HMAC_DRBG documentation improvements
* State explicit whether several numbers are in bits or bytes.
* Clarify whether buffer pointer parameters can be NULL.
* Minor wording and formatting improvements.
diff --git a/include/mbedtls/hmac_drbg.h b/include/mbedtls/hmac_drbg.h
index f090ac4..f253b9a 100644
--- a/include/mbedtls/hmac_drbg.h
+++ b/include/mbedtls/hmac_drbg.h
@@ -108,38 +108,65 @@
} mbedtls_hmac_drbg_context;
/**
- * \brief HMAC_DRBG context initialization
- * Makes the context ready for mbedtls_hmac_drbg_seed(),
- * mbedtls_hmac_drbg_seed_buf() or
- * mbedtls_hmac_drbg_free().
+ * \brief HMAC_DRBG context initialization.
*
- * \param ctx HMAC_DRBG context to be initialized
+ * This function makes the context ready for mbedtls_hmac_drbg_seed(),
+ * mbedtls_hmac_drbg_seed_buf() or mbedtls_hmac_drbg_free().
+ *
+ * \param ctx HMAC_DRBG context to be initialized.
*/
void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
/**
- * \brief HMAC_DRBG initial seeding
- * Seed and setup entropy source for future reseeds.
+ * \brief HMAC_DRBG initial seeding.
*
- * \param ctx HMAC_DRBG context to be seeded
- * \param md_info MD algorithm to use for HMAC_DRBG
- * \param f_entropy Entropy callback (p_entropy, buffer to fill, buffer
- * length)
- * \param p_entropy Entropy context
- * \param custom Personalization data (Device specific identifiers)
- * (Can be NULL)
- * \param len Length of personalization data
+ * Set the initial seed and set up the entropy source for future reseeds.
*
- * \note The "security strength" as defined by NIST is set to:
- * 128 bits if md_alg is SHA-1,
- * 192 bits if md_alg is SHA-224,
- * 256 bits if md_alg is SHA-256 or higher.
+ * You can provide a personalization string in addition to the
+ * entropy source, to make this instantiation as unique as possible.
+ *
+ * \note By default, the security strength as defined by NIST is:
+ * - 128 bits if \p md_info is SHA-1;
+ * - 192 bits if \p md_info is SHA-224;
+ * - 256 bits if \p md_info is SHA-256, SHA-384 or SHA-512.
* Note that SHA-256 is just as efficient as SHA-224.
+ * The security strength can be reduced if a smaller
+ * entropy length is set with
+ * mbedtls_hmac_drbg_set_entropy_len().
*
- * \return 0 if successful, or
- * MBEDTLS_ERR_MD_BAD_INPUT_DATA, or
- * MBEDTLS_ERR_MD_ALLOC_FAILED, or
- * MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED.
+ * \note The default entropy length is the security strength
+ * (converted from bits to bytes). You can override
+ * it mbedtls_hmac_drbg_set_entropy_len().
+ * \p f_entropy is always called with a length that is
+ * less than or equal to the entropy length.
+ *
+ * \note During the initial seeding, this function calls
+ * the entropy source to obtain a nonce
+ * whose length is half the entropy length.
+ *
+ * \param ctx HMAC_DRBG context to be seeded.
+ * \param md_info MD algorithm to use for HMAC_DRBG.
+ * \param f_entropy The entropy callback, taking as arguments the
+ * \p p_entropy context, the buffer to fill, and the
+ * length of the buffer.
+ * \param p_entropy The entropy context to pass to \p f_entropy.
+ * \param custom The personalization string.
+ * This can be \c NULL, in which case the personalization
+ * string is empty regardless of the value of \p len.
+ * \param len The length of the personalization string.
+ * This must be at most #MBEDTLS_HMAC_DRBG_MAX_INPUT
+ * and also at most
+ * #MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT - \p entropy_len * 3 / 2
+ * where \p entropy_len is the entropy length
+ * described above.
+ *
+ * \return \c 0 if successful.
+ * \return #MBEDTLS_ERR_MD_BAD_INPUT_DATA if \p md_info is
+ * invalid.
+ * \return #MBEDTLS_ERR_MD_ALLOC_FAILED if there was not enough
+ * memory to allocate context data.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * if the call to \p f_entropy failed.
*/
int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
const mbedtls_md_info_t * md_info,
@@ -150,66 +177,84 @@
/**
* \brief Initilisation of simpified HMAC_DRBG (never reseeds).
- * (For use with deterministic ECDSA.)
*
- * \param ctx HMAC_DRBG context to be initialised
- * \param md_info MD algorithm to use for HMAC_DRBG
- * \param data Concatenation of entropy string and additional data
- * \param data_len Length of data in bytes
+ * This function is meant for use in algorithms that need a pseudorandom
+ * input such as deterministic ECDSA.
*
- * \return 0 if successful, or
- * MBEDTLS_ERR_MD_BAD_INPUT_DATA, or
- * MBEDTLS_ERR_MD_ALLOC_FAILED.
