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CTR_DRBG: support set_entropy_len() before seed()

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mbedtls_ctr_drbg_seed() always set the entropy length to the default,
so a call to mbedtls_ctr_drbg_set_entropy_len() before seed() had no
effect. Change this to the more intuitive behavior that
set_entropy_len() sets the entropy length and seed() respects that and
only uses the default entropy length if there was no call to
set_entropy_len().

The former test-only function mbedtls_ctr_drbg_seed_entropy_len() is
no longer used, but keep it for strict ABI compatibility.
This commit is contained in:
Gilles Peskine 2019-10-04 12:15:55 +02:00
parent c4280acfa0
commit 912ffe414e
3 changed files with 35 additions and 40 deletions
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16
include/mbedtls/ctr_drbg.h
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@ -214,11 +214,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
* with mbedtls_entropy_init() (which registers the platform's default
* entropy sources).
*
* \p f_entropy is always called with a buffer size equal to the entropy
* length. The entropy length is initially #MBEDTLS_CTR_DRBG_ENTROPY_LEN
* and this value is always used for the initial seeding. You can change
* the entropy length for subsequent seeding by calling
* mbedtls_ctr_drbg_set_entropy_len() after this function.
* The entropy length is #MBEDTLS_CTR_DRBG_ENTROPY_LEN by default.
* You can override it by calling mbedtls_ctr_drbg_set_entropy_len().
*
* You can provide a personalization string in addition to the
* entropy source, to make this instantiation as unique as possible.
@ -255,6 +252,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_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 buffer size
* 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
@ -298,15 +297,10 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
/**
* \brief This function sets the amount of entropy grabbed on each
* subsequent reseed.
* seed or reseed.
*
* The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
*
* \note mbedtls_ctr_drbg_seed() always sets the entropy length
* to #MBEDTLS_CTR_DRBG_ENTROPY_LEN, so this function
* only has an effect when it is called after
* mbedtls_ctr_drbg_seed().
*
* \note The security strength of CTR_DRBG is bounded by the
* entropy length. Thus:
* - When using AES-256