You are not understanding.  The reset value always goes up.  When it increments, the seconds restart at 0, but together they always go up.  I actually refuse to accept the command if the reset (I call it the epoch) has changed.

I don't know how an attacker could force me to send a packet.  But as I say, I would not plan to use this feature.  Feel free to read my paper about this in the AMSAT Symposium Procedings (maybe 2016 or 17).  I'm not saying it is perfect, but I think the issues are smaller than you suggest.

73,

Burns Fisher, WB1FJ
AMSAT(R) Engineering -- Flight Software


On Tue, Sep 19, 2023 at 6:13 PM Corey Minyard <minyard@acm.org> wrote:
Comments inline...

On Tue, Sep 19, 2023 at 09:11:22PM +0000, Burns Fisher (AMSAT) via pacsat-dev wrote:
> See below.
>
> 73,
>
> Burns Fisher, WB1FJ
> *AMSAT(R) Engineering -- Flight Software*
>
>
> On Tue, Sep 19, 2023 at 4:59 PM Corey Minyard <minyard@acm.org> wrote:
>
> > On Tue, Sep 19, 2023 at 07:19:27PM +0000, Burns Fisher (AMSAT) via
> > pacsat-dev wrote:
> >
> > >
> > > One thing that a serial number does not prevent is a replay attack that
> > > happens after the ground station sends a command that was not received by
> > > the s/c.  Suppose we wanted to turn on some device like a camera over a
> > > certain area.  Send the command as the s/c approaches the area, but for
> > > whatever reason the s/c does not receive it (but the bad guy does).  The
> > > bad guy then sends the command later...maybe he even sends the info to a
> > > conspirator around the world and that person sends it.  I think with
> > just a
> > > serial number the s/c would still respond to it if there had been no
> > > command in between.  Depending on the exact command it may cause no
> > > problem, but that is what the "no later than 300 seconds after the s/c
> > > time" is for.
> >
> > This is true.  I've never had to deal with a situation like this, so
> > maybe that's where the weirdness comes from.
> >
> > It also seems to me that relying on s/c time is not a robust solution.
> > I don't know how robust ground information about that time is valid, but
> > a protocol that didn't rely on that would be better, I think.
> >
> >
> Every telemetry frame downlink includes the spacecraft time.  300 seconds
> gives plenty of slack.  And as I said AmCom can do this more-or-less
> automatically.

It sounds like you cannot use the s/c time for a sequence number,
because it can be reset.  You cannot rely on it always going up.  So you
would need to send two values, one sequence number and the spacecraft
time as separate values.  In that case, I believe this works.

But that's only the case if the telemetry is signed somehow so it
cannot be spoofed by an attacker.  If an attacker can spoof the
telemetry, it can send a future time to the ground station and cause it
to send the command with that time, which could then be used later.

>
>
>
> > >
> > > I'm not saying reset/seconds is the only way...just mentioning a few
> > things
> > > that we do to make it easier:  AmCom keeps track of the spacecraft time
> > (if
> > > the s/c does not reset, and then has to wait till we determine time 0
> > (the
> > > UTC that corresponds to the start of the new reset epoch).  If the s/c is
> > > unresponsive, I think I'd send "turn off replay protection" in the blind
> > > and go from there.  (Note: Fox-1E and HuskySat has/had this capability,
> > but
> > > we never turned it on!)
> >
> > Would not the "turn off replay protection" be subject to the same issue?
> >
>
> I believe that Chris said earlier that this particular command does not
> require the time.  But it is also (hopefully) never used so one could not
> easily record it.  I don't really like this, but after/If we get confidence
> in this replay protection, it could be removed.  As I said, it has been on
> a few s/c, but never actually used.

From what I can tell, it would be easy for an attacker to force you to
send this packet.

I'm not sure how much we really need to worry about this.  But if we
need to worry about it, we need a robust protocol.

I'm not a professional cryptographer, so I'm not sure how robust either
the time or two command protocol is against attack.  I also haven't
spent a long time thinking about it.  But I know that if you rely on
anything unauthenticated from the spacecraft, you are in trouble.