+ * \param ctx HMAC_DRBG context to be initialised.
+ * \param md_info MD algorithm to use for HMAC_DRBG.
+ * \param data Concatenation of the initial entropy string and
+ * the additional data.
+ * \param data_len Length of \p data in bytes.
+ *
+ * \return \c 0 if successful. or
+ * \return #MBEDTLS_ERR_MD_BAD_INPUT_DATA if \p md_info is
+ * invalid.
+ * \return #MBEDTLS_ERR_MD_ALLOC_FAILED if there was not enough
+ * memory to allocate context data.
*/
int mbedtls_hmac_drbg_seed_buf( mbedtls_hmac_drbg_context *ctx,
const mbedtls_md_info_t * md_info,
const unsigned char *data, size_t data_len );
/**
- * \brief Enable / disable prediction resistance (Default: Off)
+ * \brief This function turns prediction resistance on or off.
+ * The default value is off.
*
- * Note: If enabled, entropy is used for ctx->entropy_len before each call!
- * Only use this if you have ample supply of good entropy!
+ * \note If enabled, entropy is gathered at the beginning of
+ * every call to mbedtls_hmac_drbg_random_with_add()
+ * or mbedtls_hmac_drbg_random().
+ * Only use this if your entropy source has sufficient
+ * throughput.
*
- * \param ctx HMAC_DRBG context
- * \param resistance MBEDTLS_HMAC_DRBG_PR_ON or MBEDTLS_HMAC_DRBG_PR_OFF
+ * \param ctx The HMAC_DRBG context.
+ * \param resistance #MBEDTLS_HMAC_DRBG_PR_ON or #MBEDTLS_HMAC_DRBG_PR_OFF.
*/
void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx,
int resistance );
/**
- * \brief Set the amount of entropy grabbed on each reseed
- * (Default: given by the security strength, which
- * depends on the hash used, see \c mbedtls_hmac_drbg_init() )
+ * \brief This function sets the amount of entropy grabbed on each
+ * seed or reseed.
*
- * \param ctx HMAC_DRBG context
- * \param len Amount of entropy to grab, in bytes
+ * During the initial seeding, mbedtls_hmac_drbg_seed() additionally grabs
+ * half this amount to create the nonce.
+ *
+ * The default value is given by the security strength, which depends on the
+ * hash used. See the documentation of mbedtls_hmac_drbg_seed() for details.
+ *
+ * \param ctx The HMAC_DRBG context.
+ * \param len The amount of entropy to grab, in bytes.
*/
void mbedtls_hmac_drbg_set_entropy_len( mbedtls_hmac_drbg_context *ctx,
size_t len );
/**
- * \brief Set the reseed interval
- * (Default: MBEDTLS_HMAC_DRBG_RESEED_INTERVAL)
+ * \brief Set the reseed interval.
*
- * \param ctx HMAC_DRBG context
- * \param interval Reseed interval
+ * The reseed interval is the number of calls to mbedtls_hmac_drbg_random()
+ * or mbedtls_hmac_drbg_random_with_add() after which the entropy function
+ * is called again.
+ *
+ * The default value is #MBEDTLS_HMAC_DRBG_RESEED_INTERVAL.
+ *
+ * \param ctx The HMAC_DRBG context.
+ * \param interval The reseed interval.
*/
void mbedtls_hmac_drbg_set_reseed_interval( mbedtls_hmac_drbg_context *ctx,
int interval );
/**
- * \brief HMAC_DRBG update state
+ * \brief This function updates the state of the HMAC_DRBG context.
*
- * \param ctx HMAC_DRBG context
- * \param additional Additional data to update state with, or NULL
- * \param add_len Length of additional data, or 0
+ * \param ctx The HMAC_DRBG context.
+ * \param additional The data to update the state with.
+ * If this is \c NULL, there is no additional data.
+ * \param add_len Length of \p additional in bytes.
+ * Unused if \p additional is \c NULL.
*
* \return \c 0 on success, or an error from the underlying
* hash calculation.
- *
- * \note Additional data is optional, pass NULL and 0 as second
- * third argument if no additional data is being used.
*/
int mbedtls_hmac_drbg_update_ret( mbedtls_hmac_drbg_context *ctx,
const unsigned char *additional, size_t add_len );
@@ -232,33 +277,52 @@
size_t add_len );
/**
- * \brief HMAC_DRBG reseeding (extracts data from entropy source)
+ * \brief This function reseeds the HMAC_DRBG context, that is
+ * extracts data from the entropy source.
*
- * \param ctx HMAC_DRBG context
- * \param additional Additional data to add to state (Can be NULL)
- * \param len Length of additional data
+ * \param ctx The HMAC_DRBG context.
+ * \param additional Additional data to add to the state.
+ * If this is \c NULL, there is no additional data
+ * and \p len should be \c 0.
+ * \param len The length of the additional data.
