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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 derive_key_block(self, master_secret, server_random, client_random, req_len):
 """ Perform the derivation of master_secret into a key_block of req_len requested length. See RFC 5246, section 6.3. """

        seed = server_random + client_random
        if self.tls_version <= 0x0300:
            return self.prf(master_secret, seed, req_len)
        else:
            return self.prf(master_secret, b"key expansion", seed, req_len)