Random number generator (RNG) module#
The Random number generator (RNG) module provides random number generation, see mbedtls_ctr_drbg_random().
The block-cipher counter-mode based deterministic random bit generator (CTR_DBRG) as specified in NIST SP800-90. It needs an external source of entropy. For these purposes mbedtls_entropy_func() can be used. This is an implementation based on a simple entropy accumulator design.
Meaning that there seems to be no practical algorithm that can guess the next bit with a probability larger than 1/2 in an output sequence.
This module can be used to generate random numbers.
Silicon Labs Default Entropy Source Plugin#
This plugin supports using the default entropy source(s) on Silicon Labs devices, and plugs in to mbed TLS' entropy collector interface when enabled.
This plugin can be enabled by selecting Mbed TLS support for default hardware entropy source component from Platform Security category.
Silicon Labs Hardware Entropy Source Plugins#
These plugins support getting hardware-provided entropy, and provide an implementation of mbedtls_hardware_poll, which the mbed TLS DRBG entropy collector interfaces with for collector updates.
True Random Number Generator (TRNG)#
The TRNG peripheral collects data from from a dedicated NIST-800-90B compliant source. The TRNG peripheral may either be stand-alone or integrated in the SE or CRYPTOACC peripheral depending on the target device. It is used automatically on compatible devices for entropy collection through mbed TLS when the Silicon Labs Default Entropy Source Plugin is enabled.
Radio (RAIL) Entropy Source#
The RAIL (radio) entropy source is used as a fallback entropy source for mbedTLS, which is used on devices that do not have a functional TRNG. This would provide entropy for applications needing random numbers or applications indirectly using mbedTLS modules that depend on the random number generation interfaces of mbed TLS, even when a TRNG is not present or accessible. It does, however, require that the part supports radio communications, and that the Silicon Labs RAIL library is compiled in to and initialised by the application.
Fallback mode is used automatically when the Silicon Labs Default Entropy Source Plugin is enabled and the device has a radio module, but no functional TRNG.
Note
The user is responsible for initializing the radio properly before calling into mbedtls API functions that will use the RAIL entropy module. Normally the radio initialization is performed by calling the RAIL API. The wireless stacks from Silicon Labs perform radio initialization internally, in which case the user should just make sure the wireless stacks initializes before user code can call into mbedtls API functions that will use the RAIL entropy module.
ADC Entropy Source Plugin#
This plugin supports getting entropy from a Silicon Labs ADC peripheral, instead of the default entropy source. ADC entropy collection is not automatically added to the mbed TLS entropy collector upon inclusion of this plugin, so if this is to be used with the mbed TLS DRBG collector, or for use in mbed TLS modules relying on entropy, the user needs to take extra steps in order to correctly set up the plugin.
This plugin currently only supports the ADC peripheral on Series-1 devices. See ADC Entropy Source Plugin for more details.
Mbed TLS Support for non-volatile entropy seed#
In absence of one of the hardware entropy sources, the default entropy source plugin falls back to generating entropy based off of a non-volatile seed.
This functionality is dependent on having a NVM3 default instance included in the application, since that is where the seed gets stored. On first boot, a seed gets generated by hashing device-unique information. On subsequent cycles, the seed gets rolled over and mixed with potential other entropy sources, meaning that the amount of reboot cycles and reinitialisations of the entropy collector becomes part of the entropy.
Warnings
The seed generated at first boot is considered low quality. If high quality entropy is required, but the device does not support any of the Silicon Labs Hardware Entropy Source Plugins then the application developer is strongly encouraged to write a high-quality seed during factory production by calling
mbedtls_nv_seed_writeand providing a true-random seed of size #MBEDTLS_ENTROPY_BLOCK_SIZEUsers are strongly encouraged to look into using ADC entropy if at all possible for their application. See ADC Entropy Source Plugin and AN1311.