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+-- This script may be used with the auth-filter.
+--
+-- Requirements:
+--      luacrypto >= 0.3
+--      <http://mkottman.github.io/luacrypto/>
+--      luaposix
+--      <https://github.com/luaposix/luaposix>
+--
+local sysstat = require("posix.sys.stat")
+local unistd = require("posix.unistd")
+local crypto = require("crypto")
+
+
+-- This file should contain a series of lines in the form of:
+--      username1:hash1
+--      username2:hash2
+--      username3:hash3
+--      ...
+-- Hashes can be generated using something like `mkpasswd -m sha-512 -R 300000`.
+-- This file should not be world-readable.
+local users_filename = "/etc/cgit-auth/users"
+
+-- This file should contain a series of lines in the form of:
+--      groupname1:username1,username2,username3,...
+--      ...
+local groups_filename = "/etc/cgit-auth/groups"
+
+-- This file should contain a series of lines in the form of:
+--      reponame1:groupname1,groupname2,groupname3,...
+--      ...
+local repos_filename = "/etc/cgit-auth/repos"
+
+-- Set this to a path this script can write to for storing a persistent
+-- cookie secret, which should not be world-readable.
+local secret_filename = "/var/cache/cgit/auth-secret"
+
+--
+--
+-- Authentication functions follow below. Swap these out if you want different authentication semantics.
+--
+--
+
+-- Looks up a hash for a given user.
+function lookup_hash(user)
+        local line
+        for line in io.lines(users_filename) do
+                local u, h = string.match(line, "(.-):(.+)")
+                if u:lower() == user:lower() then
+                        return h
+                end
+        end
+        return nil
+end
+
+-- Looks up users for a given repo.
+function lookup_users(repo)
+        local users = nil
+        local groups = nil
+        local line, group, user
+        for line in io.lines(repos_filename) do
+                local r, g = string.match(line, "(.-):(.+)")
+                if r == repo then
+                        groups = { }
+                        for group in string.gmatch(g, "([^,]+)") do
+                                groups[group:lower()] = true
+                        end
+                        break
+                end
+        end
+        if groups == nil then
+                return nil
+        end
+        for line in io.lines(groups_filename) do
+                local g, u = string.match(line, "(.-):(.+)")
+                if groups[g:lower()] then
+                        if users == nil then
+                                users = { }
+                        end
+                        for user in string.gmatch(u, "([^,]+)") do
+                                users[user:lower()] = true
+                        end
+                end
+        end
+        return users
+end
+
+
+-- Sets HTTP cookie headers based on post and sets up redirection.
+function authenticate_post()
+        local hash = lookup_hash(post["username"])
+        local redirect = validate_value("redirect", post["redirect"])
+
+        if redirect == nil then
+                not_found()
+                return 0
+        end
+
+        redirect_to(redirect)
+
+        if hash == nil or hash ~= unistd.crypt(post["password"], hash) then
+                set_cookie("cgitauth", "")
+        else
+                -- One week expiration time
+                local username = secure_value("username", post["username"], os.time() + 604800)
+                set_cookie("cgitauth", username)
+        end
+
+        html("\n")
+        return 0
+end
+
+
+-- Returns 1 if the cookie is valid and 0 if it is not.
+function authenticate_cookie()
+        accepted_users = lookup_users(cgit["repo"])
+        if accepted_users == nil then
+                -- We return as valid if the repo is not protected.
+                return 1
+        end
+
+        local username = validate_value("username", get_cookie(http["cookie"], "cgitauth"))
+        if username == nil or not accepted_users[username:lower()] 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("<input type='hidden' name='redirect' value='")
+        html_attr(secure_value("redirect", cgit["url"], 0))
+        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
+
+
+
+--
+--
+-- Wrapper around filter API, 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"] = select(12, ...)
