Messages in this thread Patch in this message | | | From | Stefan Berger <> | Subject | [PATCH v4 03/12] crypto: ecdsa - Extend res.x mod n calculation for NIST P521 | Date | Thu, 29 Feb 2024 21:19:58 -0500 |
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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 <stefanb@linux.ibm.com> --- 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
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