diff options
| author | Stefan Berger <stefanb@linux.ibm.com> | 2024-04-04 10:18:47 -0400 |
|---|---|---|
| committer | Herbert Xu <herbert@gondor.apana.org.au> | 2024-04-12 15:07:52 +0800 |
| commit | 48e8d3a5f4f902f2bcd70cc88ff784aa7d6d2c63 (patch) | |
| tree | e4d068323469e4f34150bf6786f86e9133904843 | |
| parent | dcee6068d4ab503bdb3eed6561c4ce7c7e2175f6 (diff) | |
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>
| -rw-r--r-- | crypto/ecdsa.c | 2 |
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)) |