Re: Using SHA256 for authentication
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.
73 Chris
On Sat, Sept 16, 2023, 12:49 Corey Minyard via pacsat-dev < [email protected]> 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 [email protected] 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 [email protected] wrote:
On Fri, Sep 15, 2023 at 10:06 AM Chris Thompson via pacsat-dev [email protected] 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 Thompson