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HMAC_DRBG: improve the documentation of the entropy length

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This commit is contained in:
Gilles Peskine 2019-09-30 15:20:52 +02:00
parent 74efcd2b71
commit 3457b5e05e
1 changed files with 28 additions and 8 deletions
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36
include/mbedtls/hmac_drbg.h
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@ -122,6 +122,32 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
* *
* Set the initial seed and set up the entropy source for future reseeds. * Set the initial seed and set up the entropy source for future reseeds.
* *
* A typical choice for the \p f_entropy and \p p_entropy parameters is
* to use the entropy module:
* - \p f_entropy is mbedtls_entropy_func();
* - \p p_entropy is an instance of ::mbedtls_entropy_context initialized
* with mbedtls_entropy_init() (which registers the platform's default
* entropy sources).
*
* \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().
*
* \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 ctx HMAC_DRBG context to be seeded.
* \param md_info MD algorithm to use for HMAC_DRBG. * \param md_info MD algorithm to use for HMAC_DRBG.
* \param f_entropy The entropy callback, taking as arguments the * \param f_entropy The entropy callback, taking as arguments the
@ -137,13 +163,7 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
* and also at most * and also at most
* #MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT - \p entropy_len * 3 / 2 * #MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT - \p entropy_len * 3 / 2
* where \p entropy_len is the entropy length * where \p entropy_len is the entropy length
* (see mbedtls_hmac_drbg_set_entropy_len()). * described above.
*
* \note The "security strength" as defined by NIST is set to:
* 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.
* *
* \return 0 if successful. * \return 0 if successful.
* \return #MBEDTLS_ERR_MD_BAD_INPUT_DATA if \p md_info is * \return #MBEDTLS_ERR_MD_BAD_INPUT_DATA if \p md_info is
@ -203,7 +223,7 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
* seed or reseed. * seed or reseed.
* *
* The default value is given by the security strength, which depends on the * The default value is given by the security strength, which depends on the
* hash used. See mbedtls_hmac_drbg_init(). * hash used. See the documentation of mbedtls_hmac_drbg_seed() for details.
* *
* \param ctx The HMAC_DRBG context. * \param ctx The HMAC_DRBG context.
* \param len The amount of entropy to grab, in bytes. * \param len The amount of entropy to grab, in bytes.