From e730ff68ee9fef8705e75a3fc2311d2da5eeeb4e Mon Sep 17 00:00:00 2001
From: Andres Amaya Garcia <andres.amayagarcia@arm.com>
Date: Mon, 8 Oct 2018 19:44:55 +0100
Subject: [PATCH] Improve docs for ASN.1 bitstrings and their usage

---
 include/mbedtls/asn1write.h | 18 ++++++++++--------
 include/mbedtls/x509_csr.h  |  8 ++++++++
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index d62853ebf..083601af3 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -189,16 +189,18 @@ int mbedtls_asn1_write_ia5_string( unsigned char **p, unsigned char *start,
                            const char *text, size_t text_len );
 
 /**
- * \brief           Write a bitstring tag (MBEDTLS_ASN1_BIT_STRING) and
- *                  value in ASN.1 format
- *                  Note: function works backwards in data buffer
+ * \brief           Write a bitstring tag (#MBEDTLS_ASN1_BIT_STRING) and
+ *                  value in ASN.1 format.
  *
- * \param p         reference to current position pointer
- * \param start     start of the buffer (for bounds-checking)
- * \param buf       the bitstring
- * \param bits      the total number of bits in the bitstring
+ * \note            This function works backwards in data buffer.
  *
- * \return          the length written or a negative error code
+ * \param p         The reference to the current position pointer.
+ * \param start     The start of the buffer, for bounds-checking.
+ * \param buf       The bitstring to write.
+ * \param bits      The total number of bits in the bitstring.
+ *
+ * \return          The number of bytes written to \p p on success.
+ * \return          A negative error code on failure.
  */
 int mbedtls_asn1_write_bitstring( unsigned char **p, unsigned char *start,
                           const unsigned char *buf, size_t bits );
diff --git a/include/mbedtls/x509_csr.h b/include/mbedtls/x509_csr.h
index 0c6ccad78..a3c28048e 100644
--- a/include/mbedtls/x509_csr.h
+++ b/include/mbedtls/x509_csr.h
@@ -205,6 +205,14 @@ void mbedtls_x509write_csr_set_md_alg( mbedtls_x509write_csr *ctx, mbedtls_md_ty
  * \param key_usage key usage flags to set
  *
  * \return          0 if successful, or MBEDTLS_ERR_X509_ALLOC_FAILED
+ *
+ * \note            The <code>decipherOnly</code> flag from the Key Usage
+ *                  extension is represented by bit 8 (i.e.
+ *                  <code>0x8000</code>), which cannot typically be represented
+ *                  in an unsigned char. Therefore, the flag
+ *                  <code>decipherOnly</code> (i.e.
+ *                  #MBEDTLS_X509_KU_DECIPHER_ONLY) cannot be set using this
+ *                  function.
  */
 int mbedtls_x509write_csr_set_key_usage( mbedtls_x509write_csr *ctx, unsigned char key_usage );