import hmac
import hashlib

class Signer:
    """Simple HMAC-based signer for MVP. In a real system, replace with public-key cryptography."""
    def __init__(self, key: bytes):
        if isinstance(key, str):
            key = key.encode('utf-8')
        self._key = key

    def sign(self, data: bytes) -> bytes:
        return hmac.new(self._key, data, hashlib.sha256).digest()

    def verify(self, data: bytes, signature: bytes) -> bool:
        expected = self.sign(data)
        return hmac.compare_digest(expected, signature)

def digest(data: bytes) -> bytes:
    return hashlib.sha256(data).digest()