What is MASC Encryption?

MASC (Marconi Advanced Scrambler) is a secure form of analogue encryption manufactured by Marconi Secure Systems, based on research carried out at Bristol University's Centre for Communications Research.

It is in use by the majority of police forces in the UK, in fact it is the only security system recommended by the Association of Chief Police Officers for use on police radios.

In the late eighties / early nineties, police forces up and down the country paid more attention to the fact that anybody with a scanner (including criminals) were able to receive police radio frequencies. This was obviously a concern and ways to counteract the problem were sought.

As is always the case with public services, cost was a major factor. A number of different voice encryption products were tested with varying degrees of success. Motorola's Securenet products and MASC seemed to offer the best level of protection at minimal cost.

After extensive trials MASC was settled on as the system of choice. Being an analogue based system it is nowhere as secure as some of the latest digital encryption systems, however, it satisfied the requirement of preventing casual monitoring of police channels.

A great advantage of MASC is that it is possible to retro-fit existing radios with the module, thus saving the huge costs involved in replacing entire fleets of radios.

How Does It Work

MASC uses a system called "variable band inversion", it is sometimes referred to as "rolling code inversion". This is a system whereby the transmitted audio is inverted around a constantly varying inversion point. The sequence by which this inversion point varies is pseudo-random and is determined by a "key code". As there are over 100 billion possible key codes it would be safe to say that the technique is safe from all but the most determined hacker.

For an excellent description of voice encryption techniques and sound samples of the various systems click here.

At the beginning and end of each transmission a data burst is sent, along with small data bursts several times a second throughout the transmission. The purpose of these is to enable the transmitting and receiving radios to remain synchronised to each other and therefore capable of decoding the signal. They also carry information such as the identity of the transmitting radio.

This allows many advanced features. Whenever an officer starts transmitting, his radio ID is immediately displayed on the dispatchers screen thus preventing rogue transmissions . The radios incorporate a personal attack button which an officer can press if he is in need of urgent assistance. An alarm will sound on the dispatchers panel along with the callsign and details of the current call he is responding to, thus backup can be sent, and all this without the officer having to actually say anything over the radio!

Likewise, should an encrypted radio be lost or stolen, it can be rendered useless by performing an over the air stun. Knowing the ID code of the radio allows the control room to send a stun command over the air. This will disable the radio and force it to immediately erase its key code.

One disadvantage of virtually all voice encryption systems is that range is reduced by up to 30%. Also they are very susceptible to interference. On a normal unencrypted radio, interference will still allow the signal to be received, all that will happen is that you may hear a few crackles and pops on top of the signal. However when this happens on an encrypted radio it has a much greater effect. As mentioned before, it is essential that both the transmitting and receiving radio remain synchronised to each other. This is achieved by a constant stream of data bursts several times a second. The crackles and pops can now have the effect of corrupting any one of these data bursts. If even just one becomes corrupted this will cause the radios to fall out of synchronisation and therefore the received signal will become unintelligible.

If you regularly scan around Northumbria Police's frequencies, you will often be able to clearly receive channels that are normally encrypted. If sufficient interference is being received to disrupt the MASC encryption then the radio operators will send a command over the air that will force all of the radios on a channel into clear mode. This often happens in the summer when tropospheric ducting results in interference being received from stations many miles away. In fact, a number of years ago before GSM mobile phones become common it was often the case that the police signals would be interfered with by Dutch and German radio telephone systems operating in the 450/460 MHz band!

Note : MASC in not used on the VHF channels due to the increased vulnerability to interference as a result of AM modulation being used.

Can it be Cracked?

In short, no! MASC is not the most secure encryption method by far, but it is nevertheless very resilient to non-professional cracking attempts. I doubt if foreign intelligence services would have much difficulty in cracking a MASC code but the chances of you or I doing it are as near impossible as to bear thinking about.

I have spoken to a few people who claim that a friend of a friend has the equipment to decode it, but I have yet to see any evidence with my own eyes! One person who claimed to have the know how was the owner of an electronics store in Tyne and Wear. However despite numerous requests to see (and hear) the decoder module, I am still waiting. I will leave you to draw your own conclusions!