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# rng

Generate random bytes on-chain using the RNG precompile at `0x64`.

## Overview

`rng()` and `async_rng()` return randomness as a Python `int`.

Input encoding is:

* `num_bytes` as a 4-byte big-endian `uint32`
* optional `pers` bytes appended after it

## Signature

```python
def rng(
    w3: Web3,
    *,
    num_bytes: int,
    pers: bytes = b"",
) -> int

async def async_rng(
    w3: AsyncWeb3,
    *,
    num_bytes: int,
    pers: bytes = b"",
) -> int
```

## Parameters

| Parameter   | Type                 | Required | Description                                 |
| ----------- | -------------------- | -------- | ------------------------------------------- |
| `w3`        | `Web3` / `AsyncWeb3` | Yes      | Connected Seismic client                    |
| `num_bytes` | `int`                | Yes      | Number of random bytes to request (`1..32`) |
| `pers`      | `bytes`              | No       | Optional personalization bytes              |

## Returns

| Type  | Description                                             |
| ----- | ------------------------------------------------------- |
| `int` | Random value interpreted as big-endian unsigned integer |

## Examples

### Basic Usage

```python
from seismic_web3 import create_public_client
from seismic_web3 import precompiles as sp

w3 = create_public_client("https://testnet-1.seismictest.net/rpc")

value = sp.rng(w3, num_bytes=32)
print(value)
```

### With Personalization

```python
value = sp.rng(w3, num_bytes=16, pers=b"my-domain")
```

### Async Usage

```python
from seismic_web3 import create_async_public_client
from seismic_web3 import precompiles as sp

async def main():
    w3 = create_async_public_client("https://testnet-1.seismictest.net/rpc")
    value = await sp.async_rng(w3, num_bytes=32)
    print(value)
```

## Gas Cost

The SDK uses:

```python
from math import ceil

init_cost = 3500 + ceil(len(pers) / 32) * 5
fill_cost = ceil(num_bytes / 32) * 5
total_gas = init_cost + fill_cost
```

Examples:

* `num_bytes=1`, empty `pers`: `3505`
* `num_bytes=32`, empty `pers`: `3505`
* `num_bytes=16`, `len(pers)=64`: `3515`

## Notes

* `num_bytes` outside `1..32` raises `ValueError` before RPC.
* Output is right-padded to 32 bytes and decoded as a `uint256`-style integer.
* This function is for node-provided randomness; do not use it as consensus-critical randomness.

## See Also

* [RNG — Precompiles Reference](/reference/precompiles/rng.md)
* [ecdh](/clients/python/precompiles/ecdh.md)
* [hkdf](/clients/python/precompiles/hkdf.md)


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