From: Stefan Berger <[email protected]>
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.
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 <[email protected]>
Tested-by: Lukas Wunner <[email protected]>
---
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))
--
2.43.0
On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> From: Stefan Berger <[email protected]>
>
> 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 <[email protected]>
> Tested-by: Lukas Wunner <[email protected]>
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
On Mon, Mar 18, 2024 at 10:33:47PM +0200, Jarkko Sakkinen wrote:
> On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> > Signed-off-by: Stefan Berger <[email protected]>
> > Tested-by: Lukas Wunner <[email protected]>
>
> What was there to test in this anyway? I see only comment change below.
The full series was tested, irrespective of the content of the individual
patches.
Thanks,
Lukas