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ecp_curves: Extended documentation for CURVE25519.

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Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
This commit is contained in:
Minos Galanakis 2023-05-22 11:23:56 +01:00
parent 47249fd9ec
commit d6beda7af9
2 changed files with 8 additions and 3 deletions
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9
library/ecp_curves.c
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@ -5455,7 +5455,7 @@ int mbedtls_ecp_mod_p255_raw(mbedtls_mpi_uint *X, size_t X_Limbs)
memset(A1, 0, sizeof(mbedtls_mpi_uint) * A1_limbs);
}
/* Step 2: Reduce to <p
/* Step 2: Reduce to <2p
* Split as A0 + 2^255*c, with c a scalar, and compute A0 + 19*c */
*carry <<= 1;
*carry += (X[P255_WIDTH - 1] >> (biL - 1));
@ -5463,6 +5463,13 @@ int mbedtls_ecp_mod_p255_raw(mbedtls_mpi_uint *X, size_t X_Limbs)
/* Clear top bit */
X[P255_WIDTH - 1] <<= 1; X[P255_WIDTH - 1] >>= 1;
/* Since the top bit for X has been cleared 0 + 0 + Carry
* will not overflow.
*
* Furthermore for 2p = 2^256-38. When a carry propagation on the highest
* limb occurs, X > 2^255 and all the remaining bits on the limb are zero.
* - If X < 2^255 ==> X < 2p
* - If X > 2^255 ==> X < 2^256 - 2^255 < 2p */
(void) mbedtls_mpi_core_add(X, X, carry, P255_WIDTH);
mbedtls_free(carry);

2
library/ecp_invasive.h
View File

@ -250,8 +250,6 @@ int mbedtls_ecp_mod_p256k1_raw(mbedtls_mpi_uint *X, size_t X_limbs);
* (double the bitlength of the modulus).
* Upon return holds the reduced value which is
* in range `0 <= X < 2 * N` (where N is the modulus).
* The bitlength of the reduced value is the same as
* that of the modulus (255 bits).
* \param[in] X_limbs The length of \p X in limbs.
*
* \return \c 0 on success.