lkml.org 
[lkml]   [2024]   [Mar]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [PATCH v6 04/13] crypto: ecdsa - Extend res.x mod n calculation for NIST P521
    From
    On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
    > From: Stefan Berger <stefanb@linux.ibm.com>
    >
    > 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@linux.ibm.com>
    > Tested-by: Lukas Wunner <lukas@wunner.de>

    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

    \
     
     \ /
      Last update: 2024-05-27 15:53    [W:2.871 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site