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Merge pull request #726 from mpg/protect-base-blinding-2.7-restricted

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[Backport 2.7] Protect base blinding in RSA and DHM
This commit is contained in:
Gilles Peskine 2020-07-29 10:58:59 +02:00 committed by GitHub
commit 388de18e45
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GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 79 additions and 19 deletions
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6
ChangeLog.d/protect-base-blinding.txt Normal file
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@ -0,0 +1,6 @@
Security
* Fix side channel in RSA private key operations and static (finite-field)
Diffie-Hellman. An adversary with precise enough timing and memory access
information (typically an untrusted operating system attacking a secure
enclave) could bypass an existing counter-measure (base blinding) and
potentially fully recover the private key.

56
library/dhm.c
View File

@ -335,6 +335,32 @@ 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,
@ -344,7 +370,10 @@ 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;
mbedtls_mpi R;
mbedtls_mpi_init( &R );
/*
* Don't use any blinding the first time a particular X is used,
@ -379,24 +408,23 @@ 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 ) );
MBEDTLS_MPI_CHK( dhm_random_below( &ctx->Vi, &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 ) );
/* Vf = Vi^-X mod P
* First compute Vi^-1 = R * (R Vi)^-1, (avoiding leaks from inv_mod),
* 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 ) );
if( count++ > 10 )
return( MBEDTLS_ERR_MPI_NOT_ACCEPTABLE );
}
while( mbedtls_mpi_cmp_int( &ctx->Vi, 1 ) <= 0 );
/* Vf = Vi^-X mod P */
MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vi, &ctx->P ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->Vf, &ctx->Vf, &ctx->X, &ctx->P, &ctx->RP ) );
cleanup:
mbedtls_mpi_free( &R );
return( ret );
}

36
library/rsa.c
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@ -746,6 +746,9 @@ static int rsa_prepare_blinding( mbedtls_rsa_context *ctx,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret, count = 0;
mbedtls_mpi R;
mbedtls_mpi_init( &R );
if( ctx->Vf.p != NULL )
{
@ -761,18 +764,41 @@ static int rsa_prepare_blinding( mbedtls_rsa_context *ctx,
/* Unblinding value: Vf = random number, invertible mod N */
do {
if( count++ > 10 )
return( MBEDTLS_ERR_RSA_RNG_FAILED );
{
ret = MBEDTLS_ERR_RSA_RNG_FAILED;
goto cleanup;
}
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->Vf, ctx->len - 1, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_gcd( &ctx->Vi, &ctx->Vf, &ctx->N ) );
} while( mbedtls_mpi_cmp_int( &ctx->Vi, 1 ) != 0 );
/* Blinding value: Vi = Vf^(-e) mod N */
MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vi, &ctx->Vf, &ctx->N ) );
/* Compute Vf^-1 as R * (R Vf)^-1 to avoid leaks from inv_mod. */
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &R, ctx->len - 1, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vf, &R ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->N ) );
/* At this point, Vi is invertible mod N if and only if both Vf and R
* are invertible mod N. If one of them isn't, we don't need to know
* which one, we just loop and choose new values for both of them.
* (Each iteration succeeds with overwhelming probability.) */
ret = mbedtls_mpi_inv_mod( &ctx->Vi, &ctx->Vi, &ctx->N );
if( ret == MBEDTLS_ERR_MPI_NOT_ACCEPTABLE )
continue;
if( ret != 0 )
goto cleanup;
/* Finish the computation of Vf^-1 = R * (R Vf)^-1 */
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vi, &R ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->N ) );
} while( 0 );
/* Blinding value: Vi = Vf^(-e) mod N
* (Vi already contains Vf^-1 at this point) */
MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->Vi, &ctx->Vi, &ctx->E, &ctx->N, &ctx->RN ) );
cleanup:
mbedtls_mpi_free( &R );
return( ret );
}