-corey

>
>
>
> >
> > Thinking this through a little, the way I would design this would be to
> > allow single commands for things where this isn't important.
> >
> > For things where an attacker could cause issues with a intercept and
> > delay tactic, have a two-command operation.  The first command says
> > "turn on the camera".  The spacecraft then returns some sort of indicator
> > that the command is pending.  After receiving the indicator, the second
> > command is sent to commit the operation.  If the commit is not received
> > after a short period of time, the command is discarded.
> >
> > The problem is that the operation is now more likely to fail from a
> > missed command.  Oh, and the attacker could spoof the indicator from the
> > s/c, so you would have to have something in that indicator (like using
> > sha256) to prevent that.
> >
> > I'm pretty sure (but I don't have a formal proof, just gut feel from
> > stuff I worked on) that you cannot design a reasonable protocol without
> > some sort of shared state (time on the s/c) or some sort of indication
> > from the s/c.
> >
> > -corey - AE5KM
> >
> > >
> > > 73,
> > >
> > > Burns Fisher, WB1FJ
> > > *AMSAT(R) Engineering -- Flight Software*
> > >
> > >
> > > On Tue, Sep 19, 2023 at 1:58 PM Chris Thompson via pacsat-dev <
> > > pacsat-dev@amsat.org> wrote:
> > >
> > > > Corey, I was delayed by another bug but managed to look at this
> > today.  It
> > > > looks very good.
> > > >
> > > > I can calculate the same hash value with the default key using the Java
> > > > Mac class and the HmacSHA256 algorithm.  So that is great.  It uses the
> > > > public algorithm.
> > > >
> > > > I did notice that the default key in the code you implemented is 28
> > bytes
> > > > and not 32 bytes as you stated in an earlier email.  Is that correct?
> > It
> > > > works with the 28 byte key.
> > > >
> > > > One other thing.  The existing code prevents a replay attack by sending
> > > > the reset/uptime on the spacecraft in the command.  We have to send the
> > > > same reset and an uptime that is the same or no more than 300 seconds
> > after
> > > > the actual uptime on the spacecraft.  I think this could cause issues
> > with
> > > > an unresponsive spacecraft where we don't know the time onboard. I'm
> > sure
> > > > that is why there is a special command that goes around the
> > requirement and
> > > > turns it off.
> > > >
> > > > I would instead propose we send a serial number in the command, with no
> > > > other meaning.  Any command can only be used once and we store the
> > highest
> > > > serial number used so far.  The serial number would be in the hash of
> > the
> > > > command of course.  To make it easy to sync the serial numbers across
> > > > different command stations I propose that the serial number be unix
> > time in
> > > > UTC, or the time based on a later epoch if we want something with less
> > than
> > > > 32 bits.  It will just be treated as a serial number that is checked
> > for
> > > > duplicates when received at the spacecraft.  Specifically all commands
> > must
> > > > have a serial number greater than the largest one received so far.
> > > >
> > > > Maybe that is not secure enough.  Let me know.
> > > >
> > > > 73
> > > > Chris
> > > >
> > > >
> > > > On Sat, Sep 16, 2023 at 6:56 PM Chris Thompson via pacsat-dev <
> > > > pacsat-dev@amsat.org> wrote:
> > > >
> > > >> Sounds very good.  I will try to have a look at it tomorrow or Monday.
> > > >>
> > > >> 73
> > > >> Chris
> > > >>
> > > >> On Sat, Sept 16, 2023, 15:53 Corey Minyard <minyard@acm.org> wrote:
> > > >>
> > > >>> On Sat, Sep 16, 2023 at 1:58 PM Chris Thompson via pacsat-dev
> > > >>> <pacsat-dev@amsat.org> wrote:
> > > >>> >
> > > >>> > Just for reference, the sha.c algorithm did match what I get from
> > Java
> > > >>> with the standard sha-256 digest.  So it does work for the 18 bytes
> > of our
> > > >>> commands.  But it sounds like it will not work if we append the
> > secret key
> > > >>> twice.
> > > >>>
> > > >>> Yes, my impetus for replacing it, plus the new algorithm allows
> > > >>> partial adds of data to make the process easier.
> > > >>>
> > > >>> I have pushed up an update-sha-hmac branch that reworks all the
> > > >>> keying.  It uses the hmac-sha256 algorithm for authentication, it
> > adds
> > > >>> the code to load the key from NVRAM, and reworks the keysize to be 32
> > > >>> bytes.  Reworking the key size will rearrange NVRAM, unfortunately.
> > > >>> With this, you should be able to add a key from the command line and
> > > >>> have it write it to NVRAM, and use it for the hmac-sha256
> > > >>> authentication.  It does remove the AES code.
> > > >>>
> > > >>> -corey - AE5KM
> > > >>>
> > > >>> >
> > > >>> > 73
> > > >>> > Chris
> > > >>> >
> > > >>> > On Sat, Sept 16, 2023, 12:49 Corey Minyard via pacsat-dev <
> > > >>> pacsat-dev@amsat.org> wrote:
> > > >>> >>
> > > >>> >> I just pulled Authenticate/src/sha.c out of the code and moved it
> > into
> > > >>> >> a separate file and played with it a bit.  It wasn't matching the
> > > >>> >> results from sha256sum, and looking at the code, I realized that
> > the
> > > >>> >> implementation only accepts up to 64 bytes of data.  It works for
