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DHM: use CT modinv for blinding

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
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Manuel Pégourié-Gonnard
2025-07-10 21:48:41 +02:00
parent a56a05b015
commit c2d210ea0d
1 changed files with 3 additions and 10 deletions
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13
library/dhm.c
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@@ -18,6 +18,7 @@
#if defined(MBEDTLS_DHM_C) #if defined(MBEDTLS_DHM_C)
#include "mbedtls/dhm.h" #include "mbedtls/dhm.h"
#include "bignum_internal.h"
#include "mbedtls/platform_util.h" #include "mbedtls/platform_util.h"
#include "mbedtls/error.h" #include "mbedtls/error.h"
@@ -381,16 +382,8 @@ static int dhm_update_blinding(mbedtls_dhm_context *ctx,
/* Vi = random( 2, P-2 ) */ /* Vi = random( 2, P-2 ) */
MBEDTLS_MPI_CHK(dhm_random_below(&ctx->Vi, &ctx->P, f_rng, p_rng)); MBEDTLS_MPI_CHK(dhm_random_below(&ctx->Vi, &ctx->P, f_rng, p_rng));
/* Vf = Vi^-X mod P /* Vf = Vi^-X mod P */
* First compute Vi^-1 = R * (R Vi)^-1, (avoiding leaks from inv_mod), MBEDTLS_MPI_CHK(mbedtls_mpi_gcd_modinv_odd(NULL, &ctx->Vf, &ctx->Vi, &ctx->P));
* then elevate to the Xth power. */
MBEDTLS_MPI_CHK(dhm_random_below(&R, &ctx->P, f_rng, p_rng));
MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(&ctx->Vf, &ctx->Vi, &R));
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(&ctx->Vf, &ctx->Vf, &ctx->P));
MBEDTLS_MPI_CHK(mbedtls_mpi_inv_mod(&ctx->Vf, &ctx->Vf, &ctx->P));
MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(&ctx->Vf, &ctx->Vf, &R));
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(&ctx->Vf, &ctx->Vf, &ctx->P));
MBEDTLS_MPI_CHK(mbedtls_mpi_exp_mod(&ctx->Vf, &ctx->Vf, &ctx->X, &ctx->P, &ctx->RP)); MBEDTLS_MPI_CHK(mbedtls_mpi_exp_mod(&ctx->Vf, &ctx->Vf, &ctx->X, &ctx->P, &ctx->RP));
cleanup: cleanup: