> For the complete documentation index, see [llms.txt](https://k4k3ru.gitbook.io/k4k3ru-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://k4k3ru.gitbook.io/k4k3ru-docs/api/authentication/sign-request-with-python.md).

# Sign a Request with Python

Build an authenticated K4K3RU JSON-RPC request with Python. The standard library handles HMAC-SHA256; the `cryptography` package provides Ed25519 support.

## Prerequisites

* Python 3.10 or later.
* An API credential created with `hmac-sha256` or `ed25519`.
* `K4K3RU_API_KEY`, `K4K3RU_SECRET_KEY`, and `K4K3RU_SIGNATURE_ALGORITHM` set in your environment.

Install the Ed25519 dependency when you use an `ed25519` credential:

```bash
python -m pip install cryptography
```

## Example

Save this program as `sign_request.py`:

```python
import base64
import hashlib
import hmac
import json
import os
import secrets
import time


def base64url_decode(value: str) -> bytes:
    padding = "=" * (-len(value) % 4)
    return base64.urlsafe_b64decode(value + padding)


def base64url_encode(value: bytes) -> str:
    return base64.urlsafe_b64encode(value).decode("ascii").rstrip("=")


def canonical_json(value: object) -> str:
    return json.dumps(value, sort_keys=True, separators=(",", ":"), ensure_ascii=False)


def sign(algorithm: str, secret: bytes, payload: bytes) -> bytes:
    if algorithm == "hmac-sha256":
        return hmac.new(secret, payload, hashlib.sha256).digest()
    if algorithm == "ed25519":
        if len(secret) != 64:
            raise ValueError(f"invalid Ed25519 private key length: {len(secret)}")
        from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey

        return Ed25519PrivateKey.from_private_bytes(secret[:32]).sign(payload)
    raise ValueError(f"unsupported signature algorithm: {algorithm}")


api_key = os.environ["K4K3RU_API_KEY"]
secret = base64url_decode(os.environ["K4K3RU_SECRET_KEY"])
algorithm = os.environ["K4K3RU_SIGNATURE_ALGORITHM"]
method = "PaymentOnchain.CreateIntent"
params = {"productName": "usdc-base-mainnet-1"}
timestamp = int(time.time())
nonce = base64url_encode(secrets.token_bytes(16))
canonical_params = canonical_json(params)
payload = f"{method}\n{timestamp}\n{nonce}\n{canonical_params}".encode("utf-8")
signature = base64url_encode(sign(algorithm, secret, payload))

request = {
    "id": "1",
    "method": method,
    "params": params,
    "auth": {
        "apiKey": api_key,
        "timestamp": timestamp,
        "nonce": nonce,
        "signature": signature,
    },
}
print(json.dumps(request, indent=2, ensure_ascii=False))
```

Run it with:

```bash
python sign_request.py
```

The program prints a request body that you can send to `https://api.k4k3ru.com/` with `Content-Type: application/json`. Sign the same `params` value that you place in the request; do not independently modify or reparse it after signing.

## Next Steps

See [API Request Authentication](/k4k3ru-docs/api/authentication.md) for canonicalization, freshness, replay-protection, and troubleshooting requirements.


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