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mbedtls_hmac_drbg_set_entropy_len() only matters when reseeding

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The documentation of HMAC_DRBG erroneously claimed that
mbedtls_hmac_drbg_set_entropy_len() had an impact on the initial
seeding. This is in fact not the case: mbedtls_hmac_drbg_seed() forces
the entropy length to its chosen value. Fix the documentation.
This commit is contained in:
Gilles Peskine 2019-10-04 11:52:22 +02:00
parent cc74872ba9
commit 8b424397b9
1 changed files with 14 additions and 11 deletions
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25
include/mbedtls/hmac_drbg.h
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@ -139,13 +139,13 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
* 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().
* mbedtls_hmac_drbg_set_entropy_len() afterwards.
*
* \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 The entropy length for the initial seeding is
* the security strength (converted from bits to bytes).
* You can set a different entropy length for subsequent
* seeding by calling mbedtls_hmac_drbg_set_entropy_len()
* after this function.
*
* \note During the initial seeding, this function calls
* the entropy source to obtain a nonce
@ -156,6 +156,8 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
* \param f_entropy The entropy callback, taking as arguments the
* \p p_entropy context, the buffer to fill, and the
* length of the buffer.
* \p f_entropy is always called with a length that is
* less than or equal to the entropy length.
* \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
@ -222,13 +224,14 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
/**
* \brief This function sets the amount of entropy grabbed on each
* seed or reseed.
* reseed.
*
* During the initial seeding, mbedtls_hmac_drbg_seed() additionally grabs
* half this amount to create the nonce.
* The default value is set by mbedtls_hmac_drbg_seed().
*
* 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.
* \note mbedtls_hmac_drbg_seed() always sets the entropy length
* to the default value based on the chosen MD algorithm,
* so this function only has an effect if it is called
* after mbedtls_hmac_drbg_seed().
*
* \param ctx The HMAC_DRBG context.
* \param len The amount of entropy to grab, in bytes.