Tor design proposals

Filename: 119-controlport-auth.txt
Title: New PROTOCOLINFO command for controllers
Author: Roger Dingledine
Created: 14-Aug-2007
Status: Closed
Implemented-In: 0.2.0.x

Overview:

  Here we describe how to help controllers locate the cookie
  authentication file when authenticating to Tor, so we can a) require
  authentication by default for Tor controllers and b) still keep
  things usable.  Also, we propose an extensible, general-purpose mechanism
  for controllers to learn about a Tor instance's protocol and
  authentication requirements before authenticating.

The Problem:

  When we first added the controller protocol, we wanted to make it
  easy for people to play with it, so by default we didn't require any
  authentication from controller programs. We allowed requests only from
  localhost as a stopgap measure for security.

  Due to an increasing number of vulnerabilities based on this approach,
  it's time to add authentication in default configurations.

  We have a number of goals:
  - We want the default Vidalia bundles to transparently work. That
    means we don't want the users to have to type in or know a password.
  - We want to allow multiple controller applications to connect to the
    control port. So if Vidalia is launching Tor, it can't just keep the
    secrets to itself.

  Right now there are three authentication approaches supported
  by the control protocol: NULL, CookieAuthentication, and
  HashedControlPassword. See Sec 5.1 in control-spec.txt for details.

  There are a couple of challenges here. The first is: if the controller
  launches Tor, how should we teach Tor what authentication approach
  it should require, and the secret that goes along with it? Next is:
  how should this work when the controller attaches to an existing Tor,
  rather than launching Tor itself?

  Cookie authentication seems most amenable to letting multiple controller
  applications interact with Tor. But that brings in yet another question:
  how does the controller guess where to look for the cookie file,
  without first knowing what DataDirectory Tor is using?

Design:

  We should add a new controller command PROTOCOLINFO that can be sent
  as a valid first command (the others being AUTHENTICATE and QUIT). If
  PROTOCOLINFO is sent as the first command, the second command must be
  either a successful AUTHENTICATE or a QUIT.

  If the initial command sequence is not valid, Tor closes the connection.


Spec:

  C:  "PROTOCOLINFO" *(SP PIVERSION) CRLF
  S:  "250+PROTOCOLINFO" SP PIVERSION CRLF *InfoLine "250 OK" CRLF

    InfoLine = AuthLine / VersionLine / OtherLine

     AuthLine = "250-AUTH" SP "METHODS=" AuthMethod *(",")AuthMethod
                       *(SP "COOKIEFILE=" AuthCookieFile) CRLF
     VersionLine = "250-VERSION" SP "Tor=" TorVersion [SP Arguments] CRLF

     AuthMethod =
      "NULL"           / ; No authentication is required
      "HASHEDPASSWORD" / ; A controller must supply the original password
      "COOKIE"         / ; A controller must supply the contents of a cookie

     AuthCookieFile = QuotedString
     TorVersion = QuotedString

     OtherLine = "250-" Keyword [SP Arguments] CRLF

  For example:

  C: PROTOCOLINFO CRLF
  S: "250+PROTOCOLINFO 1" CRLF
  S: "250-AUTH Methods=HASHEDPASSWORD,COOKIE COOKIEFILE="/tor/cookie"" CRLF
  S: "250-VERSION Tor=0.2.0.5-alpha" CRLF
  S: "250 OK" CRLF

  Tor MAY give its InfoLines in any order; controllers MUST ignore InfoLines
  with keywords it does not recognize.  Controllers MUST ignore extraneous
  data on any InfoLine.

  PIVERSION is there in case we drastically change the syntax one day. For
  now it should always be "1", for the controller protocol.  Controllers MAY
  provide a list of the protocol versions they support; Tor MAY select a
  version that the controller does not support.

  Right now only two "topics" (AUTH and VERSION) are included, but more
  may be included in the future. Controllers must accept lines with
  unexpected topics.

  AuthCookieFile = QuotedString

  AuthMethod is used to specify one or more control authentication
  methods that Tor currently accepts.

  AuthCookieFile specifies the absolute path and filename of the
  authentication cookie that Tor is expecting and is provided iff
  the METHODS field contains the method "COOKIE".  Controllers MUST handle
  escape sequences inside this string.

  The VERSION line contains the Tor version.

  [What else might we want to include that could be useful? -RD]

Compatibility:

  Tor 0.1.2.16 and 0.2.0.4-alpha hang up after the first failed
  command. Earlier Tors don't know about this command but don't hang
  up. That means controllers will need a mechanism for distinguishing
  whether they're talking to a Tor that speaks PROTOCOLINFO or not.

  I suggest that the controllers attempt a PROTOCOLINFO. Then:
    - If it works, great. Authenticate as required.
    - If they get hung up on, reconnect and do a NULL AUTHENTICATE.
    - If it's unrecognized but they're not hung up on, do a NULL
      AUTHENTICATE.

Unsolved problems:

  If Torbutton wants to be a Tor controller one day... talking TCP is
  bad enough, but reading from the filesystem is even harder. Is there
  a way to let simple programs work with the controller port without
  needing all the auth infrastructure?

  Once we put this approach in place, the next vulnerability we see will
  involve an attacker somehow getting read access to the victim's files
  --- and then we're back where we started. This means we still need
  to think about how to demand password-based authentication without
  bothering the user about it.