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ecp_curves: Hardcod Montgomery const for curve25519

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This patch adds two embedded constants used by `ecp_use_curve25519()`.
The method has been updated to read that into an mpi instead of
calculating it on the fly.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
This commit is contained in:
Minos Galanakis 2023-01-16 16:16:49 +00:00
parent 52a7aeebf3
commit bececeb0b9
1 changed files with 18 additions and 11 deletions
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29
library/ecp_curves.c
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@ -4627,9 +4627,21 @@ static int ecp_mod_p256k1(mbedtls_mpi *);
#if defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
/* Constants used by ecp_use_curve25519() */
static const mbedtls_mpi_sint curve25519_a24 = 0x01DB42;
static const unsigned char curve25519_part_of_n[] = {
0x14, 0xDE, 0xF9, 0xDE, 0xA2, 0xF7, 0x9C, 0xD6,
0x58, 0x12, 0x63, 0x1A, 0x5C, 0xF5, 0xD3, 0xED,
/* P = 2^255 - 19 */
static const mbedtls_mpi_uint curve25519_p[] = {
MBEDTLS_BYTES_TO_T_UINT_8(0xED, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF),
MBEDTLS_BYTES_TO_T_UINT_8(0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF),
MBEDTLS_BYTES_TO_T_UINT_8(0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF),
MBEDTLS_BYTES_TO_T_UINT_8(0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0X7F)
};
/* N = 2^252 + 27742317777372353535851937790883648493 */
static const mbedtls_mpi_uint curve25519_n[] = {
MBEDTLS_BYTES_TO_T_UINT_8(0XED, 0XD3, 0XF5, 0X5C, 0X1A, 0X63, 0X12, 0X58),
MBEDTLS_BYTES_TO_T_UINT_8(0XD6, 0X9C, 0XF7, 0XA2, 0XDE, 0XF9, 0XDE, 0X14),
MBEDTLS_BYTES_TO_T_UINT_8(0X00, 0X00, 0X00, 0X00, 0x00, 0x00, 0x00, 0x00),
MBEDTLS_BYTES_TO_T_UINT_8(0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10)
};
/*
@ -4642,16 +4654,11 @@ static int ecp_use_curve25519(mbedtls_ecp_group *grp)
/* Actually ( A + 2 ) / 4 */
MBEDTLS_MPI_CHK(mbedtls_mpi_lset(&grp->A, curve25519_a24));
/* P = 2^255 - 19 */
MBEDTLS_MPI_CHK(mbedtls_mpi_lset(&grp->P, 1));
MBEDTLS_MPI_CHK(mbedtls_mpi_shift_l(&grp->P, 255));
MBEDTLS_MPI_CHK(mbedtls_mpi_sub_int(&grp->P, &grp->P, 19));
ecp_mpi_load(&grp->P, curve25519_p, sizeof(curve25519_p));
grp->pbits = mbedtls_mpi_bitlen(&grp->P);
/* N = 2^252 + 27742317777372353535851937790883648493 */
MBEDTLS_MPI_CHK(mbedtls_mpi_read_binary(&grp->N,
curve25519_part_of_n, sizeof(curve25519_part_of_n)));
MBEDTLS_MPI_CHK(mbedtls_mpi_set_bit(&grp->N, 252, 1));
ecp_mpi_load(&grp->N, curve25519_n, sizeof(curve25519_n));
/* Y intentionally not set, since we use x/z coordinates.
* This is used as a marker to identify Montgomery curves! */