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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 postprocess_key_for_export(self, key, client_random, server_random, con_end, read_or_write, req_len):
 """ Postprocess cipher key for EXPORT ciphersuite, i.e. weakens it. An export key generation example is given in section 6.3.1 of RFC 2246. See also page 86 of EKR's book. """

        s = con_end + read_or_write
        s = (s == "clientwrite" or s == "serverread")

        if self.tls_version < 0x0300:
            return None
        elif self.tls_version == 0x0300:
            if s:
                tbh = key + client_random + server_random
            else:
                tbh = key + server_random + client_random
            export_key = _tls_hash_algs["MD5"]().digest(tbh)[:req_len]
        else:
            if s:
                tag = b"client write key"
            else:
                tag = b"server write key"
            export_key = self.prf(key,
                                  tag,
                                  client_random + server_random,
                                  req_len)
        return export_key