'Returns the cipher for self.server_name.'

def _GetServerCipher(self):

    if (self.server_cipher is not None):
        expiry = (self.server_cipher_age + rdfvalue.Duration('1d'))
        if (expiry > rdfvalue.RDFDatetime.Now()):
            return self.server_cipher
    remote_public_key = self._GetRemotePublicKey(self.server_name)
    self.server_cipher = Cipher(self.common_name, self.private_key, remote_public_key)
    self.server_cipher_age = rdfvalue.RDFDatetime.Now()
    return self.server_cipher
