http://en.wikipedia.org/wiki/Elliptic_Curve_DSA
"order" in the code you pasted = "n" in the wiki = "N" in your mail.
The code you pasted refers to GFp so the points (x,y) satisfy 0 <= x <
p and 0 <= y < p. That's probably what you mean by "q". Anyway, it's
used in the underlying elliptic curve operations but is not
necessarily needed at the ECDSA level -- all that needs is the outputs
from scalar multiplication (i.e., p is used under the hood in
EC_POINT_mul and EC_POINT_get_affine_coordinates_GFp).
BBB
On Thu, Dec 19, 2013 at 2:52 AM, Paddy <stonecold...@gmail.com> wrote:
> Hi,
>
> >From my understanding ECDSA has a modulus N and a field size of q.
>
> When generating the co-ordinates (x,y) using the generator and random value
> k - g * k = (x,y) - the x and y values should be restricted to the range x =
> [1, ... , q-1] and y = [1, ... , q -1].
>
> Then of course to get r we do:
>
> r = x mod N
>
> >From what I can see in the implementation (ecs_ossl.c) when using
> ecdsa_sign_setup - the 'q' field size is never used!
>
> /*
> * Does the multiplciation of G (generator) * k to produce curve point (x,y)
> */
> EC_POINT_mul(group, temp_point, k, NULL, NULL, ctx)
>
> /*
> * Retrieve BIGNUM for X value (don't need y)
> */
> if (EC_METHOD_get_field_type(EC_GROUP_method_of(group)) ==
> NID_X9_62_prime_field)
> {
> if (!EC_POINT_get_affine_coordinates_GFp(group,
> tmp_point, X, NULL, ctx))
> {
> ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP,ERR_R_EC_LIB);
> goto err;
> }
> }
>
> /*
> * Get r value by doing r = x mod N
> */
> if (!BN_nnmod(r, X, order, ctx))
> {
> ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_BN_LIB);
> goto err;
> }
>
> As well, in the verification of a signature 'q' should be used - but from
> the implementation in the do_verify method, when 'mod q' should be executed
> - it is actually doing it on the order N:
>
> /* u1 = m * tmp mod order */
> if (!BN_mod_mul(u1, m, u2, order, ctx))
> {
> ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
> goto err;
> }
>
> /* u2 = r * w mod q */
> if (!BN_mod_mul(u2, sig->r, u2, order, ctx))
> {
> ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
> goto err;
> }
>
> --------------------
>
> To sum up - when does the field size 'q' get used (as depicted in the
> algorithm description)? As it makes the r = x mod N operation pointless?
>
>
>
> --
> View this message in context:
> http://openssl.6102.n7.nabble.com/ECDSA-OpenSSL-Implementation-using-the-modulus-N-instead-of-field-size-q-tp47743.html
> Sent from the OpenSSL - User mailing list archive at Nabble.com.
> ______________________________________________________________________
> OpenSSL Project http://www.openssl.org
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