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authorStefan Berger <stefanb@linux.ibm.com>2024-04-04 17:18:47 +0300
committerHerbert Xu <herbert@gondor.apana.org.au>2024-04-12 10:07:52 +0300
commit48e8d3a5f4f902f2bcd70cc88ff784aa7d6d2c63 (patch)
treee4d068323469e4f34150bf6786f86e9133904843 /crypto/ecdsa.c
parentdcee6068d4ab503bdb3eed6561c4ce7c7e2175f6 (diff)
downloadlinux-48e8d3a5f4f902f2bcd70cc88ff784aa7d6d2c63.tar.xz
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 <lukas@wunner.de> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Diffstat (limited to 'crypto/ecdsa.c')
-rw-r--r--crypto/ecdsa.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
index 9462a4d640a7..9b84ab503276 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))