Update documentation
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 45b0006..3d1321c 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -99,12 +99,21 @@
/** Fast quasi-reduction modulo p521 = 2^521 - 1 (FIPS 186-3 D.2.5)
*
- * \param[in,out] N_p The address of the MPI to be converted.
- * Must have 2 * N - 1 limbs, where N is the modulus.
- * \param[in] N_n The length of \p N_p in limbs.
+ * \param[in,out] X The address of the MPI to be converted.
+ * Must have twice as many limbs as the modulus
+ * (the modulus is 521 bits long). Upon return this
+ * holds the reduced value. The reduced value is
+ * in range `0 <= X < 2 * N` (where N is the modulus).
+ * and its the bitlength is one plus the bitlength
+ * of the modulus.
+ * \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 does not have
+ * twice as many limbs as the modulus.
*/
MBEDTLS_STATIC_TESTABLE
-int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *N_p, size_t N_n);
+int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *X, size_t X_limbs);
#endif /* MBEDTLS_ECP_DP_SECP521R1_ENABLED */