CTR_DRBG documentation improvements
* State explicit whether several numbers are in bits or bytes.
* Clarify whether buffer pointer parameters can be NULL.
* Clarify some terminology.
* Minor wording and formatting improvements.
diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 5a32843..f1b4f7c 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -1,13 +1,16 @@
/**
* \file ctr_drbg.h
*
- * \brief CTR_DRBG is based on AES-256, as defined in <em>NIST SP 800-90A:
- * Recommendation for Random Number Generation Using Deterministic
- * Random Bit Generators</em>.
+ * \brief The CTR_DRBG pseudorandom generator.
+ *
+ * CTR_DRBG is a standardized way of building a PRNG from a block-cipher
+ * in counter mode operation, as defined in <em>NIST SP 800-90A:
+ * Recommendation for Random Number Generation Using Deterministic Random
+ * Bit Generators</em>.
*
*/
/*
- * Copyright (C) 2006-2018, Arm Limited (or its affiliates), All Rights Reserved
+ * Copyright (C) 2006-2019, Arm Limited (or its affiliates), All Rights Reserved
* SPDX-License-Identifier: Apache-2.0
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may
@@ -92,7 +95,7 @@
#if !defined(MBEDTLS_CTR_DRBG_MAX_SEED_INPUT)
#define MBEDTLS_CTR_DRBG_MAX_SEED_INPUT 384
-/**< The maximum size of seed or reseed buffer. */
+/**< The maximum size of seed or reseed buffer in bytes. */
#endif
/* \} name SECTION: Module settings */
@@ -150,17 +153,21 @@
* \brief This function seeds and sets up the CTR_DRBG
* entropy source for future reseeds.
*
- * \note Personalization data can be provided in addition to the more generic
- * entropy source, to make this instantiation as unique as possible.
+ * You can provide a personalization string in addition to the
+ * entropy source, to make this instantiation as unique as possible.
+ *
*
* \param ctx The CTR_DRBG context to seed.
* \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.
- * \param custom Personalization data, that is device-specific
- identifiers. Can be NULL.
- * \param len The length of the personalization data.
+ * 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_CTR_DRBG_MAX_SEED_INPUT / 2.
*
* \return \c 0 on success, or
* #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.
@@ -183,7 +190,8 @@
* The default value is off.
*
* \note If enabled, entropy is gathered at the beginning of
- * every call to mbedtls_ctr_drbg_random_with_add().
+ * every call to mbedtls_ctr_drbg_random_with_add()
+ * or mbedtls_ctr_drbg_random().
* Only use this if your entropy source has sufficient
* throughput.
*
@@ -199,14 +207,20 @@
* #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
*
* \param ctx The CTR_DRBG context.
- * \param len The amount of entropy to grab.
+ * \param len The amount of entropy to grab, in bytes.
+ * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
*/
void mbedtls_ctr_drbg_set_entropy_len( mbedtls_ctr_drbg_context *ctx,
size_t len );
/**
* \brief This function sets the reseed interval.
- * The default value is #MBEDTLS_CTR_DRBG_RESEED_INTERVAL.
+ *
+ * The reseed interval is the number of calls to mbedtls_ctr_drbg_random()
+ * or mbedtls_ctr_drbg_random_with_add() after which the entropy function
+ * is called again.
+ *
+ * The default value is #MBEDTLS_CTR_DRBG_RESEED_INTERVAL.
*
* \param ctx The CTR_DRBG context.
* \param interval The reseed interval.
@@ -219,8 +233,12 @@
* extracts data from the entropy source.
*
* \param ctx The CTR_DRBG context.
- * \param additional Additional data to add to the state. Can be NULL.
+ * \param additional Additional data to add to the state. Can be \c NULL.
* \param len The length of the additional data.
+ * This must be less than
+ * #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - \c entropy_len
+ * where \c entropy_len is the entropy length
+ * configured for the context.
*
* \return \c 0 on success, or
* #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.
@@ -232,7 +250,8 @@
* \brief This function updates the state of the CTR_DRBG context.
*
* \param ctx The CTR_DRBG context.
- * \param additional The data to update the state with.
+ * \param additional The data to update the state with. This must not be
+ * \c NULL unless \p add_len is \c 0.
* \param add_len Length of \p additional in bytes. This must be at
* most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
*
@@ -258,8 +277,10 @@
* The remaining Bytes are silently discarded.
*
* \param ctx The CTR_DRBG context.
- * \param additional The data to update the state with.
- * \param add_len Length of \p additional data.
+ * \param additional The data to update the state with. This must not be
+ * \c NULL unless \p add_len is \c 0.
+ * \param add_len Length of \p additional data. This must be at
+ * most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
*/
void mbedtls_ctr_drbg_update( mbedtls_ctr_drbg_context *ctx,
const unsigned char *additional,
@@ -269,17 +290,26 @@
* \brief This function updates a CTR_DRBG instance with additional
* data and uses it to generate random data.
*
- * \note The function automatically reseeds if the reseed counter is exceeded.
+ * This function automatically reseeds if the reseed counter is exceeded
+ * or prediction resistance is enabled.
*
* \param p_rng The CTR_DRBG context. This must be a pointer to a
* #mbedtls_ctr_drbg_context structure.
* \param output The buffer to fill.
- * \param output_len The length of the buffer.
- * \param additional Additional data to update. Can be NULL.
- * \param add_len The length of the additional data.
+ * \param output_len The length of the buffer in bytes.
+ * \param additional Additional data to update. Can be \c NULL, in which
+ * case the additional data is empty regardless of
+ * the value of \p add_len.
+ * \param add_len The length of the additional data
+ * if \p additional is not \c NULL.
+ * This must be less than #MBEDTLS_CTR_DRBG_MAX_INPUT
+ * and less than
+ * #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - \c entropy_len
+ * where \c entropy_len is the entropy length
+ * configured for the context.
*
- * \return \c 0 on success, or
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
* #MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG on failure.
*/
int mbedtls_ctr_drbg_random_with_add( void *p_rng,
@@ -289,15 +319,17 @@
/**
* \brief This function uses CTR_DRBG to generate random data.
*
- * \note The function automatically reseeds if the reseed counter is exceeded.
+ * This function automatically reseeds if the reseed counter is exceeded
+ * or prediction resistance is enabled.
+ *
*
* \param p_rng The CTR_DRBG context. This must be a pointer to a
* #mbedtls_ctr_drbg_context structure.
* \param output The buffer to fill.
- * \param output_len The length of the buffer.
+ * \param output_len The length of the buffer in bytes.
*
- * \return \c 0 on success, or
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
* #MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG on failure.
*/
int mbedtls_ctr_drbg_random( void *p_rng,
@@ -310,9 +342,9 @@
* \param ctx The CTR_DRBG context.
* \param path The name of the file.
*
- * \return \c 0 on success,
- * #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error, or
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on reseed
* failure.
*/
int mbedtls_ctr_drbg_write_seed_file( mbedtls_ctr_drbg_context *ctx, const char *path );
@@ -324,10 +356,12 @@
* \param ctx The CTR_DRBG context.
* \param path The name of the file.
*
- * \return \c 0 on success,
- * #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error,
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
- * #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG on failure.
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on
+ * reseed failure.
+ * \return #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG if the existing
+ * seed file is too large.
*/
int mbedtls_ctr_drbg_update_seed_file( mbedtls_ctr_drbg_context *ctx, const char *path );
#endif /* MBEDTLS_FS_IO */