Hi Chris,

Just a thought or two:

First, the replay attack code is initially turned off.  If you turn it on with a command, and it does not hear another command for n minutes/hours (I don't remember) it turns it back off.  Once it hears a command for the first time after replay prevention is turned on, then it stays on until/unless it gets that special command to turn it off.

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.

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!)

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
>> >
>> >
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--
Chris E. Thompson
chrisethompson@gmail.com
g0kla@arrl.net

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