Add mbedtls_mpi_core_check_zero_ct() and tests

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

library/bignum_core.c

@@ -742,6 +742,17 @@ mbedtls_mpi_uint mbedtls_mpi_core_sub_int( mbedtls_mpi_uint *X,
     return( c );
 }
 
+mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct( const mbedtls_mpi_uint *A,
+                                                 size_t limbs )
+{
+    mbedtls_mpi_uint bits = 0;
+
+    for( size_t i = 0; i < limbs; i++ )
+        bits |= A[i];
+
+    return( bits );
+}
+
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */

library/bignum_core.h

@@ -572,6 +572,19 @@ mbedtls_mpi_uint mbedtls_mpi_core_sub_int( mbedtls_mpi_uint *X,
                                            mbedtls_mpi_uint b,
                                            size_t limbs );
 
+/**
+ * \brief Determine if a given MPI has the value \c 0 in constant time with
+ *        respect to the value (but not with respect to the number of limbs).
+ *
+ * \param[in] A   The MPI to test.
+ * \param limbs   Number of limbs in \p A.
+ *
+ * \return        0 if `A == 0`
+ *                non-0 (may be any value) if `A != 0`.
+ */
+mbedtls_mpi_uint mbedtls_mpi_core_check_zero_ct( const mbedtls_mpi_uint *A,
+                                                 size_t limbs );
+
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */

scripts/mbedtls_dev/bignum_core.py

@@ -818,6 +818,20 @@ class BignumCoreSubInt(BignumCoreTarget, bignum_common.OperationCommon):
             str(-borrow)
         ]
 
+class BignumCoreZeroCheckCT(BignumCoreTarget, bignum_common.OperationCommon):
+    """Test cases for bignum core zero check (constant flow)."""
+    count = 0
+    symbol = "== 0"
+    test_function = "mpi_core_check_zero_ct"
+    test_name = "mpi_core_check_zero_ct"
+    input_style = "variable"
+    arity = 1
+    suffix = True
+
+    def result(self) -> List[str]:
+        result = 1 if self.int_a == 0 else 0
+        return [str(result)]
+
 # END MERGE SLOT 3
 
 # BEGIN MERGE SLOT 4

tests/suites/test_suite_bignum_core.function

@@ -1162,6 +1162,25 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void mpi_core_check_zero_ct( char *input_X, int expected_is_zero )
+{
+    mbedtls_mpi_uint *X = NULL;
+    size_t X_limbs;
+
+    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &X, &X_limbs, input_X ) );
+
+    TEST_CF_SECRET( X, X_limbs * sizeof( mbedtls_mpi_uint ) );
+
+    mbedtls_mpi_uint check = mbedtls_mpi_core_check_zero_ct( X, X_limbs );
+    int is_zero = (check == 0);
+    TEST_EQUAL( is_zero, expected_is_zero );
+
+exit:
+    mbedtls_free( X );
+}
+/* END_CASE */
+
 /* END MERGE SLOT 3 */
 
 /* BEGIN MERGE SLOT 4 */