When 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 is done in a single subtraction since these curves' 'n' use all the 64bit digits. This is also significantly faster than a modulo operation. For NIST P521 the same could take multiple subtractions. However, during testing with varying NIST P521 keys it was never necessary to do any subtraction at all. Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx> --- crypto/ecdsa.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c index 228f675ac2ed..c9b867a9cbb9 100644 --- a/crypto/ecdsa.c +++ b/crypto/ecdsa.c @@ -121,8 +121,8 @@ static int _ecdsa_verify(struct ecc_ctx *ctx, const u64 *hash, const u64 *r, con ecc_point_mult_shamir(&res, u1, &curve->g, u2, &ctx->pub_key, curve); /* 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 */ + while (unlikely(vli_cmp(res.x, curve->n, ndigits) == 1)) + /* 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
