diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index 21743e0..2d7fc81 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -26,11 +26,40 @@
 #include "mbedtls/bignum.h"
 #endif
 
+/** Import X from unsigned binary data.
+ *
+ * This function is guaranteed to return an MPI with exactly the necessary
+ * number of limbs (in particular, it does not skip 0s in the input).
+ *
+ * \param X      The address of the MPI. The size is determined by \p m.
+ * \param m      The address of a modulus related to \p X.
+ * \param buf    The input buffer to import from.
+ * \param buflen Tne length in bytes of \p buf.
+ *
+ * \return       \c 0 if successful.
+ * \return       #MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if \p X isn't
+ *               large enough to hold the value in \p buf.
+ * \return       #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if the external reprezentation
+ *               of \p m is invalid or \p X is less then \p m.
+ */
 int mbedtls_mpi_mod_raw_read( mbedtls_mpi_uint *X,
                               mbedtls_mpi_mod_modulus *m,
                               unsigned char *buf,
                               size_t buflen );
 
+/** Export X into unsigned binary data.
+ *
+ * \param X      The address of the MPI. The size is determined by \p m.
+ * \param m      The address of a modulus related to \p X.
+ * \param buf    The output buffer to import.
+ * \param buflen Tne length in bytes of \p buf.
+ *
+ * \return       \c 0 if successful.
+ * \return       #MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if \p buf isn't
+ *               large enough to hold the value of \p X.
+ * \return       #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if the external reprezentation
+ *               of \p m is invalid.
+ */
 int mbedtls_mpi_mod_raw_write( mbedtls_mpi_uint *X,
                                mbedtls_mpi_mod_modulus *m,
                                unsigned char *buf,