diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 1c86072..b57f2a1 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -560,6 +560,25 @@
 int mbedtls_mpi_cmp_mpi( const mbedtls_mpi *X, const mbedtls_mpi *Y );
 
 /**
+ * \brief          Compare two MPIs in constant time.
+ *
+ * \param X        The left-hand MPI. This must point to an initialized MPI
+ *                 with the same allocated length as Y.
+ * \param Y        The right-hand MPI. This must point to an initialized MPI
+ *                 with the same allocated length as X.
+ * \param ret      The result of the comparison:
+ *                 \c 1 if \p X is greater than \p Y.
+ *                 \c -1 if \p X is lesser than \p Y.
+ *                 \c 0 if \p X is equal to \p Y.
+ *
+ * \return         0 on success.
+ * \return         MBEDTLS_ERR_MPI_BAD_INPUT_DATA if the allocated length of
+ *                 the two input MPIs is not the same.
+ */
+int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
+        int *ret );
+
+/**
  * \brief          Compare an MPI with an integer.
  *
  * \param X        The left-hand MPI. This must point to an initialized MPI.