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.
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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