+
+end
+
+function filter_close()
+        return action()
+end
+
+function filter_write(str)
+        post = parse_qs(str)
+end
+
+
+--
+--
+-- Utility functions 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 url_encode(str)
+        if not str then
+                return ""
+        end
+        str = string.gsub(str, "\n", "\r\n")
+        str = string.gsub(str, "([^%w ])", function(c) return string.format("%%%02X", string.byte(c)) end)
+        str = string.gsub(str, " ", "+")
+        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
+
+
+--
+--
+-- Cookie construction and validation helpers.
+--
+--
+
+local secret = nil
+
+-- Loads a secret from a file, creates a secret, or returns one from memory.
+function get_secret()
+        if secret ~= nil then
+                return secret
+        end
+        local secret_file = io.open(secret_filename, "r")
+        if secret_file == nil then
+                local old_umask = sysstat.umask(63)
+                local temporary_filename = secret_filename .. ".tmp." .. crypto.hex(crypto.rand.bytes(16))
+                local temporary_file = io.open(temporary_filename, "w")
+                if temporary_file == nil then
+                        os.exit(177)
+                end
+                temporary_file:write(crypto.hex(crypto.rand.bytes(32)))
+                temporary_file:close()
+                unistd.link(temporary_filename, secret_filename) -- Intentionally fails in the case that another process is doing the same.
+                unistd.unlink(temporary_filename)
+                sysstat.umask(old_umask)
+                secret_file = io.open(secret_filename, "r")
+        end
+        if secret_file == nil then
+                os.exit(177)
+        end
+        secret = secret_file:read()
+        secret_file:close()
+        if secret:len() ~= 64 then
+                os.exit(177)
+        end
+        return secret
+end
+
+-- Returns value of cookie if cookie is valid. Otherwise returns nil.
+function validate_value(expected_field, cookie)
+        local i = 0
+        local value = ""
+        local field = ""
+        local expiration = 0
+        local salt = ""
+        local hmac = ""
+
+        if cookie == nil or cookie:len() < 3 or cookie:sub(1, 1) == "|" then
+                return nil
+        end
+
+        for component in string.gmatch(cookie, "[^|]+") do
+                if i == 0 then
+                        field = component
+                elseif i == 1 then
+                        value = component
+                elseif i == 2 then
+                        expiration = tonumber(component)
+                        if expiration == nil then
+                                expiration = -1
+                        end
+                elseif i == 3 then
+                        salt = component
+                elseif i == 4 then
+                        hmac = component
+                else
+                        break
+                end
+                i = i + 1
+        end
+
+        if hmac == nil or hmac:len() == 0 then
+                return nil
+        end
+
+        -- Lua hashes strings, so these comparisons are time invariant.
+        if hmac ~= crypto.hmac.digest("sha256", field .. "|" .. value .. "|" .. tostring(expiration) .. "|" .. salt, get_secret()) then
+                return nil
+        end
+
+        if expiration == -1 or (expiration ~= 0 and expiration <= os.time()) then
+                return nil
+        end
+
+        if url_decode(field) ~= expected_field then
+                return nil
+        end
+
+        return url_decode(value)
+end
+
+function secure_value(field, value, expiration)
+        if value == nil or value:len() <= 0 then
+                return ""
+        end
+
+        local authstr = ""
+        local salt = crypto.hex(crypto.rand.bytes(16))
+        value = url_encode(value)
+        field = url_encode(field)
+        authstr = field .. "|" .. value .. "|" .. tostring(expiration) .. "|" .. salt
+        authstr = authstr .. "|" .. crypto.hmac.digest("sha256", authstr, get_secret())
+        return authstr
+end
+
+function set_cookie(cookie, value)
+        html("Set-Cookie: " .. cookie .. "=" .. value .. "; HttpOnly")
+        if http["https"] == "yes" or http["https"] == "on" or http["https"] == "1" then
+                html("; secure")
+        end
+        html("\n")
+end
+
+function redirect_to(url)
+        html("Status: 302 Redirect\n")
+        html("Cache-Control: no-cache, no-store\n")
+        html("Location: " .. url .. "\n")
+end
+
+function not_found()
+        html("Status: 404 Not Found\n")
+        html("Cache-Control: no-cache, no-store\n\n")
+end