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On 4/21/22 14:57, Stefan Berger wrote:

Fix incorrect usage of vli_cmp when calculating the value of res.x. For

signature verification to succeed, res.x must be the same as the r

component of the signature which is in the range of [1..n-1] with 'n'

being the order of the curve. Therefore, when res.x equals n calculate

res.x = res.x - n as well. Signature verification could have previously

unnecessarily failed in extremely rare cases.

Actually, I am withdrawing this patch. Before this patch res.x could equal 'n' and then wouldn't match r due to the range of r being [1..n-1]. Now if res.x equals 'n' then res.x - n will be 0 and again will not match 'r' due to the range of r being [1..n-1]. So it makes no difference whether vli_cmp() == 1 or vli_cmp() >= 1 and the concern above about rare cases not verifying the signature is wrong.

Stefan

Fixes: 4e6602916bc6 ("crypto: ecdsa - Add support for ECDSA signature verification")

Cc: <stable@xxxxxxxxxxxxxxx>

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 b3a8a6b572ba..674ab9275366 100644

--- a/crypto/ecdsa.c

+++ b/crypto/ecdsa.c

@@ -120,8 +120,8 @@ static int _ecdsa_verify(struct ecc_ctx *ctx, const u64 *hash, const u64 *r, con

/* res = u1*G + u2 * pub_key */

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))

+ /* res.x = res.x mod n (if res.x >= order) */

+ if (unlikely(vli_cmp(res.x, curve->n, ndigits) >= 0))

/* faster alternative for NIST p384, p256 & p192 */

vli_sub(res.x, res.x, curve->n, ndigits);

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