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✏️ Editing: cmac.py

# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.

from cryptography import utils
from cryptography.exceptions import (
    AlreadyFinalized,
    UnsupportedAlgorithm,
    _Reasons,
)
from cryptography.hazmat.backends import _get_backend
from cryptography.hazmat.backends.interfaces import CMACBackend
from cryptography.hazmat.primitives import ciphers

class CMAC(object):
    def __init__(
        self, algorithm: ciphers.BlockCipherAlgorithm, backend=None, ctx=None
    ):
        backend = _get_backend(backend)
        if not isinstance(backend, CMACBackend):
            raise UnsupportedAlgorithm(
                "Backend object does not implement CMACBackend.",
                _Reasons.BACKEND_MISSING_INTERFACE,
            )

if not isinstance(algorithm, ciphers.BlockCipherAlgorithm):
            raise TypeError("Expected instance of BlockCipherAlgorithm.")
        self._algorithm = algorithm

self._backend = backend
        if ctx is None:
            self._ctx = self._backend.create_cmac_ctx(self._algorithm)
        else:
            self._ctx = ctx

def update(self, data: bytes) -> None:
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")

utils._check_bytes("data", data)
        self._ctx.update(data)

def finalize(self) -> bytes:
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")
        digest = self._ctx.finalize()
        self._ctx = None
        return digest

def verify(self, signature: bytes) -> None:
        utils._check_bytes("signature", signature)
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")

ctx, self._ctx = self._ctx, None
        ctx.verify(signature)

def copy(self) -> "CMAC":
        if self._ctx is None:
            raise AlreadyFinalized("Context was already finalized.")
        return CMAC(
            self._algorithm, backend=self._backend, ctx=self._ctx.copy()
        )

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