diff --git a/include/mbedtls/chacha20.h b/include/mbedtls/chacha20.h
index 529f22d..2ae5e6e 100644
--- a/include/mbedtls/chacha20.h
+++ b/include/mbedtls/chacha20.h
@@ -83,13 +83,18 @@
  *                  \c mbedtls_chacha20_free().
  *
  * \param ctx       The ChaCha20 context to initialize.
+ *                  This must not be \c NULL.
  */
 void mbedtls_chacha20_init( mbedtls_chacha20_context *ctx );
 
 /**
- * \brief           This function releases and clears the specified ChaCha20 context.
+ * \brief           This function releases and clears the specified
+ *                  ChaCha20 context.
  *
- * \param ctx       The ChaCha20 context to clear.
+ * \param ctx       The ChaCha20 context to clear. This may be \c NULL,
+ *                  in which case this function is a no-op. If it is not
+ *                  \c NULL, it must point to an initialized context.
+ *
  */
 void mbedtls_chacha20_free( mbedtls_chacha20_context *ctx );
 
@@ -102,7 +107,9 @@
  *                  \c mbedtls_chacha_update().
  *
  * \param ctx       The ChaCha20 context to which the key should be bound.
- * \param key       The encryption/decryption key. Must be 32 bytes in length.
+ *                  It must be initialized.
+ * \param key       The encryption/decryption key. This must be \c 32 Bytes
+ *                  in length.
  *
  * \return          \c 0 on success.
  * \return          #MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA if ctx or key is NULL.
@@ -121,8 +128,9 @@
  *                  messages encrypted with the same nonce and key.
  *
  * \param ctx       The ChaCha20 context to which the nonce should be bound.
- * \param nonce     The nonce. Must be 12 bytes in size.
- * \param counter   The initial counter value. This is usually 0.
+ *                  It must be initialized and bound to a key.
+ * \param nonce     The nonce. This must be \c 12 Bytes in size.
+ * \param counter   The initial counter value. This is usually \c 0.
  *
  * \return          \c 0 on success.
  * \return          #MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA if ctx or nonce is
@@ -150,16 +158,16 @@
  *                  key and nonce.
  *
  * \param ctx       The ChaCha20 context to use for encryption or decryption.
- * \param size      The length of the input data in bytes.
+ *                  It must be initialized and bound to a key and nonce.
+ * \param size      The length of the input data in Bytes.
  * \param input     The buffer holding the input data.
- *                  This pointer can be NULL if size == 0.
+ *                  This pointer can be \c NULL if `size == 0`.
  * \param output    The buffer holding the output data.
- *                  Must be able to hold \p size bytes.
- *                  This pointer can be NULL if size == 0.
+ *                  This must be able to hold \p size Bytes.
+ *                  This pointer can be \c NULL if `size == 0`.
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA if the ctx, input, or
- *                  output pointers are NULL.
+ * \return          A negative error code on failure.
  */
 int mbedtls_chacha20_update( mbedtls_chacha20_context *ctx,
                              size_t size,
@@ -180,19 +188,19 @@
  * \note            The \p input and \p output pointers must either be equal or
  *                  point to non-overlapping buffers.
  *
- * \param key       The encryption/decryption key. Must be 32 bytes in length.
- * \param nonce     The nonce. Must be 12 bytes in size.
- * \param counter   The initial counter value. This is usually 0.
- * \param size      The length of the input data in bytes.
+ * \param key       The encryption/decryption key.
+ *                  This must be \c 32 Bytes in length.
+ * \param nonce     The nonce. This must be \c 12 Bytes in size.
+ * \param counter   The initial counter value. This is usually \c 0.
+ * \param size      The length of the input data in Bytes.
  * \param input     The buffer holding the input data.
- *                  This pointer can be NULL if size == 0.
+ *                  This pointer can be \c NULL if `size == 0`.
  * \param output    The buffer holding the output data.
- *                  Must be able to hold \p size bytes.
- *                  This pointer can be NULL if size == 0.
+ *                  This must be able to hold \p size Bytes.
+ *                  This pointer can be \c NULL if `size == 0`.
  *
  * \return          \c 0 on success.
- * \return          #MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA if key, nonce, input,
- *                  or output is NULL.
+ * \return          A negative error code on failure.
  */
 int mbedtls_chacha20_crypt( const unsigned char key[32],
                             const unsigned char nonce[12],