---
title: "NanoCrypto external entropy injection"
source_url: https://dev.digicert.com/trustcore-sdk/nanocrypto/nanocrypto-external-entropy-injection.html
---

This section describes three different ways to inject external entropy for the random number generator.

## API usage with user-provided entropy source

### Build flags

The following build flag must not be defined:

- `__DISABLE_MOCANA_ADD_ENTROPY__`

#### Sample code

```c
status = RANDOM_setEntropySource(ENTROPY_SRC_EXTERNAL);
if (OK != status) goto err;

status = MOCANA_initMocana();
if (OK != status) goto err;

/* Repeat to add more entropy. User must add 48 full bytes of entropy
before it is injected into the context via a reseed operation */
status = MOCANA_addEntropy32Bits(entropyBytes);
if (OK != status) goto err;

/* g_pRandomContext has been seeded with the External Entropy.
This context is good for 2^48 requests before a reseed is needed. */
```

## Use /dev/random as the external entropy source in MOCANA_initMocana

The `MOCANA_initMocana()` function internally invokes the `Mocana_addExternalEntropy` method to use `/dev/random` as the entropy source.

### Build flags

The following build flags must be defined:

- `__MOCANA_FORCE_ENTROPY__`
- `__RTOS_LINUX__` or `__RTOS_VXWORKS__`

The following build flag must not be defined:

- `__DISABLE_MOCANA_ADD_ENTROPY__`

## Provide external entropy material via user-defined function

The `MOCANA_initMocana()` function internally invokes the user defined function `MOCANA_CUSTOM_getEntropy()`. The function implementation must fill the provided buffer with adequate entropy material for a full re-seed. By default, 48 bytes of entropy are required for a re-seed. If less than 48 bytes are provided, a re-seed is not triggered.

The integrator must implement the `MOCANA_CUSTOM_getEntropy()` function located in the source file `mss/src/examples/custom_entropy.c`.

### Build flags

The following build flags must be defined:

- `__ENABLE_MOCANA_CUSTOM_ENTROPY_INJECT__`
- `__RTOS_LINUX__` or `__RTOS_VXWORKS__`

The following build flag must not be defined:

- `__DISABLE_MOCANA_ADD_ENTROPY__`