On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote: > From: Stefan Berger <stefanb@xxxxxxxxxxxxx> > > res.x has been calculated by ecc_point_mult_shamir, which uses > 'mod curve_prime'. The curve_prime 'p' is typically larger than the > curve_order 'n' and therefore it is possible that p > res.x >= n. The first sentence is an incomplete sentence. A lot of cross-referencing from e.g. elixir is required to "decipher" this commit message :-) I do get that math here is complicated but for that matter each commit message should be written with care, minimizing the require cross- referencing. These commit messages are adding extra layer of salt. > > If res.x >= n then res.x mod n can be calculated by iteratively sub- > tracting n from res.x until n > res.x. 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 => true for the NIST curves > > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx> > Tested-by: Lukas Wunner <lukas@xxxxxxxxx> What was there to test in this anyway? I see only comment change below. > --- > 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)) BR, Jarkko
