diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index f738e91..c57372c 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -155,7 +155,20 @@
 /* END MERGE SLOT 4 */
 
 /* BEGIN MERGE SLOT 5 */
-
+/**
+ * \brief Perform a known-size modular addition.
+ *
+ * Calculate A + B mod N.
+ *
+ * \param[out] X        The result of the modular addition.
+ * \param[in] A         The left operand. This must be smaller than \p N.
+ * \param[in] B         The right operand. This must be smaller than \p N.
+ * \param[in] N         The modulus.
+ * \param n             Number of limbs of \p X, \p A, \p B and \p N.
+ */
+void MPI_CORE(add_mod)( mbedtls_mpi_uint *X, mbedtls_mpi_uint const *A,
+                        mbedtls_mpi_uint const *B, const mbedtls_mpi_uint *N,
+                        size_t n );
 /* END MERGE SLOT 5 */
 
 /* BEGIN MERGE SLOT 6 */