diff --git a/library/bignum_core.c b/library/bignum_core.c
index 1ce8457..75cce05 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -742,6 +742,17 @@
     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 */
diff --git a/library/bignum_core.h b/library/bignum_core.h
index b7af4d0..7f5375e 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -572,6 +572,19 @@
                                            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 */