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Have mbedtls_mpi_core_exp_mod() take a temporary instead of allocatin…
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…g memory

Last PR needed for Mbed-TLS#6293

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
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tom-cosgrove-arm committed Dec 6, 2022
1 parent 21d459d commit 0a0dded
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Showing 3 changed files with 93 additions and 54 deletions.
57 changes: 29 additions & 28 deletions library/bignum_core.c
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Original file line number Diff line number Diff line change
Expand Up @@ -596,6 +596,19 @@ static size_t exp_mod_get_window_size( size_t Ebits )
return( wsize );
}

size_t mbedtls_mpi_core_exp_mod_working_limbs( size_t AN_limbs, size_t E_limbs )
{
const size_t wsize = exp_mod_get_window_size( E_limbs * biL );
const size_t welem = ( (size_t) 1 ) << wsize;

/* How big does each part of the working memory pool need to be? */
const size_t table_limbs = welem * AN_limbs;
const size_t select_limbs = AN_limbs;
const size_t temp_limbs = 2 * AN_limbs + 1;

return( table_limbs + select_limbs + temp_limbs );
}

static void exp_mod_precompute_window( const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
size_t AN_limbs,
Expand Down Expand Up @@ -636,35 +649,27 @@ static void exp_mod_precompute_window( const mbedtls_mpi_uint *A,
* (The difference is that the body in our loop processes a single bit instead
* of a full window.)
*/
int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
size_t AN_limbs,
const mbedtls_mpi_uint *E,
size_t E_limbs,
const mbedtls_mpi_uint *RR )
void mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
size_t AN_limbs,
const mbedtls_mpi_uint *E,
size_t E_limbs,
const mbedtls_mpi_uint *RR,
mbedtls_mpi_uint *T )
{
const size_t wsize = exp_mod_get_window_size( E_limbs * biL );
const size_t welem = ( (size_t) 1 ) << wsize;

/* Allocate memory pool and set pointers to parts of it */
const size_t table_limbs = welem * AN_limbs;
const size_t temp_limbs = 2 * AN_limbs + 1;
const size_t select_limbs = AN_limbs;
const size_t total_limbs = table_limbs + temp_limbs + select_limbs;
/* This is how we will use the temporary storage T, which must have space
* for table_limbs, select_limbs and (2 * AN_limbs + 1) for montmul. */
const size_t table_limbs = welem * AN_limbs;
const size_t select_limbs = AN_limbs;

/* heap allocated memory pool */
mbedtls_mpi_uint *mempool =
mbedtls_calloc( total_limbs, sizeof(mbedtls_mpi_uint) );
if( mempool == NULL )
{
return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
}

/* pointers to temporaries within memory pool */
mbedtls_mpi_uint *const Wtable = mempool;
mbedtls_mpi_uint *const Wselect = Wtable + table_limbs;
mbedtls_mpi_uint *const temp = Wselect + select_limbs;
/* Pointers to specific parts of the temporary working memory pool */
mbedtls_mpi_uint *const Wtable = T;
mbedtls_mpi_uint *const Wselect = Wtable + table_limbs;
mbedtls_mpi_uint *const temp = Wselect + select_limbs;

/*
* Window precomputation
Expand Down Expand Up @@ -731,10 +736,6 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
}
}
while( ! ( E_bit_index == 0 && E_limb_index == 0 ) );

mbedtls_platform_zeroize( mempool, total_limbs * sizeof(mbedtls_mpi_uint) );
mbedtls_free( mempool );
return( 0 );
}

/* END MERGE SLOT 1 */
Expand Down
62 changes: 42 additions & 20 deletions library/bignum_core.h
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Original file line number Diff line number Diff line change
Expand Up @@ -499,28 +499,50 @@ int mbedtls_mpi_core_fill_random( mbedtls_mpi_uint *X, size_t X_limbs,
/* BEGIN MERGE SLOT 1 */

/**
* \brief Perform a modular exponentiation with secret exponent:
* X = A^E mod N, where \p A is already in Montgomery form.
*
* \param[out] X The destination MPI, as a little endian array of length
* \p AN_limbs.
* \param[in] A The base MPI, as a little endian array of length \p AN_limbs.
* Must be in Montgomery form.
* \param[in] N The modulus, as a little endian array of length \p AN_limbs.
* \param AN_limbs The number of limbs in \p X, \p A, \p N, \p RR.
* \param[in] E The exponent, as a little endian array of length \p E_limbs.
* \param E_limbs The number of limbs in \p E.
* \param[in] RR The precomputed residue of 2^{2*biL} modulo N, as a little
* endian array of length \p AN_limbs.
* \brief Returns the number of limbs of working memory required for
* a call to `mbedtls_mpi_core_exp_mod()`.
*
* \return \c 0 if successful.
* \return #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
* \param AN_limbs The number of limbs in the input `A` and the modulus `N`
* (they must be the same size) that will be given to
* `mbedtls_mpi_core_exp_mod()`.
* \param E_limbs The number of limbs in the exponent `E` that will be given
* to `mbedtls_mpi_core_exp_mod()`.
*
* \return The number of limbs of working memory required by
* `mbedtls_mpi_core_exp_mod()`.
*/
int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N, size_t AN_limbs,
const mbedtls_mpi_uint *E, size_t E_limbs,
const mbedtls_mpi_uint *RR );
size_t mbedtls_mpi_core_exp_mod_working_limbs( size_t AN_limbs, size_t E_limbs );

