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Explain the design of mbedtls_psa_get_random better

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-01-05 16:03:55 +01:00
parent 0c59ba88cb
commit 9c3e060253
2 changed files with 20 additions and 2 deletions
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18
library/psa_crypto.c
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@ -6412,14 +6412,28 @@ psa_status_t psa_generate_random( uint8_t *output,
} }
/* Wrapper function allowing the classic API to use the PSA RNG. /* Wrapper function allowing the classic API to use the PSA RNG.
* In the non-external case, mbedtls_psa_get_random is defined *
* as a constant function pointer in psa_crypto_random_impl.h. * `mbedtls_psa_get_random(MBEDTLS_PSA_RANDOM_STATE, ...)` calls
* `psa_generate_random(...)`. The state parameter is ignored since the
* PSA API doesn't support passing an explicit state.
*
* In the non-external case, psa_generate_random() calls an
* `mbedtls_xxx_drbg_random` function which has exactly the same signature
* and semantics as mbedtls_psa_get_random(). As an optimization,
* instead of doing this back-and-forth between the PSA API and the
* classic API, psa_crypto_random_impl.h defines `mbedtls_psa_get_random`
* as a constant function pointer to `mbedtls_xxx_drbg_random`.
*/ */
#if defined (MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG) #if defined (MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
int mbedtls_psa_get_random( void *p_rng, int mbedtls_psa_get_random( void *p_rng,
unsigned char *output, unsigned char *output,
size_t output_size ) size_t output_size )
{ {
/* This function takes a pointer to the RNG state because that's what
* classic mbedtls functions using an RNG expect. The PSA RNG manages
* its own state internally and doesn't let the caller access that state.
* So we just ignore the state parameter, and in practice we'll pass
* NULL. */
(void) p_rng; (void) p_rng;
psa_status_t status = psa_generate_random( output, output_size ); psa_status_t status = psa_generate_random( output, output_size );
if( status == PSA_SUCCESS ) if( status == PSA_SUCCESS )

4
library/psa_crypto_random_impl.h
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@ -30,10 +30,12 @@
typedef mbedtls_psa_external_random_context_t mbedtls_psa_random_context_t; typedef mbedtls_psa_external_random_context_t mbedtls_psa_random_context_t;
/* Trivial wrapper around psa_generate_random(). */
int mbedtls_psa_get_random( void *p_rng, int mbedtls_psa_get_random( void *p_rng,
unsigned char *output, unsigned char *output,
size_t output_size ); size_t output_size );
/* The PSA RNG API doesn't need any externally maintained state. */
#define MBEDTLS_PSA_RANDOM_STATE NULL #define MBEDTLS_PSA_RANDOM_STATE NULL
#else /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */ #else /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
@ -163,6 +165,8 @@ static mbedtls_f_rng_t *const mbedtls_psa_get_random = mbedtls_hmac_drbg_random;
* This variable is only intended to be used through the macro * This variable is only intended to be used through the macro
* #MBEDTLS_PSA_RANDOM_STATE. * #MBEDTLS_PSA_RANDOM_STATE.
*/ */
/* psa_crypto.c sets this variable to a pointer to the DRBG state in the
* global PSA crypto state. */
extern mbedtls_psa_drbg_context_t *const mbedtls_psa_random_state; extern mbedtls_psa_drbg_context_t *const mbedtls_psa_random_state;
/** A pointer to the PSA DRBG state. /** A pointer to the PSA DRBG state.