Hardware random overview
This article gives information about the hardware random (HWRNG) framework.
- 1 Framework purpose
- 2 System overview
- 3 Configuration
- 4 How to use the framework
- 5 How to trace and debug the framework
- 6 Source code location
- 7 To go further
- 8 References
1 Framework purpose
The Hardware random framework is integrated in the kernel. It provides access to RNG peripherals and focuses on supporting the hardware number generator.
2 System overview
The HW random framework allows retrieving random numbers in userland.
2.1 Component description
- HW random core (Kernel space)
Generic interface in kernel space. This layer is in charge of creating the character device (char device) and sysfs to access hw_random.
- RNG (Kernel space)
Hardware random Linux® drivers handling the HW blocks.
- RNG (Hardware)
HW blocks handling the RNG peripheral.
2.2 API description
The Hardware random framework uses char device API ioctl operations. For additional information, refer to:
- sysfs interface.
- Kernel Documentation directory
3.1 Kernel configuration
The Hardware random support is activated by default in ST deliveries. No specific configuration is required apart from enabling or disabling peripheral support using Linux® Menuconfig tool. Refer to Menuconfig or how to configure kernel and select:
[*] Device Drivers ---> [*] Character devices ---> [*] Hardware Random Number Generator Core support ---> [*] STMicroelectronics STM32 random number generator
3.2 Device tree configuration
DT configuration can be done thanks to the STM32CubeMX.
A detailed device tree configuration is described in RNG device tree configuration.
4 How to use the framework
The framework provides external interfaces from userland : How to control RNG.
4.1 How to use from char device
The community tool for using Hardware random framework is rng_tools which provides a complete set of utilities related to random number generators:
- rngd: runs a background daemon that opens /dev/hwrng file (default) to connect and retrieve random numbers.
- rngtest: runs different tests that check the entropy and verify the compliance regarding FIPS 140-2 standard.
4.2 How to use from sysfs
Available devices compatible with Hardware framework can be listed using sysfs commands:
Board $> cat /sys/class/misc/hw_random/rng_available stm32-rng
The selected device is shown here:
Board $> cat /sys/class/misc/hw_random/rng_current stm32-rng
To select a different device:
Board $> echo "stm32-rng"> /sys/class/misc/hw_random/rng_current
5 How to trace and debug the framework
Light information on the framework can be accessed by using sysfs.
By default, the framework does not provide any specific debug output or dynamic debugging tool.
6 Source code location
Hardware random drivers and framework are available here.
7 To go further
Code examples are directly available from rng-tools github.
Random Number Generator
Application programming interface