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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