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Document that MBEDTLS_PSA_HMAC_DRBG_MD_TYPE does not force HMAC

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MBEDTLS_PSA_HMAC_DRBG_MD_TYPE was documented and announced as causing the
PSA DRBG to be HMAC_DRBG. However, that was never actually implemented:
CTR_DRBG is prioritized if enabled.

Since there is a simple workaround of disabling MBEDTLS_CTR_DRBG_C if you
want to use HMAC_DRBG, we have decided to accept the actual behavior and fix
the documentation.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2024-07-01 21:14:45 +02:00
parent f6275b745f
commit 3f21ca7f16
2 changed files with 16 additions and 5 deletions
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4
ChangeLog.d/MBEDTLS_PSA_HMAC_DRBG_MD_TYPE.txt Normal file
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@ -0,0 +1,4 @@
Security
* Unlike previously documented, enabling MBEDTLS_PSA_HMAC_DRBG_MD_TYPE does
not cause the PSA subsystem to use HMAC_DRBG: it uses HMAC_DRBG only when
MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG and MBEDTLS_CTR_DRBG_C are disabled.

17
include/mbedtls/mbedtls_config.h
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@ -4016,11 +4016,18 @@
* Use HMAC_DRBG with the specified hash algorithm for HMAC_DRBG for the
* PSA crypto subsystem.
*
* If this option is unset:
* - If CTR_DRBG is available, the PSA subsystem uses it rather than HMAC_DRBG.
* - Otherwise, the PSA subsystem uses HMAC_DRBG with either
* #MBEDTLS_MD_SHA512 or #MBEDTLS_MD_SHA256 based on availability and
* on unspecified heuristics.
* If this option is unset, the library chooses a hash (currently between
* #MBEDTLS_MD_SHA512 and #MBEDTLS_MD_SHA256) based on availability and
* unspecified heuristics.
*
* \note The PSA crypto subsystem uses the first available mechanism amongst
* the following:
* - #MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG if enabled;
* - Entropy from #MBEDTLS_ENTROPY_C plus CTR_DRBG with AES
* if #MBEDTLS_CTR_DRBG_C is enabled;
* - Entropy from #MBEDTLS_ENTROPY_C plus HMAC_DRBG.
*
* A future version may reevaluate the prioritization of DRBG mechanisms.
*/
//#define MBEDTLS_PSA_HMAC_DRBG_MD_TYPE MBEDTLS_MD_SHA256