Merge pull request #6743 from minosgalanakis/bignum/implement_modular_negation

Bignum: Implement fixed width modular negation

library/bignum_mod_raw.c

@@ -213,6 +213,18 @@ int mbedtls_mpi_mod_raw_from_mont_rep( mbedtls_mpi_uint *X,
     mbedtls_free( T );
     return( 0 );
 }
+
+void mbedtls_mpi_mod_raw_neg( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_mod_modulus *m )
+{
+    mbedtls_mpi_core_sub( X, m->p, A, m->limbs );
+
+    /* If A=0 initially, then X=N now. Detect this by
+     * subtracting N and catching the carry. */
+    mbedtls_mpi_uint borrow = mbedtls_mpi_core_sub( X, X, m->p, m->limbs );
+    (void) mbedtls_mpi_core_add_if( X, m->p, m->limbs, (unsigned) borrow  );
+}
 /* END MERGE SLOT 7 */
 
 /* BEGIN MERGE SLOT 8 */

library/bignum_mod_raw.h

@@ -278,6 +278,23 @@ int mbedtls_mpi_mod_raw_to_mont_rep( mbedtls_mpi_uint *X,
  */
 int mbedtls_mpi_mod_raw_from_mont_rep( mbedtls_mpi_uint *X,
                                        const mbedtls_mpi_mod_modulus *m );
+
+/** \brief  Perform fixed width modular negation.
+ *
+ * The size of the operation is determined by \p m. \p A must have
+ * the same number of limbs as \p m.
+ *
+ * \p X may be aliased to \p A.
+ *
+ * \param[out] X        The result of the modular negation.
+ *                      This must be initialized.
+ * \param[in] A         Little-endian presentation of the input operand. This
+ *                      must be less than or equal to \p m.
+ * \param[in] m         The modulus to use.
+ */
+void mbedtls_mpi_mod_raw_neg( mbedtls_mpi_uint *X,
+                              const mbedtls_mpi_uint *A,
+                              const mbedtls_mpi_mod_modulus *m);
 /* END MERGE SLOT 7 */
 
 /* BEGIN MERGE SLOT 8 */

scripts/mbedtls_dev/bignum_mod_raw.py

@@ -137,7 +137,18 @@ class BignumModRawConvertFromMont(bignum_common.ModOperationCommon,
         result = self.from_montgomery(self.int_a)
         return [self.format_result(result)]
 
+class BignumModRawModNegate(bignum_common.ModOperationCommon,
+                            BignumModRawTarget):
+    """ Test cases for mpi_mod_raw_neg(). """
+    test_function = "mpi_mod_raw_neg"
+    test_name = "Modular negation: "
+    symbol = "-"
+    input_style = "arch_split"
+    arity = 1
 
+    def result(self) -> List[str]:
+        result = (self.int_n - self.int_a) % self.int_n
+        return [self.format_result(result)]
 # END MERGE SLOT 7
 
 # BEGIN MERGE SLOT 8

tests/suites/test_suite_bignum_mod_raw.function

@@ -595,6 +595,60 @@ exit:
     mbedtls_free( X );
 }
 /* END_CASE */
+
+/* BEGIN_CASE */
+void mpi_mod_raw_neg( char * input_N, char * input_A, char * input_X )
+{
+    mbedtls_mpi_uint *N = NULL;
+    mbedtls_mpi_uint *A = NULL;
+    mbedtls_mpi_uint *X = NULL;
+    mbedtls_mpi_uint *R = NULL;
+    mbedtls_mpi_uint *Z = NULL;
+    size_t n_limbs, a_limbs, x_limbs, bytes;
+
+    mbedtls_mpi_mod_modulus m;
+    mbedtls_mpi_mod_modulus_init( &m );
+
+    /* Read inputs */
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &N, &n_limbs, input_N ) );
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &A, &a_limbs, input_A ) );
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &X, &x_limbs, input_X ) );
+
+    TEST_EQUAL( a_limbs, n_limbs );
+    TEST_EQUAL( x_limbs, n_limbs );
+    bytes = n_limbs * sizeof( mbedtls_mpi_uint );
+
+    ASSERT_ALLOC( R, n_limbs );
+    ASSERT_ALLOC( Z, n_limbs );
+
+    TEST_EQUAL( 0, mbedtls_mpi_mod_modulus_setup( &m, N, n_limbs,
+            MBEDTLS_MPI_MOD_REP_MONTGOMERY ) );
+
+    /* Neg( A == 0 ) => Zero result */
+    mbedtls_mpi_mod_raw_neg( R, Z, &m );
+    ASSERT_COMPARE( R, bytes, Z, bytes );
+
+    /* Neg( A == N ) => Zero result */
+    mbedtls_mpi_mod_raw_neg( R, N, &m );
+    ASSERT_COMPARE( R, bytes, Z, bytes );
+
+    /* Neg( A ) => Correct result */
+    mbedtls_mpi_mod_raw_neg( R, A, &m );
+    ASSERT_COMPARE( R, bytes, X, bytes );
+
+    /* Neg( A ): alias A to R => Correct result */
+    mbedtls_mpi_mod_raw_neg( A, A, &m );
+    ASSERT_COMPARE( A, bytes, X, bytes );
+exit:
+    mbedtls_mpi_mod_modulus_free( &m );
+    mbedtls_free( N );
+    mbedtls_free( A );
+    mbedtls_free( X );
+    mbedtls_free( R );
+    mbedtls_free( Z );
+}
+/* END_CASE */
+
 /* END MERGE SLOT 7 */
 
 /* BEGIN MERGE SLOT 8 */