On 25/05/12 14:41, Khuc, Chuong D. wrote:
Wow, that is a lot of good information. Thanks, Matt. And I am still trying to
digest the first paragraph. So do you mean the R value that I mentioned is
actually the public key?
No, R is just a random point...different for every signature. The public
key is also a point - but different to R.
And if I was provided
with a private key, are the following lines of code appropriate to compute
the public key and get its x-y coordinates?
EC_KEY *eckey = EC_KEY_new_by_curve_name(NID_secp256k1);
if(eckey == NULL)
{
printf("ERROR: NULL EC_KEY!\r\n");
}
else
{
BIGNUM *retbn;
retbn = BN_bin2bn(tempkey,32,NULL); // tempkey is an array of 32 u_char
if(EC_KEY_set_private_key(eckey,retbn)!=1)
{
printf("ERROR: Fail to set private key!\r\n");
}
You probably want to goto an error location after the printf...you don't
want to fall through to the next section of code! Same for the error
conditions below.
EC_POINT* pub_key = EC_POINT_new(EC_KEY_get0_group(eckey));
if(EC_POINT_mul(EC_KEY_get0_group(eckey),pub_key,retbn,NULL,NULL,NULL) !=
1)
{
printf("ERROR: Fail to generate public key!\r\n");
}
if(EC_KEY_set_public_key(eckey,pub_key) != 1)
{
printf("ERROR: Can't set public key!\r\n");
}
Yes - all of this looks fine at a glance.
BIGNUM *x_coord = BN_new();
BIGNUM *y_coord = BN_new();
// IS THIS THE RIGHT WAY TO GET X-Y COORDINATES?
if(EC_POINT_get_affine_coordinates_GFp(EC_KEY_get0_group(eckey),
EC_KEY_get0_public_keyeckey), x_coord, y_coord, NULL)!=1)
{
printf("ERROR: Fail to get affine coordinates!\r\n");
}
}
Yes - that is a valid way to get the x/y coords - as long as you are
using a GFp curve (which in this example code you are). Other curves may
require you to call EC_POINT_get_affine_coordinates_GF2m.
However if your intention is to write out the public key in a
"compressed" form as discussed in the previous email then you don't need
to actually get the coords. You can just use:
BIGNUM *EC_POINT_point2bn(const EC_GROUP *, const EC_POINT *,
point_conversion_form_t form, BIGNUM *, BN_CTX *);
or
size_t EC_POINT_point2oct(const EC_GROUP *group, const EC_POINT *p,
point_conversion_form_t form,
unsigned char *buf, size_t len, BN_CTX *ctx);
Matt
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