Merge pull request #6606 from gabor-mezei-arm/6222_bignum_low_level_s…

…ubtraction

Bignum: Add low level subtraction

library/bignum_mod_raw.c

@@ -108,6 +108,16 @@ int mbedtls_mpi_mod_raw_write( const mbedtls_mpi_uint *A,
 
 /* BEGIN MERGE SLOT 2 */
 
+void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              const mbedtls_mpi_mod_modulus *N )
+{
+    mbedtls_mpi_uint c = mbedtls_mpi_core_sub( X, A, B, N->limbs );
+
+    (void) mbedtls_mpi_core_add_if( X, N->p, N->limbs, (unsigned) c );
+}
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */

library/bignum_mod_raw.h

@@ -144,6 +144,28 @@ int mbedtls_mpi_mod_raw_write( const mbedtls_mpi_uint *A,
 
 /* BEGIN MERGE SLOT 2 */
 
+/** \brief  Subtract two MPIs, returning the residue modulo the specified
+ *          modulus.
+ *
+ * The size of the operation is determined by \p N. \p A and \p B must have
+ * the same number of limbs as \p N.
+ *
+ * \p X may be aliased to \p A or \p B, or even both, but may not overlap
+ * either otherwise.
+ *
+ * \param[out] X        The address of the result MPI.
+ *                      This must be initialized. Must have enough limbs to
+ *                      store the full value of the result.
+ * \param[in]  A        The address of the first MPI. This must be initialized.
+ * \param[in]  B        The address of the second MPI. This must be initialized.
+ * \param[in]  N        The address of the modulus. Used to perform a modulo
+ *                      operation on the result of the subtraction.
+ */
+void mbedtls_mpi_mod_raw_sub( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_uint *B,
+                              const mbedtls_mpi_mod_modulus *N );
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */

scripts/mbedtls_dev/bignum_mod_raw.py

@@ -30,6 +30,25 @@ class BignumModRawTarget(test_data_generation.BaseTarget):
 
 # BEGIN MERGE SLOT 2
 
+class BignumModRawSub(bignum_common.ModOperationCommon,
+                      BignumModRawTarget):
+    """Test cases for bignum mpi_mod_raw_sub()."""
+    symbol = "-"
+    test_function = "mpi_mod_raw_sub"
+    test_name = "mbedtls_mpi_mod_raw_sub"
+    input_style = "fixed"
+    arity = 2
+
+    def arguments(self) -> List[str]:
+        return [bignum_common.quote_str(n) for n in [self.arg_a,
+                                                     self.arg_b,
+                                                     self.arg_n]
+               ] + self.result()
+
+    def result(self) -> List[str]:
+        result = (self.int_a - self.int_b) % self.int_n
+        return [self.format_result(result)]
+
 # END MERGE SLOT 2
 
 # BEGIN MERGE SLOT 3

tests/CMakeLists.txt

@@ -70,6 +70,7 @@ if(GEN_FILES)
             ${CMAKE_CURRENT_SOURCE_DIR}/../tests/scripts/generate_bignum_tests.py
             ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/mbedtls_dev/bignum_common.py
             ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/mbedtls_dev/bignum_core.py
+            ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/mbedtls_dev/bignum_mod_raw.py
             ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/mbedtls_dev/test_case.py
             ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/mbedtls_dev/test_data_generation.py
     )

tests/Makefile

@@ -94,6 +94,7 @@ $(GENERATED_BIGNUM_DATA_FILES): generated_bignum_test_data
 generated_bignum_test_data: scripts/generate_bignum_tests.py
 generated_bignum_test_data: ../scripts/mbedtls_dev/bignum_common.py
 generated_bignum_test_data: ../scripts/mbedtls_dev/bignum_core.py
+generated_bignum_test_data: ../scripts/mbedtls_dev/bignum_mod_raw.py
 generated_bignum_test_data: ../scripts/mbedtls_dev/test_case.py
 generated_bignum_test_data: ../scripts/mbedtls_dev/test_data_generation.py
 generated_bignum_test_data:

tests/suites/test_suite_bignum_mod_raw.function

@@ -275,6 +275,79 @@ exit:
 
 /* BEGIN MERGE SLOT 2 */
 
+/* BEGIN_CASE */
+void mpi_mod_raw_sub( char * input_A,
+                      char * input_B,
+                      char * input_N,
+                      char * result )
+{
+    mbedtls_mpi_uint *A = NULL;
+    mbedtls_mpi_uint *B = NULL;
+    mbedtls_mpi_uint *N = NULL;
+    mbedtls_mpi_uint *X = NULL;
+    mbedtls_mpi_uint *res = NULL;
+    size_t limbs_A;
+    size_t limbs_B;
+    size_t limbs_N;
+    size_t limbs_res;
+
+    mbedtls_mpi_mod_modulus m;
+    mbedtls_mpi_mod_modulus_init( &m );
+
+    TEST_EQUAL( mbedtls_test_read_mpi_core( &A,   &limbs_A,   input_A ), 0 );
+    TEST_EQUAL( mbedtls_test_read_mpi_core( &B,   &limbs_B,   input_B ), 0 );
+    TEST_EQUAL( mbedtls_test_read_mpi_core( &N,   &limbs_N,   input_N ), 0 );
+    TEST_EQUAL( mbedtls_test_read_mpi_core( &res, &limbs_res, result  ), 0 );
+
+    size_t limbs = limbs_N;
+    size_t bytes = limbs * sizeof( mbedtls_mpi_uint );
+
+    TEST_EQUAL( limbs_A,   limbs );
+    TEST_EQUAL( limbs_B,   limbs );
+    TEST_EQUAL( limbs_res, limbs );
+
+    ASSERT_ALLOC( X, limbs );
+
+    TEST_EQUAL( mbedtls_mpi_mod_modulus_setup(
+                        &m, N, limbs,
+                        MBEDTLS_MPI_MOD_EXT_REP_BE,
+                        MBEDTLS_MPI_MOD_REP_MONTGOMERY ), 0 );
+
+    mbedtls_mpi_mod_raw_sub( X, A, B, &m );
+    ASSERT_COMPARE( X, bytes, res, bytes );
+
+    /* alias X to A */
+    memcpy( X, A, bytes );
+    mbedtls_mpi_mod_raw_sub( X, X, B, &m );
+    ASSERT_COMPARE( X, bytes, res, bytes );
+
+    /* alias X to B */
+    memcpy( X, B, bytes );
+    mbedtls_mpi_mod_raw_sub( X, A, X, &m );
+    ASSERT_COMPARE( X, bytes, res, bytes );
+
+    /* A == B: alias A and B */
+    if( memcmp( A, B, bytes ) == 0 )
+    {
+        mbedtls_mpi_mod_raw_sub( X, A, A, &m );
+        ASSERT_COMPARE( X, bytes, res, bytes );
+
+        /* X, A, B all aliased together */
+        memcpy( X, A, bytes );
+        mbedtls_mpi_mod_raw_sub( X, X, X, &m );
+        ASSERT_COMPARE( X, bytes, res, bytes );
+    }
+exit:
+    mbedtls_free( A );
+    mbedtls_free( B );
+    mbedtls_free( X );
+    mbedtls_free( res );
+
+    mbedtls_mpi_mod_modulus_free( &m );
+    mbedtls_free( N );
+}
+/* END_CASE */
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */