# wolfSSL Raspberry Pi Pico Acceleration

wolfSSL supports RNG acceleration on the Raspberry Pi RP2040 and RP2350
microcontrollers. Everything here assumes you are using the standard [Raspberry
Pi Pico SDK](https://github.com/raspberrypi/pico-sdk).

It has only been tested with the ARM cores of the RP2350, not the RISC-V cores.

## RNG Acceleration

The Pico SDK has RNG functions for both the RP2040 and RP2350. In the RP2040
this is an optimised PRNG method, in the RP2350 it uses a built-in TRNG. The
same API is used for both.

## Compiling wolfSSL

In your `user_settings.h`, you should set the following to add support in
wolfSSL:

```c
#define WOLFSSL_RPIPICO
```

Then for an RP2040, enable the ARM Thumb instructions:

```c
#define WOLFSSL_SP_ARM_THUMB_ASM
```

or for an RP2350, the Cortex-M instructions should be used:

```c
#define WOLFSSL_SP_ARM_CORTEX_M_ASM
```

To enable the RNG acceleration add the following:

```c
#define WC_NO_HASHDRBG
#define CUSTOM_RAND_GENERATE_BLOCK wc_pico_rng_gen_block
#define WC_RESEED_INTERVAL (1000000)
```
NOTE: the value for `WC_RESEED_INTERVAL` here is just an example. You should find what is
most appropriate for your application and use case.

In CMake you should add the following linking to both wolfSSL and the end
application:

```cmake
target_link_libraries(wolfssl
    pico_stdlib
    pico_rand
)
```

A full example can be found in the
[`RPi-Pico`](https://github.com/wolfSSL/wolfssl-examples/tree/master/RPi-Pico)
directory of the
[`wolfssl-examples`](https://github.com/wolfSSL/wolfssl-examples) GitHub
repository.

## Note on RP2350 SHA256

Although RP2350 has SHA256 acceleration, we cannot use this. It is because
we need to get an intermediate result using `wc_Sha256GetHash()`. The hardware
will only deal with 64byte packets of data, so to get a result we need to do
SHA padding. Once the SHA padding is done, it is not in a state to add more
data.
The only real workaround would be to cache everything being sent into the
hardware and replay it as a new instance when trying to get an intermediate
result. This would not very efficient for an embedded device.

## Support

For questions please email support@wolfssl.com