SMS encryption over GSM

Question

I have read this somewhere:

Most mobile operators encrypt all mobile communication data, including SMS messages In GSM, messages are encrypted using A5/1 but even when encrypted, the data held by SMS is readable for the operator. Mobile phone operators have the ability to filter and modify short messages during delivery. Also, it is possible that the operator might not filter messages on purpose but might use equipment that cannot handle encrypted messages.

I want to know..is it true..? Can someone explain how this filtering is done..? and is there any solution to avoid such loss of messages on the network..?

Answer 1

A5/1 is being used on the radio link between mobile and base station controller (BSC, the network entity entity that manages the radio resources). The radio link transports a couple of higher level protocols, among them MAP which is used to transport SMS.

The BSC is relaying SMS over MAP into the core network. The protocol stack between BSC and core network is not encrypted as well as the communication inside the core network. This was deemed as not needed at time GSM was designed, the links are supposed to be mobile operators very own property and territory and therefore assumed being secure.

The core network typically delivers SMS to an SMSC (short message service center) which is reponsible for routing messages to receipients.

A network operator can read SMS in clear text in various places, e.g.

• With a protocol analyzer, tapping links between network nodes
• On the SMSC, in message queues (databases...) or even log files
• On an MSC when tracing MAP messages

Message filtering and modification may happen on the SMSC, depending on the network operator needs.