From 43f6548dd86122c7e87a19c854ea6c54c845ba2e Mon Sep 17 00:00:00 2001 From: Stefan Berger Date: Thu, 4 Apr 2024 10:18:47 -0400 Subject: crypto: ecdsa - Extend res.x mod n calculation for NIST P521 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 Tested-by: Lukas Wunner Reviewed-by: Jarkko Sakkinen Signed-off-by: Stefan Berger Signed-off-by: Herbert Xu --- crypto/ecdsa.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c index 9462a4d6..9b84ab50 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)) -- cgit v1.2.3