On Wed Mar 20, 2024 at 1:47 PM EET, Stefan Berger wrote: > res.x has been calculated by ecc_point_mult_shamir, which uses > 'mod curve_prime' on res.x and therefore p > res.x with 'p' being the > curve_prime. Further, it is true that for the NIST curves p > n with 'n' > being the 'curve_order' and therefore the following may be true as well: > p > res.x >= n. > > If res.x >= n then res.x mod n can be calculated by iteratively sub- > tracting n from res.x until res.x < n. For NIST P192/256/384 this can be > done in a single subtraction. This can also be done in a single > subtraction for NIST P521. > > The mathematical reason why a single subtraction is sufficient is due to > the values of 'p' and 'n' of the NIST curves where the following holds > true: > > note: max(res.x) = p - 1 > > max(res.x) - n < n > p - 1 - n < n > p - 1 < 2n => holds true for the NIST curves > > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx> > Tested-by: Lukas Wunner <lukas@xxxxxxxxx> > --- > crypto/ecdsa.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c > index 64e1e69d53ba..1814f009f971 100644 > --- a/crypto/ecdsa.c > +++ b/crypto/ecdsa.c > @@ -122,7 +122,7 @@ static int _ecdsa_verify(struct ecc_ctx *ctx, const u64 *hash, const u64 *r, con > > /* res.x = res.x mod n (if res.x > order) */ > if (unlikely(vli_cmp(res.x, curve->n, ndigits) == 1)) > - /* faster alternative for NIST p384, p256 & p192 */ > + /* faster alternative for NIST p521, p384, p256 & p192 */ > vli_sub(res.x, res.x, curve->n, ndigits); > > if (!vli_cmp(res.x, r, ndigits)) Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx> BR, Jarkko