/**
* \brief Perform a modular exponentiation with secret exponent:
* X = A^E mod N, where \p A is already in Montgomery form.
*
* \param[out] X The destination MPI, as a little endian array of length
* \p AN_limbs.
* \param[in] A The base MPI, as a little endian array of length \p AN_limbs.
* Must be in Montgomery form.
* \param[in] N The modulus, as a little endian array of length \p AN_limbs.
* \param AN_limbs The number of limbs in \p X, \p A, \p N, \p RR.
* \param[in] E The exponent, as a little endian array of length \p E_limbs.
* \param E_limbs The number of limbs in \p E.
* \param[in] RR The precomputed residue of 2^{2*biL} modulo N, as a little
* endian array of length \p AN_limbs.
* \param[in,out] T Temporary storage of at least the number of limbs returned
* by `mbedtls_mpi_core_exp_mod_working_limbs()`.
* Its initial content is unused and its final content is
* indeterminate.
* It must not alias or otherwise overlap any of the other
* parameters.
* It is up to the caller to zeroize \p T when it is no
* longer needed, and before freeing it if it was dynamically
* allocated.
*/
void mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N, size_t AN_limbs,
const mbedtls_mpi_uint *E, size_t E_limbs,
const mbedtls_mpi_uint *RR,
mbedtls_mpi_uint *T );

/* END MERGE SLOT 1 */

Expand Down
28 changes: 22 additions & 6 deletions tests/suites/test_suite_bignum_core.function
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Original file line number Diff line number Diff line change
Expand Up @@ -1046,15 +1046,13 @@ void mpi_core_exp_mod( char * input_N, char * input_A,
char * input_E, char * input_X )
{
mbedtls_mpi_uint *A = NULL;
size_t A_limbs;
mbedtls_mpi_uint *E = NULL;
size_t E_limbs;
mbedtls_mpi_uint *N = NULL;
size_t N_limbs;
mbedtls_mpi_uint *X = NULL;
size_t X_limbs;
size_t A_limbs, E_limbs, N_limbs, X_limbs;
const mbedtls_mpi_uint *R2 = NULL;
mbedtls_mpi_uint *Y = NULL;
mbedtls_mpi_uint *T = NULL;
/* Legacy MPIs for computing R2 */
mbedtls_mpi N_mpi;
mbedtls_mpi_init( &N_mpi );
Expand All @@ -1078,11 +1076,29 @@ void mpi_core_exp_mod( char * input_N, char * input_A,
TEST_EQUAL( 0, mbedtls_mpi_grow( &R2_mpi, N_limbs ) );
R2 = R2_mpi.p;

TEST_EQUAL( 0,
mbedtls_mpi_core_exp_mod( Y, A, N, N_limbs, E, E_limbs, R2 ) );
size_t working_limbs = mbedtls_mpi_core_exp_mod_working_limbs( N_limbs,
E_limbs );

/* No point exactly duplicating the code in mbedtls_mpi_core_exp_mod_working_limbs()
* to see if the output is correct, but we can check that it's in a
* reasonable range. The current calculation works out as
* `1 + N_limbs * (welem + 3)`, where welem is the number of elements in
* the window (1 << 1 up to 1 << 6).
*/
size_t min_expected_working_limbs = 1 + N_limbs * 4;
size_t max_expected_working_limbs = 1 + N_limbs * 67;

TEST_LE_U( min_expected_working_limbs, working_limbs );
TEST_LE_U( working_limbs, max_expected_working_limbs );

ASSERT_ALLOC( T, working_limbs );

mbedtls_mpi_core_exp_mod( Y, A, N, N_limbs, E, E_limbs, R2, T );

TEST_EQUAL( 0, memcmp( X, Y, N_limbs * sizeof( mbedtls_mpi_uint ) ) );

exit:
mbedtls_free( T );
mbedtls_free( A );
mbedtls_free( E );
mbedtls_free( N );
Expand Down

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