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DHM: make drawing of blinding value a function

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In the next commit, we'll need to draw a second random value, in order to
blind modular inversion. Having a function for that will avoid repetition.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2020-06-25 12:34:58 +02:00
parent a586099fd3
commit 41ee83972b
1 changed files with 28 additions and 13 deletions
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41
library/dhm.c
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@ -350,6 +350,32 @@ cleanup:
return( 0 ); return( 0 );
} }
/*
* Pick a random R in the range [2, M) for blinding purposes
*/
static int dhm_random_below( mbedtls_mpi *R, const mbedtls_mpi *M,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret, count;
count = 0;
do
{
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( R, mbedtls_mpi_size( M ), f_rng, p_rng ) );
while( mbedtls_mpi_cmp_mpi( R, M ) >= 0 )
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( R, 1 ) );
if( count++ > 10 )
return( MBEDTLS_ERR_MPI_NOT_ACCEPTABLE );
}
while( mbedtls_mpi_cmp_int( R, 1 ) <= 0 );
cleanup:
return( ret );
}
/* /*
* Use the blinding method and optimisation suggested in section 10 of: * Use the blinding method and optimisation suggested in section 10 of:
* KOCHER, Paul C. Timing attacks on implementations of Diffie-Hellman, RSA, * KOCHER, Paul C. Timing attacks on implementations of Diffie-Hellman, RSA,
@ -359,7 +385,7 @@ cleanup:
static int dhm_update_blinding( mbedtls_dhm_context *ctx, static int dhm_update_blinding( mbedtls_dhm_context *ctx,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng ) int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{ {
int ret, count; int ret;
/* /*
* Don't use any blinding the first time a particular X is used, * Don't use any blinding the first time a particular X is used,
@ -394,18 +420,7 @@ static int dhm_update_blinding( mbedtls_dhm_context *ctx,
*/ */
/* Vi = random( 2, P-1 ) */ /* Vi = random( 2, P-1 ) */
count = 0; MBEDTLS_MPI_CHK( dhm_random_below( &ctx->Vi, &ctx->P, f_rng, p_rng ) );
do
{
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->Vi, mbedtls_mpi_size( &ctx->P ), f_rng, p_rng ) );
while( mbedtls_mpi_cmp_mpi( &ctx->Vi, &ctx->P ) >= 0 )
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &ctx->Vi, 1 ) );
if( count++ > 10 )
return( MBEDTLS_ERR_MPI_NOT_ACCEPTABLE );
}
while( mbedtls_mpi_cmp_int( &ctx->Vi, 1 ) <= 0 );
/* Vf = Vi^-X mod P */ /* Vf = Vi^-X mod P */
MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vi, &ctx->P ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vi, &ctx->P ) );