HMAC-SHA256,

**HMAC-SHA256 in Python**
=====================================

HMAC-SHA256 is a cryptographic hash function that uses the SHA-256 algorithm with a secret key for message authentication. Here's an example implementation in Python:

### Using the `hmac` and `hashlib` libraries

```python
import hmac
import hashlib

def hmac_sha256(key, message):
    """
    Compute the HMAC-SHA256 of a message using a secret key.

    Args:
        key (bytes): The secret key.
        message (bytes): The message to be hashed.

    Returns:
        bytes: The HMAC-SHA256 digest.
    """
    return hmac.new(key, message, hashlib.sha256).digest()

# Example usage:
key = b"my_secret_key"
message = b"Hello, World!"
digest = hmac_sha256(key, message)
print(digest.hex())
```

### Using the `pycryptodome` library

If you need more advanced cryptographic functionality, consider using the `pycryptodome` library, which provides a more comprehensive implementation of HMAC-SHA256:

```python
from Crypto.Hash import HMAC, SHA256

def hmac_sha256(key, message):
    """
    Compute the HMAC-SHA256 of a message using a secret key.

    Args:
        key (bytes): The secret key.
        message (bytes): The message to be hashed.

    Returns:
        bytes: The HMAC-SHA256 digest.
    """
    h = HMAC.new(key, digestmod=SHA256)
    h.update(message)
    return h.digest()

# Example usage:
key = b"my_secret_key"
message = b"Hello, World!"
digest = hmac_sha256(key, message)
print(digest.hex())
```

### Verifying a message

To verify a message, you can compute the HMAC-SHA256 of the message and the expected digest, and compare the results:

```python
def verify_message(key, message, expected_digest):
    """
    Verify a message using HMAC-SHA256.

    Args:
        key (bytes): The secret key.
        message (bytes): The message to be verified.
        expected_digest (bytes): The expected HMAC-SHA256 digest.

    Returns:
        bool: True if the message is valid, False otherwise.
    """
    computed_digest = hmac_sha256(key, message)
    return hmac.compare_digest(computed_digest, expected_digest)

# Example usage:
key = b"my_secret_key"
message = b"Hello, World!"
expected_digest = b"expected_digest_value"
is_valid = verify_message(key, message, expected_digest)
print(is_valid)
```

Note that in the `verify_message` function, we use the `hmac.compare_digest` function to compare the computed digest with the expected digest. This function is designed to prevent timing attacks.