Re: [OPSAWG] Declaring something to be a controller in MUD

Michael Richardson <> Tue, 25 June 2019 19:52 UTC

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From: Michael Richardson <>
To: Eliot Lear <>
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Date: Tue, 25 Jun 2019 15:52:26 -0400
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Subject: Re: [OPSAWG] Declaring something to be a controller in MUD
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Eliot Lear <> wrote:
    > A few of us are just trying to put out an initial draft that addresses
    > one gap in MUD (there are several).

    > In a MUD file one can say that one
    > wants to access a controller in two ways: either "my-controller”
    > meaning a controller that services devices of a particular MUD URL or a
    > “controller” class that services devices based on a particular class
    > name of controller.

I think that we have two potential avenues for security attacks here:
  1) a device that claims to be a controller in order to gain access to
  2) devices that claim to be belong to a controller in order to attack

To my mind there are some different things we could do:

1) insist that to user the my-controller connections that the two devices
   have to be signed by the "same" entity.  ["same" could mean literal the
   same key, the same certificate Issuer/DN,  or something more complex]

2) we could have devices declare in an MUD extension the
   DN/certificate/entity which must sign their controller device.

3) (2) above, but with some level of indirection through some URL.

    > In either case, right now the administrator has to manually know and
    > populate information, to say - some device is a controller,
    > either for MUD URL <> or
    > a class <>.
    > That can be laborious.  To assist, we are examining ways to have a
    > controller declare itself as a candidate controller.  That at least
    > provides a hint to the administrator that this particular device is
    > capable of serving in a particular role.

I think that anything that requires administrator activity to be a fail for
residential use.  It's too complex.

    > To make that declaration, the device must- Form the declaration; Find
    > the MUD manager; and Send it.

    > Finding the MUD manager depends on one question: Was the device built
    > to be a controller or is it a general purpose device that has an app
    > that is intended to be a controller?

    > If the device was built to be a controller, we can simply cram the
    > declaration into that devices own MUD file as an extension.  If the
    > device is a general purpose computer, things get a bit more
    > interesting.

Yes... but I think that we have to solve the multi-purpose computer MUD
anyway.  The intelligent speakers (Echo,Home,Mycroft,etc.) need to gain new
MUD definitions as they gain "skills", and I think that we can treat a
smartphone in a similar way.

This might be a place where IPv6 wins, if we can split off each skill into a
new provisioning domain, giving it a new IPv6 IID.  I was thinking that maybe
we can associate the private key that signed the MUD file to the IID via
something like SEND/CGA, but I'm not sure how many private keys we have
(one for the app developer, or one per app installed on each device).

    > In this case we have two choices:

    > Either create a MUD file that points somewhere internally - this
    > doesn’t seem very plug and play.  Make the declaration directly to the
    > MUD manager.

    > I’m going to focus on the latter for the moment.  It is easy enough to
    > create a RESTful interface for this purpose, but it requires a
    > mechanism to discovered the MUD manager, which up until now has been an
    > internal part of the network infrastructure.

    > Let me call this out plainly: letting the app itself directly call the
    > MUD manager requires that the MUD manager itself become exposed to the
    > user infrastructure, which is a change.

And that the MUD manager have some reason to trust the device is not p0wned,
and sending a bogus MUD file up.   A certificate chain back to the
manufacturer is not enough, it has to be signed by a key that an attacker
can't get access to.  So that requires attested keys if they are "local", or
for the signature to be done elsewhere.

    > One possibility to address this is to incorporate the new RESTful
    > endpoint into an ANIMA BRSKI join registrar, which may already be
    > exposed.  But that requires that ANIMA BRSKI be in play, which it may
    > not.

It is, however, a really good idea for the case where it is in play.

    > My thinking is that we do this work in two stages.  First handle the
    > easy case, which is the MUD file extension, and then figure out how to
    > do the app version of this.

    > Thoughts?


Michael Richardson <>ca>, Sandelman Software Works
 -= IPv6 IoT consulting =-