diff --git a/library/bignum.c b/library/bignum.c
index a7e3fa3..288f859 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -785,6 +785,9 @@
 
 /*
  * Import X from unsigned binary data, little endian
+ *
+ * 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).
  */
 int mbedtls_mpi_read_binary_le( mbedtls_mpi *X,
                                 const unsigned char *buf, size_t buflen )
@@ -811,6 +814,9 @@
 
 /*
  * Import X from unsigned binary data, big endian
+ *
+ * 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).
  */
 int mbedtls_mpi_read_binary( mbedtls_mpi *X, const unsigned char *buf, size_t buflen )
 {