es/bytearray/memoryview
    The secret key to use in the symmetric cipher.
    It must be 16 (e.g. *AES-128*), 24 (e.g. *AES-192*)
    or 32 (e.g. *AES-256*) bytes long.

  nonce : bytes/bytearray/memoryview
    A value that must never be reused for any other encryption.

    There are no restrictions on its length,
    but it is recommended to use at least 16 bytes.

    The nonce shall never repeat for two
    different messages encrypted with the same key,
    but it does not need to be random.

    If not provided, a 16 byte nonce will be randomly created.

  mac_len : integer
    Length of the MAC, in bytes.
    It must be no larger than 16 bytes (which is the default).
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