Reword documentation of CMAC operations

Change the wording of the documentation for some CMAC functions,
as the existing wording, while technically correct, can be
easy to misunderstand. The reworded docs explain the flow of
a CMAC computation a little more fully.

Signed-off-by: David Horstmann <david.horstmann@arm.com>

1 files changed, 33 insertions, 16 deletions

diff --git a/include/mbedtls/cmac.h b/include/mbedtls/cmac.h
index 94139d5..8934886 100644
--- a/include/mbedtls/cmac.h
+++ b/include/mbedtls/cmac.h
@@ -74,9 +74,17 @@ struct mbedtls_cmac_context_t
 #endif /* !MBEDTLS_CMAC_ALT */
 
 /**
- * \brief               This function sets the CMAC key, and prepares to authenticate
+ * \brief               This function starts a new CMAC computation
+ *                      by setting the CMAC key, and preparing to authenticate
  *                      the input data.
- *                      Must be called with an initialized cipher context.
+ *                      It must be called with an initialized cipher context.
+ *
+ *                      Once this function has completed, data can be supplied
+ *                      to the CMAC computation by calling
+ *                      mbedtls_cipher_cmac_update().
+ *
+ *                      To start a CMAC computation using the same key as a previous
+ *                      CMAC computation, use mbedtls_cipher_cmac_finish().
  *
  * \note                When the CMAC implementation is supplied by an alternate
  *                      implementation (through #MBEDTLS_CMAC_ALT), some ciphers
@@ -102,9 +110,15 @@ int mbedtls_cipher_cmac_starts( mbedtls_cipher_context_t *ctx,
  * \brief               This function feeds an input buffer into an ongoing CMAC
  *                      computation.
  *
- *                      It is called between mbedtls_cipher_cmac_starts() or
- *                      mbedtls_cipher_cmac_reset(), and mbedtls_cipher_cmac_finish().
- *                      Can be called repeatedly.
+ *                      The CMAC computation must have previously been started
+ *                      by calling mbedtls_cipher_cmac_starts() or
+ *                      mbedtls_cipher_cmac_reset().
+ *
+ *                      Call this function as many times as needed to input the
+ *                      data to be authenticated.
+ *                      Once all of the required data has been input,
+ *                      call mbedtls_cipher_cmac_finish() to obtain the result
+ *                      of the CMAC operation.
  *
  * \param ctx           The cipher context used for the CMAC operation.
  * \param input         The buffer holding the input data.
@@ -118,12 +132,13 @@ int mbedtls_cipher_cmac_update( mbedtls_cipher_context_t *ctx,
                                 const unsigned char *input, size_t ilen );
 
 /**
- * \brief               This function finishes the CMAC operation, and writes
- *                      the result to the output buffer.
+ * \brief               This function finishes an ongoing CMAC operation, and
+ *                      writes the result to the output buffer.
  *
- *                      It is called after mbedtls_cipher_cmac_update().
- *                      It can be followed by mbedtls_cipher_cmac_reset() and
- *                      mbedtls_cipher_cmac_update(), or mbedtls_cipher_free().
+ *                      It should be followed either by
+ *                      mbedtls_cipher_cmac_reset(), which starts another CMAC
+ *                      operation with the same key, or mbedtls_cipher_free(),
+ *                      which clears the cipher context.
  *
  * \param ctx           The cipher context used for the CMAC operation.
  * \param output        The output buffer for the CMAC checksum result.
@@ -136,12 +151,14 @@ int mbedtls_cipher_cmac_finish( mbedtls_cipher_context_t *ctx,
                                 unsigned char *output );
 
 /**
- * \brief               This function prepares the authentication of another
- *                      message with the same key as the previous CMAC
- *                      operation.
- *
- *                      It is called after mbedtls_cipher_cmac_finish()
- *                      and before mbedtls_cipher_cmac_update().
+ * \brief               This function starts a new CMAC operation with the same
+ *                      key as the previous one.
+ *
+ *                      It should be called after finishing the previous CMAC
+ *                      operation with mbedtls_cipher_cmac_finish().
+ *                      After calling this function,
+ *                      call mbedtls_cipher_cmac_update() to supply the new
+ *                      CMAC operation with data.
  *
  * \param ctx           The cipher context used for the CMAC operation.
  *