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Wed, 6 Mar 2024 22:23:03 +0000 (GMT) From: Stefan Berger To: keyrings@vger.kernel.org, linux-crypto@vger.kernel.org, herbert@gondor.apana.org.au, davem@davemloft.net Cc: linux-kernel@vger.kernel.org, saulo.alessandre@tse.jus.br, lukas@wunner.de, Stefan Berger Subject: [PATCH v5 03/12] crypto: ecdsa - Extend res.x mod n calculation for NIST P521 Date: Wed, 6 Mar 2024 17:22:48 -0500 Message-ID: <20240306222257.979304-4-stefanb@linux.ibm.com> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20240306222257.979304-1-stefanb@linux.ibm.com> References: <20240306222257.979304-1-stefanb@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: oJQvlci6Zc2qzOg4PstrJX-Imuyj4EB- X-Proofpoint-ORIG-GUID: oJQvlci6Zc2qzOg4PstrJX-Imuyj4EB- X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.1011,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2024-03-06_12,2024-03-05_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 suspectscore=0 adultscore=0 impostorscore=0 bulkscore=0 phishscore=0 mlxscore=0 mlxlogscore=999 spamscore=0 malwarescore=0 lowpriorityscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311290000 definitions=main-2403060181 res.x has been calculated by ecc_point_mult_shamir, which uses 'mod curve_prime'. The curve_prime 'p' is typically larger than the curve_order 'n' and therefore it is possible that 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 n > res.x. 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 => true for the NIST curves Signed-off-by: Stefan Berger Tested-by: Lukas Wunner --- crypto/ecdsa.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c index 64e1e69d53ba..1814f009f971 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)) -- 2.43.0