> > > >>> >> buffers less than 64 bytes.  It also won't do partial pieces,
> > which
> > > >>> >> would make the implementation of HMAC easier.
> > > >>> >>
> > > >>> >> I'm going to recommend we adapt
> > https://github.com/h5p9sl/hmac_sha256
> > > >>> >> to our needs.  I'll work on that a bit.
> > > >>> >>
> > > >>> >> Also, I couldn't find any evidence of any cryptanalysis of
> > encrypting
> > > >>> >> the sha256 output with AES.  Sometimes those things work,
> > sometimes
> > > >>> >> you get surprising results. Since the HMAC approach is well known
> > and
> > > >>> >> heavily analyzed, that would seem a better approach.
> > > >>> >>
> > > >>> >> -corey - AE5KM
> > > >>> >>
> > > >>> >> On Fri, Sep 15, 2023 at 1:21 PM Chris Thompson via pacsat-dev
> > > >>> >> <pacsat-dev@amsat.org> wrote:
> > > >>> >> >
> > > >>> >> > I did not implement it yet.  It would go in Command task.c and
> > > >>> replace or perhaps duplicate the authenticate function.
> > > >>> >> >
> > > >>> >> > Feel free to code it.
> > > >>> >> >
> > > >>> >> > I don't know if we will ultimately go this way.  I would still
> > like
> > > >>> to make the AES authentication work but I agree this could be
> > simpler and
> > > >>> faster.  So it would be good to test it.
> > > >>> >> >
> > > >>> >> > Chris
> > > >>> >> >
> > > >>> >> > On Fri, Sept 15, 2023, 11:20 Corey Minyard <minyard@acm.org>
> > wrote:
> > > >>> >> >>
> > > >>> >> >> On Fri, Sep 15, 2023 at 10:06 AM Chris Thompson via pacsat-dev
> > > >>> >> >> <pacsat-dev@amsat.org> wrote:
> > > >>> >> >> >
> > > >>> >> >> > Ok, thanks for that Corey.  Very interesting.  We may not be
> > > >>> susceptible to the length extension attack vulnerability though.  If
> > I
> > > >>> understand correctly, then a message sent as: Hash( key + "Watch the
> > > >>> enemy") could be manipulated to Hash(key + "Watch the enemy and
> > attack them
> > > >>> after 5 mins"), without knowing the key.  But our commands are fixed
> > at 18
> > > >>> bytes length (for now at least). So any extra appended message would
> > be
> > > >>> ignored.  With that said, it may not be much harder to implement the
> > scheme
> > > >>> as described.
> > > >>> >> >>
> > > >>> >> >> Yes, I was more worried about the "various security papers have
> > > >>> >> >> suggested vulnerabilities with this approach" comment in the
> > > >>> article
> > > >>> >> >> on the key || message || key approach.  It probably means
> > there are
> > > >>> >> >> other issues with the approach, possibly key extraction
> > attacks.
> > > >>> The
> > > >>> >> >> HMAC approach seems generally more cryptographically sound.
> > > >>> >> >>
> > > >>> >> >> I was going to say that I could implement it, though it's
> > pretty
> > > >>> >> >> trivial.  You've probably already done it :).
> > > >>> >> >>
> > > >>> >> >> -corey - AE5KM
> > > >>> >> >
> > > >>> >> >
> > > >>> >> > -----------------------------------------------------------
> > > >>> >> >
> > > >>> >> > pacsat-dev mailing list -- pacsat-dev@amsat.org
> > > >>> >> > View archives of this mailing list at
> > > >>> https://mailman.amsat.org/hyperkitty/list/pacsat-dev@amsat.org
> > > >>> >> > To unsubscribe send an email to pacsat-dev-leave@amsat.org
> > > >>> >> > Manage all of your AMSAT-NA mailing list preferences at
> > > >>> https://mailman.amsat.org
> > > >>> >>
> > > >>> >> -----------------------------------------------------------
> > > >>> >>
> > > >>> >> pacsat-dev mailing list -- pacsat-dev@amsat.org
> > > >>> >> View archives of this mailing list at
> > > >>> https://mailman.amsat.org/hyperkitty/list/pacsat-dev@amsat.org
> > > >>> >> To unsubscribe send an email to pacsat-dev-leave@amsat.org
> > > >>> >> Manage all of your AMSAT-NA mailing list preferences at
> > > >>> https://mailman.amsat.org
> > > >>> >
> > > >>> >
> > > >>> > -----------------------------------------------------------
> > > >>> >
> > > >>> > pacsat-dev mailing list -- pacsat-dev@amsat.org
> > > >>> > View archives of this mailing list at
> > > >>> https://mailman.amsat.org/hyperkitty/list/pacsat-dev@amsat.org
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> > > >>> > Manage all of your AMSAT-NA mailing list preferences at
> > > >>> https://mailman.amsat.org
> > > >>>
> > > >>
> > > >> -----------------------------------------------------------
> > > >>
> > > >> pacsat-dev mailing list -- pacsat-dev@amsat.org
> > > >> View archives of this mailing list at
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> > > >> Manage all of your AMSAT-NA mailing list preferences at
> > > >> https://mailman.amsat.org
> > > >>
> > > >
> > > >
> > > > --
> > > > Chris E. Thompson
> > > > chrisethompson@gmail.com
> > > > g0kla@arrl.net
> > > >
> > > > -----------------------------------------------------------
> > > >
> > > > pacsat-dev mailing list -- pacsat-dev@amsat.org
> > > > View archives of this mailing list at
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> > > > https://mailman.amsat.org
> > > >
> >
> > >
> > > -----------------------------------------------------------
> > >
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> >
> >

>
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