diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 7adbe7e..cb16d23 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -118,6 +118,21 @@
 
 #if defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED)
 
+/** Fast quasi-reduction modulo p256 (FIPS 186-3 D.2.3)
+ *
+ * \param[in,out]   X       The address of the MPI to be converted.
+ *                          Must have exact limb size that stores a 512-bit MPI
+ *                          (double the bitlength of the modulus).
+ *                          Upon return holds the reduced value which is
+ *                          in range `0 <= X < 2 * N` (where N is the modulus).
+ *                          The bitlength of the reduced value is the same as
+ *                          that of the modulus (256 bits).
+ * \param[in]       X_limbs The length of \p X in limbs.
+ *
+ * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X_limbs is not the
+ *                  limb size that sores a 512-bit MPI.
+ */
 MBEDTLS_STATIC_TESTABLE
 int mbedtls_ecp_mod_p256_raw(mbedtls_mpi_uint *X, size_t X_limbs);