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Add comments to 448 optimised reduction

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Signed-off-by: Janos Follath <janos.follath@arm.com>
This commit is contained in:
Janos Follath 2023-06-07 14:02:31 +01:00 committed by Xiaokang Qian
parent 9cce348a70
commit 035e5fc885
1 changed files with 10 additions and 5 deletions
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15
library/ecp_curves.c
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@ -5524,7 +5524,6 @@ int mbedtls_ecp_mod_p448(mbedtls_mpi_uint *X, size_t X_limbs)
/* M = A1 */
memset(M, 0, (M_limbs * ciL));
/* Do not copy into the overflow limb, as this would read past the end of
* X. */
memcpy(M, X + P448_WIDTH, ((M_limbs - 1) * ciL));
@ -5534,7 +5533,8 @@ int mbedtls_ecp_mod_p448(mbedtls_mpi_uint *X, size_t X_limbs)
X[i] = 0;
}
/* X += A1 - Carry here fits in oversize X. Oversize M means it will get
/* X = X + M = A0 + A1 */
/* Carry here fits in oversize X. Oversize M means it will get
* added in, not returned as carry. */
(void) mbedtls_mpi_core_add(X, X, M, M_limbs);
@ -5543,15 +5543,15 @@ int mbedtls_ecp_mod_p448(mbedtls_mpi_uint *X, size_t X_limbs)
mbedtls_mpi_core_sub(X, X, P, P_limbs);
}
/* Q = B1, X += B1 */
/* Q = B1 */
memcpy(Q, M, (Q_limbs * ciL));
mbedtls_mpi_core_shift_r(Q, Q_limbs, 224);
/* X = X + Q = (A0 + A1) + B1 */
/* No carry here - only max 224 bits */
(void) mbedtls_mpi_core_add(X, X, Q, Q_limbs);
/* M = (B0 + B1) * 2^224, X += M */
/* M = B0 */
if (sizeof(mbedtls_mpi_uint) > 4) {
M[P224_WIDTH_MIN] &= ((mbedtls_mpi_uint)-1) >> (P224_UNUSED_BITS);
}
@ -5559,10 +5559,15 @@ int mbedtls_ecp_mod_p448(mbedtls_mpi_uint *X, size_t X_limbs)
M[i] = 0;
}
/* M = M + Q = B0 + B1 */
(void) mbedtls_mpi_core_add(M, M, Q, Q_limbs);
/* M = (B0 + B1) * 2^224 */
/* Shifted carry bit from the addition fits in oversize M */
mbedtls_mpi_core_shift_l(M, M_limbs, 224);
/* X = X + M = (A0 + A1 + B1) + (B0 + B1) * 2^224 */
(void) mbedtls_mpi_core_add(X, X, M, M_limbs);
/* Deal with carry bit by subtracting P if necessary. */