Add an explicit mbedtls_mpi_core_montmul_working_limbs() function

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>

5 files changed, 38 insertions, 3 deletions

diff --git a/library/bignum_core.h b/library/bignum_core.h
index 7f5375e..7b5787c 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -502,6 +502,10 @@ int mbedtls_mpi_core_fill_random( mbedtls_mpi_uint *X, size_t X_limbs,
  * \brief          Returns the number of limbs of working memory required for
  *                 a call to `mbedtls_mpi_core_exp_mod()`.
  *
+ * \note           This will always be at least
+ *                 `mbedtls_mpi_core_montmul_working_limbs(AN_limbs)`,
+ *                 i.e. sufficient for a call to `mbedtls_mpi_core_montmul()`.
+ *
  * \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()`.
@@ -585,6 +589,23 @@ mbedtls_mpi_uint mbedtls_mpi_core_sub_int( mbedtls_mpi_uint *X,
 mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct( const mbedtls_mpi_uint *A,
                                                  size_t limbs );
 
+/**
+ * \brief          Returns the number of limbs of working memory required for
+ *                 a call to `mbedtls_mpi_core_montmul()`.
+ *
+ * \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_montmul()` or one of the other functions
+ *                 that specifies this as the amount of working memory needed.
+ *
+ * \return         The number of limbs of working memory required by
+ *                 `mbedtls_mpi_core_montmul()` (or other similar function).
+ */
+static inline size_t mbedtls_mpi_core_montmul_working_limbs( size_t AN_limbs )
+{
+    return( 2 * AN_limbs + 1 );
+}
+
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */
diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index c98a1c1..be8fc86 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -183,7 +183,7 @@ int mbedtls_mpi_mod_raw_to_mont_rep( mbedtls_mpi_uint *X,
                                      const mbedtls_mpi_mod_modulus *m )
 {
     mbedtls_mpi_uint *T;
-    const size_t t_limbs = m->limbs * 2 + 1;
+    const size_t t_limbs = mbedtls_mpi_core_montmul_working_limbs( m->limbs );
 
     if( ( T = (mbedtls_mpi_uint *) mbedtls_calloc( t_limbs, ciL ) ) == NULL )
         return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
@@ -200,7 +200,7 @@ int mbedtls_mpi_mod_raw_from_mont_rep( mbedtls_mpi_uint *X,
                                        const mbedtls_mpi_mod_modulus *m )
 {
     const mbedtls_mpi_uint one = 1;
-    const size_t t_limbs = m->limbs * 2 + 1;
+    const size_t t_limbs = mbedtls_mpi_core_montmul_working_limbs( m->limbs );
     mbedtls_mpi_uint *T;
 
     if( ( T = (mbedtls_mpi_uint *) mbedtls_calloc( t_limbs, ciL ) ) == NULL )
diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index f9968ba..73eaf18 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -178,6 +178,10 @@ void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
  * \brief          Returns the number of limbs of working memory required for
  *                 a call to `mbedtls_mpi_mod_raw_inv_prime()`.
  *
+ * \note           This will always be at least
+ *                 `mbedtls_mpi_core_montmul_working_limbs(AN_limbs)`,
+ *                 i.e. sufficient for a call to `mbedtls_mpi_core_montmul()`.
+ *
  * \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_mod_raw_inv_prime()`.
diff --git a/tests/suites/test_suite_bignum_core.function b/tests/suites/test_suite_bignum_core.function
index 9392f51..038ee6b 100644
--- a/tests/suites/test_suite_bignum_core.function
+++ b/tests/suites/test_suite_bignum_core.function
@@ -798,7 +798,9 @@ void mpi_core_montmul( int limbs_AN4, int limbs_B4,
     TEST_EQUAL( 0, mbedtls_mpi_grow( X, limbs_AN ) );
     TEST_EQUAL( 0, mbedtls_mpi_grow( &B, limbs_B ) );
 
-    TEST_EQUAL( 0, mbedtls_mpi_grow( &T, limbs_AN * 2 + 1 ) );
+    size_t working_limbs = mbedtls_mpi_core_montmul_working_limbs( limbs_AN );
+    TEST_EQUAL( working_limbs, limbs_AN * 2 + 1 );
+    TEST_EQUAL( 0, mbedtls_mpi_grow( &T, working_limbs ) );
 
     /* Calculate the Montgomery constant (this is unit tested separately) */
     mbedtls_mpi_uint mm = mbedtls_mpi_core_montmul_init( N.p );
@@ -1083,6 +1085,10 @@ void mpi_core_exp_mod( char * input_N, char * input_A,
     TEST_LE_U( min_expected_working_limbs, working_limbs );
     TEST_LE_U( working_limbs, max_expected_working_limbs );
 
+    /* Should also be at least mbedtls_mpi_core_montmul_working_limbs() */
+    TEST_LE_U( mbedtls_mpi_core_montmul_working_limbs( N_limbs ),
+               working_limbs );
+
     ASSERT_ALLOC( T, working_limbs );
 
     mbedtls_mpi_core_exp_mod( Y, A, N, N_limbs, E, E_limbs, R2, T );
diff --git a/tests/suites/test_suite_bignum_mod_raw.function b/tests/suites/test_suite_bignum_mod_raw.function
index 83e1f54..ef0f712 100644
--- a/tests/suites/test_suite_bignum_mod_raw.function
+++ b/tests/suites/test_suite_bignum_mod_raw.function
@@ -394,6 +394,10 @@ void mpi_mod_raw_inv_prime( char * input_N, char * input_A, char * input_X )
     TEST_LE_U( min_expected_working_limbs, working_limbs );
     TEST_LE_U( working_limbs, max_expected_working_limbs );
 
+    /* Should also be at least mbedtls_mpi_core_montmul_working_limbs() */
+    TEST_LE_U( mbedtls_mpi_core_montmul_working_limbs( N_limbs ),
+               working_limbs );
+
     ASSERT_ALLOC( T, working_limbs );
 
     mbedtls_mpi_mod_raw_inv_prime( Y, A, N, N_limbs, R2, T );