import base64
import binascii

from starlette.authentication import AuthenticationBackend, AuthenticationError


class BasicAuthBackend(AuthenticationBackend):
    async def authenticate(self, conn):
        if "Authorization" not in conn.headers:
            raise AuthenticationError('No Authorization Header')

        auth = conn.headers["Authorization"]
        try:
            scheme, creds = auth.split()
            if scheme.lower() != "basic":
                raise AuthenticationError('Invalid Auth Scheme')
            decoded = base64.b64decode(creds).decode("utf-8")
        except (ValueError, UnicodeDecodeError, binascii.Error):
            raise AuthenticationError('Invalid basic auth credentials')

        username, _, password = decoded.partition(":")
        if username == 'admin' and password == 'password':
            return None

        raise AuthenticationError('Invalid basic auth credentials')