Hi Or,
On Sun, Aug 25, 2013 at 2:28 PM, Or Yochanan ory@annapurnalabs.com wrote:
Hello,
This is my first message to the U-Boot mailing list, so please go easy on me if my syntax is flawed.. ;)
I need to use RSA decryption on my U-Boot, for a specific task i have. In order to do so - I'd like to use some of the RSA code that have been added to lib (instead of porting some other encryption code into my environment).
After going over the RSA code, I came to conclusion that the portion of the code that performs RSA verification using an RSA key (rsa_verify_key) is exactly what I need, since I don't intend to use FIT nor add anything to DT (it seems like an overkill for my requirements).
However, I fail to understand how to use the RSA's data structures correctly. Specifically, rsa_public_key input parameter requires arguments that aren't clear to me. I searched online, yet couldn't find any reference code to parse i.o. to understand what is required from this struct..
So my main questions are regarding the rsa_public_key struct that is also mentioned on the documentation:
- There's a requirement for bot num-bits, and r^2 - how comes? From what I
understand, r^2 can be calculated using num-bits - so currently I suspect that I got it wrong?
Yes it can be calculated. The idea here is to pre-calculate things that don't need to be done at run-time in U-Boot.
- What is n0inverse? I didn't find anything on the web that explains this
argument, nor did I find anything that resemble the calculation (-1 / N[0] mod 2^32).
If you look at rsa_get_params() you will see where it calculates these parameters from the RSA key. n0inverse is -1 / n mod 2^32.
Normally these three parameters are stored in the DT, but if you don't use that I suppose you could put them somewhere else.
If there's a possibility to better explain these arguments, or even better - provide a usage example, I'd be grateful.
The basic idea is that with these values is to make it possible to verify a signature using just the code in rsa-verify.c. There is no 'bignum' code required, nor any openssl key management/decoding code. If you look at rsa_verify_key() you will see that it only needs exponentiation (pow_mod()) and the code size is quite small.
Thanks in advance,
Or
Regards, Simon