---
title: "Random Number Generation"
source_url: https://dev.digicert.com/trustcore-sdk/crypto-interface/random-number-generation.html
---

In addition to providing a global RNG (i.e., a [pointer g_pRandomContext that is initialized upon NanoCrypto library initialization](https://dev.digicert.com/md/trustcore-sdk/nanocrypto/initialization.md#initialize-random-number-generator)), Crypto Interface provides APIs to create a local RNG (a local `randomContext` object) that may also be used in any API needing it.

The supported RNG algorithm through Crypto Interface is the NIST-CTR MODE DRBG using AES or TDES (NIST SP800-90A). The Crypto Interface does not support APIs for NIST_FIPS186, but that is supported by NanoCrypto directly.

To create and initialize a local `randomContext,` make the following declaration and call to seed the local RNG using the method configured via the initialization call (refer to [Initialization](https://dev.digicert.com/md/trustcore-sdk/nanocrypto/initialization.md#initialize-random-seed) for information on initial seeding):

```c
randomContext *pRandCtx = NULL;
CRYPTO_INTERFACE_RANDOM_acquireContextEx(&pRandCtx, MODE_DRBG_CTR);
```

The buffer is filled with pseudo-random bytes via the API:

```c
RANDOM_numberGenerator(pRandCtx, pBuffer, numBytesRequested);
```

For a cipher that requires a callback method for random entropy, the random context may be used with the shell callback `RANDOM_rngFun` (found in `${MSS_SRC_PKG}/src/common/random.h`):

```c
CRYPTO_INTERFACE_ECDSA_signDigestAux(pPrivKey, RANDOM_rngFun, pRandCtx, ...);
```

If a random context needs to be reseeded, a negative error code is returned when reseeding is required: `ERR_NIST_RNG_DBRG_RESEED_NEEDED`.

Reseeding may be performed only by first obtaining appropriate entropy (and additional data if appropriate), and then calling:

```c
CRYPTO_INTERFACE_NIST_CTRDRBG_reseed(pRandCtx, pNewEntropy,
newEntropyLen, pAdditionalInput, additionalInputLen);
```

When finished with the random context, free the allocated memory by calling:

```c
CRYPTO_INTERFACE_RANDOM_releaseContextEx(&pRandCtx);
```

A complete example may be found at:

```
${MSS_SRC_PKG}/src/crypto_interface/example/crypto_interface_random_example.c
```