From 9f58c4b6e5cfaab2fcd67ef517c5398512a008fd Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= Date: Thu, 25 Jun 2020 12:34:58 +0200 Subject: [PATCH] DHM: make drawing of blinding value a function MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- library/dhm.c | 41 ++++++++++++++++++++++++++++------------- 1 file changed, 28 insertions(+), 13 deletions(-) diff --git a/library/dhm.c b/library/dhm.c index 392ed0c15..6df40f13b 100644 --- a/library/dhm.c +++ b/library/dhm.c @@ -324,6 +324,32 @@ cleanup: 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: * KOCHER, Paul C. Timing attacks on implementations of Diffie-Hellman, RSA, @@ -333,7 +359,7 @@ cleanup: static int dhm_update_blinding( mbedtls_dhm_context *ctx, 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, @@ -368,18 +394,7 @@ static int dhm_update_blinding( mbedtls_dhm_context *ctx, */ /* Vi = random( 2, P-1 ) */ - count = 0; - 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 ); + MBEDTLS_MPI_CHK( dhm_random_below( &ctx->Vi, &ctx->P, f_rng, p_rng ) ); /* Vf = Vi^-X mod P */ MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vi, &ctx->P ) );