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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_str_alg(passwd, opslimit, memlimit, alg):
 """ Derive a cryptographic key using the ``passwd`` given as input and a random ``salt``, returning a string representation which includes the salt, the tuning parameters and the used algorithm. :param passwd: The input password :type passwd: bytes :param opslimit: computational cost :type opslimit: int :param memlimit: memory cost :type memlimit: int :param alg: The algorithm to use :type alg: int :return: serialized derived key and parameters :rtype: bytes """

    ensure(isinstance(opslimit, integer_types),
           raising=TypeError)
    ensure(isinstance(memlimit, integer_types),
           raising=TypeError)
    ensure(isinstance(passwd, bytes),
           raising=TypeError)

    _check_argon2_limits_alg(opslimit, memlimit, alg)

    outbuf = ffi.new("char[]", 128)

    ret = lib.crypto_pwhash_str_alg(outbuf, passwd, len(passwd),
                                    opslimit, memlimit, alg)

    ensure(ret == 0, 'Unexpected failure in key derivation',
           raising=exc.RuntimeError)

    return ffi.string(outbuf)