"] = body
+
+function filter_open(...)
+	action = actions[select(1, ...)]
+
+	http = {}
+	http["cookie"] = select(2, ...)
+	http["method"] = select(3, ...)
+	http["query"] = select(4, ...)
+	http["referer"] = select(5, ...)
+	http["path"] = select(6, ...)
+	http["host"] = select(7, ...)
+	http["https"] = select(8, ...)
+
+	cgit = {}
+	cgit["repo"] = select(9, ...)
+	cgit["page"] = select(10, ...)
+	cgit["url"] = select(11, ...)
+
+	cgit["login"] = ""
+	for _ in cgit["url"]:gfind("/") do
+		cgit["login"] = cgit["login"] .. "../"
+	end
+	cgit["login"] = cgit["login"] .. "?p=login"
+
+end
+
+function filter_close()
+	return action()
+end
+
+function filter_write(str)
+	post = parse_qs(str)
+end
+
+
+--
+--
+-- Utility functions follow below, based on keplerproject/wsapi.
+--
+--
+
+function url_decode(str)
+	if not str then
+		return ""
+	end
+	str = string.gsub(str, "+", " ")
+	str = string.gsub(str, "%%(%x%x)", function(h) return string.char(tonumber(h, 16)) end)
+	str = string.gsub(str, "\r\n", "\n")
+	return str
+end
+
+function parse_qs(qs)
+	local tab = {}
+	for key, val in string.gmatch(qs, "([^&=]+)=([^&=]*)&?") do
+		tab[url_decode(key)] = url_decode(val)
+	end
+	return tab
+end
+
+function get_cookie(cookies, name)
+	cookies = string.gsub(";" .. cookies .. ";", "%s*;%s*", ";")
+	return url_decode(string.match(cookies, ";" .. name .. "=(.-);"))
+end