+ * This must be at most #MBEDTLS_HMAC_DRBG_MAX_INPUT
+ * and also at most
+ * #MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT - \p entropy_len
+ * where \p entropy_len is the entropy length
+ * (see mbedtls_hmac_drbg_set_entropy_len()).
*
- * \return 0 if successful, or
- * MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * \return \c 0 if successful.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * if a call to the entropy function failed.
*/
int mbedtls_hmac_drbg_reseed( mbedtls_hmac_drbg_context *ctx,
const unsigned char *additional, size_t len );
/**
- * \brief HMAC_DRBG generate random with additional update input
+ * \brief This function updates an HMAC_DRBG instance with additional
+ * data and uses it to generate random data.
*
- * Note: Automatically reseeds if reseed_counter is reached or PR is enabled.
+ * This function automatically reseeds if the reseed counter is exceeded
+ * or prediction resistance is enabled.
*
- * \param p_rng HMAC_DRBG context
- * \param output Buffer to fill
- * \param output_len Length of the buffer
- * \param additional Additional data to update with (can be NULL)
- * \param add_len Length of additional data (can be 0)
+ * \param p_rng The HMAC_DRBG context. This must be a pointer to a
+ * #mbedtls_hmac_drbg_context structure.
+ * \param output The buffer to fill.
+ * \param output_len The length of the buffer in bytes.
+ * This must be at most #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
+ * \param additional Additional data to update with.
+ * If this is \c NULL, there is no additional data
+ * and \p add_len should be \c 0.
+ * \param add_len The length of the additional data.
+ * This must be at most #MBEDTLS_HMAC_DRBG_MAX_INPUT.
*
- * \return 0 if successful, or
- * MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED, or
- * MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG, or
- * MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG.
+ * \return \c 0 if successful.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * if a call to the entropy source failed.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG if
+ * \p output_len > #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG if
+ * \p add_len > #MBEDTLS_HMAC_DRBG_MAX_INPUT.
*/
int mbedtls_hmac_drbg_random_with_add( void *p_rng,
unsigned char *output, size_t output_len,
@@ -266,49 +330,59 @@
size_t add_len );
/**
- * \brief HMAC_DRBG generate random
+ * \brief This function uses HMAC_DRBG to generate random data.
*
- * Note: Automatically reseeds if reseed_counter is reached or PR is enabled.
+ * This function automatically reseeds if the reseed counter is exceeded
+ * or prediction resistance is enabled.
*
- * \param p_rng HMAC_DRBG context
- * \param output Buffer to fill
- * \param out_len Length of the buffer
+ * \param p_rng The HMAC_DRBG context. This must be a pointer to a
+ * #mbedtls_hmac_drbg_context structure.
+ * \param output The buffer to fill.
+ * \param out_len The length of the buffer in bytes.
+ * This must be at most #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
*
- * \return 0 if successful, or
- * MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED, or
- * MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG
+ * \return \c 0 if successful.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * if a call to the entropy source failed.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_REQUEST_TOO_BIG if
+ * \p out_len > #MBEDTLS_HMAC_DRBG_MAX_REQUEST.
*/
int mbedtls_hmac_drbg_random( void *p_rng, unsigned char *output, size_t out_len );
/**
* \brief Free an HMAC_DRBG context
*
- * \param ctx HMAC_DRBG context to free.
+ * \param ctx The HMAC_DRBG context to free.
*/
void mbedtls_hmac_drbg_free( mbedtls_hmac_drbg_context *ctx );
#if defined(MBEDTLS_FS_IO)
/**
- * \brief Write a seed file
+ * \brief This function writes a seed file.
*
- * \param ctx HMAC_DRBG context
- * \param path Name of the file
+ * \param ctx The HMAC_DRBG context.
+ * \param path The name of the file.
*
- * \return 0 if successful, 1 on file error, or
- * MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR on file error.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED on reseed
+ * failure.
*/
int mbedtls_hmac_drbg_write_seed_file( mbedtls_hmac_drbg_context *ctx, const char *path );
/**
- * \brief Read and update a seed file. Seed is added to this
- * instance
+ * \brief This function reads and updates a seed file. The seed
+ * is added to this instance.
*
- * \param ctx HMAC_DRBG context
- * \param path Name of the file
+ * \param ctx The HMAC_DRBG context.
+ * \param path The name of the file.
*
- * \return 0 if successful, 1 on file error,
- * MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED or
- * MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR on file error.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED on
+ * reseed failure.
+ * \return #MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG if the existing
+ * seed file is too large.
*/
int mbedtls_hmac_drbg_update_seed_file( mbedtls_hmac_drbg_context *ctx, const char *path );
#endif /* MBEDTLS_FS_IO */
@@ -316,9 +390,10 @@
#if defined(MBEDTLS_SELF_TEST)
/**
- * \brief Checkup routine
+ * \brief The HMAC_DRBG Checkup routine.
*
- * \return 0 if successful, or 1 if the test failed
+ * \return \c 0 if successful.
+ * \return \c 1 if the test failed.
*/
int mbedtls_hmac_drbg_self_test( int verbose );
#endif