Re: OpenSSL ECDSA public key from private

Much better, but it's still not finished...  ;-)

I had a look at the ruby code. #generate_key just calls openssl
> function "EC_KEY_generate_key". From  openssl.sourcearchive.com, I see
> that, within that function, the public key is generated with
>
>       if (eckey->pub_key == NULL)
>             {
>             pub_key = EC_POINT_new(eckey->group);
>             if (pub_key == NULL)
>                   goto err;
>             }
>
>
Thanks for looking at the source.  I guess this is just the creation
(memory allocation?) of the point object (yes, the public key is a
point with X and Y coordinates in Elliptic Curve crypto), but the
calculation of this point using the private key (which is a number,
not a point) must be somewhere in the next few lines in the source.


> EC_POINT_new is used within Ruby when creating an
> OpenSSL::PKey::EC::Point object, when passing it a 'group'. Now, I
> have no idea what a group is, but your key has a 'group'. And it is
> populated, even without calling #generate_key.
>
> If I run
>
> pub_key=OpenSSL::PKey::EC::Point.new(key.group)
>
> I get something. Again, I hope this is what you are looking for.
>

As far as I can see this creates a point with (0, 0) coordinates.
(The .to_bn converts a point to an OpenSSL::BN, which is a
Big Number, where (I think) the first N bit belongs to the X
coordinate and the other bits to the Y, but to_bn answers 0.)

According to this...

http://stackoverflow.com/questions/12480776/how-do-i-obtain-the-public-key-from-an-ecdsa-private-key-in-openssl

...I think EC_POINT_mul() could be a solution.  It's a C function
in OpenSSL, and it does the calculation I need.  But I don't know
much about how Ruby and C are married in native gems, and
I think the Ruby API does not allow me to call this C function. :(

Or does it?

The API should support private-to-public key calculations, however...