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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def crypto_pwhash_scryptsalsa208sha256_str( passwd, opslimit=SCRYPT_OPSLIMIT_INTERACTIVE, memlimit=SCRYPT_MEMLIMIT_INTERACTIVE):
 """ Derive a cryptographic key using the ``passwd`` and ``salt`` given as input, returning a string representation which includes the salt and the tuning parameters. The returned string can be directly stored as a password hash. See :py:func:`.crypto_pwhash_scryptsalsa208sha256` for a short discussion about ``opslimit`` and ``memlimit`` values. :param bytes passwd: :param int opslimit: :param int memlimit: :return: serialized key hash, including salt and tuning parameters :rtype: bytes """

    buf = ffi.new("char[]", SCRYPT_STRBYTES)

    ret = lib.crypto_pwhash_scryptsalsa208sha256_str(buf, passwd,
                                                     len(passwd),
                                                     opslimit,
                                                     memlimit)

    ensure(ret == 0, 'Unexpected failure in password hashing',
           raising=exc.RuntimeError)

    return ffi.string(buf)