Wireless alarm recommendations

Several times I have been asked which wireless alarm system I would recommend, so I thought I would write a quick blog post.

I’ll start with some simple points:

  1. Wired is always going to be more secure than wireless – so if possible, go for a wired alarm. They are also cheaper, less prone to false alarms, and need less maintainence.
  2. An alarm install is more than the equipment. You need to think about which detection devices you need, where to place them, and what the alarm does when it goes off. You can buy the best equipment, install it badly, and have a poor alarm system. You could also buy mediocre equipment, install it well, and have decent alarm system.
  3. An alarm system that you do not use might as well be replaced with a bell box. Think about usability, maintenance, lifetime and so on when choosing.

I’ll also be realistic. At this point in time, home alarms are not being actively jammed, are not seeing replay attacks, encryption broken and so on.

I’d also recommend that a lot of people go with a professional installer. Alarms aren’t always easy – it’s definitely something that needs experience, especially if you want a neat and efficient install. There are a lot of great installers out there who really know what they are doing and take great pride in their work. There are also a number who don’t, so chose with care. I only have one piece of universal advice here – avoid ADT.

Alarms to avoid

Yale Wirefree

This uses one-way OOK 434MHz. This makes it very easy to jam (intentionally and unintentionally). It means that battery life is lower than it can be. There is very limited functionality.

On the upside, it is low cost, easy to setup, and the voice dialler is cheap.

Yale don’t seem very responsive or open about the limitations of this system.

Friedland Response

Uses one-way FSK 868Mhz. Very easy to jam, and battery life not as good as it can be. Functionality is incredibly limited on a lot of the alarms – there isn’t even a LCD display on many of them.

It is cheap though, and easy to setup.

Friedland aren’t very responsive about issues with this alarm – they requested that I don’t quote any of my communications with them.

Here is what they had to say though:

We have been selling wireless alarms since 1990 and code issues have never been a problem.
Potential burglars would need extensive knowledge and equipment to overcome any alarm system.
Most domestic burglaries are done by opportunists who would be put off breaking into an alarmed home and would not attempt to interfere with the alarm as they know this in itself would cause the alarm to activate, even if it wasn’t alarmed!

Alarms I’m not sure about

Scantronic/Cooper Ion

This is a 1-way system which has many of the disadvantages of the Yale Wirefree and Friedland Response. The actual alarm is a full featured grade 2 panel though, so it has that on it’s side.

In communication with Cooper, it doesn’t seem they realise how dangerous leaking firmware can be, how easy it is to disassemble.

I expect that some really clever person with a lot of time on their hands might figure out a way to poke around in the binary stuff and – eventually – figure out a way to do something mischievous but this would be fraught with all sorts of difficulties. But we are, of course, aware that eighteen year olds can hack into FBI computers from their bedrooms and so anything is possible given enough time and effort.

Yale Easyfit

This claims to use 868MHz, rolling codes, and better anti-jam functionality. I have one to test but haven’t had much of a look – it still looks to be a one-way system which really limits what you can do with rolling codes and jamming detection. The functionality is still very basic.

Honeywell Domonial

These seem to get put into new-builds a lot. I’ve been warned by installers these are hell to program, and this is coming from people who think the Honeywell Galaxy is a walk in the park.

Visonic PowerMax

I don’t like these guys as a company. The alarm system claims to be 2-way, but only a very limited number of components (not including the detectors) are 2-way. The alarm also claims to use FHSS but at 64hops/s using only 4 frequencies i.e. it’s token and will do little to improve security or interference immunity.

Being misleading like that makes me unhappy.

Alarms to buy

Pyronix Enforcer

Uses a two-way 868MHz protocol with encryption. This means jamming can be easily detected and detectors know when the system is armed or not, extending battery life. The system is grade 2 which means it could be professionally installed and insurance companies will take note.

The control panel and keypad are integrated (which is fairly rare in grade 2 alarms). This makes installing easier, but it is large and not very attractive. The LED colours are a bit bling and very bright.

The system is quite easy to setup with few pitfalls. Learn the detectors, chose zone and type, program some users and codes and it will work as a basic alarm.

There is a wealth of functionality such as soak tests, double knock triggering etc.

The alarm is readily available on eBay and other online sellers.

The wireless protocol isn’t perfect, but it is adequate for home use.

The manufacturer is responsive and upfront about potential issues with their system.

Texecom Premier Elite (Ricochet Technology)

Uses a two-way 868MHz protocol with encryption which builds up a wireless mesh network. Jamming can be detected and detectors know when the system is armed or not, extending battery life. The system is grade 2 which means it could be professionally installed and insurance companies will take note. 

The control panel and keypad are separate. This makes installing a little harder than the Pyronix, but it is more secure as the control panel can be hidden. The keypads are more discrete as well.

The system is relatively hard to setup compared to the Pyronix Enforcer. You need to have a better idea of what you are doing to get everything set correctly. I can easily see that you can program the alarm in a way which means it either wouldn’t alarm when it needed to, or would alarm when it didn’t need to (this is why professional installers exist!).

There is a lot of functionality in the system – the smaller panels are just cut down versions of the much larger ones.

The alarm is relatively hard to get hold of – eBay is your best bet. It’s really only marketed to professional installers.

The wireless protocol is very well designed.

There is excellent software provided with the alarm that allows you to program it from a PC. This makes everything so much easier. You just need an FTDI cable to connect to the alarm.

The manufacturer actively engages with the installer community and myself about potential issues. Frequent firmware updates are available.

The alarm is designed and manufactured in the UK.

Programming a Texecom Premier Elite 12-W using a FTDI cable

The Texecom Premier Elite series of alarms can be programmed using Windows software called Wintex. This makes setting up these alarms far easier than using the keypad menus – they have hundreds of options and settings.

Texecom sell two products to connect to these alarms using Wintex – PC-COM (a serial port adapter ~£20) and USB-COM (a USB to serial adapter ~£35) . I strongly suspected these were just serial TTL converters, but I was concerned that there might be some jiggery pokery stopping this from working. Some software requires very specific VID (vendor ID) and PID (product IDs) on the USB device. Some software uses custom drivers. Others use microcontrollers and obfuscation to make sure you buy the genuine product.

As an avid hardware hacker, I have a lot of USB to TTL serial converters. The most useful (and reliable, in terms of drivers) are FTDI cables based on the FT232R chips. Genuine cables are ~£14, breakout boards can be as low as £2 on eBay. So let’s try and get connected to the Premier Elite 12-W using this cable.

There are two ports on the Premier Elite board – Com Port 1 and Com Port 2. These are 5 pin Molex connectors with only 4 pins populated. There didn’t seem to be a direct pin-out in the manual, so from the manual and with a multimeter we have:

Pin 1 – 12V

Pin 2 – nothing

Pin 3 – GND

Pin 4 – Receive

Pin 5 – Transmit

Com port 1 and 2

Com port 1 and 2

Signalling appears to be 5V. So, get out the 5V FTDI cable (they come in different voltages):

A 5V FTDI cable

A 5V FTDI cable

Pin 1 – GND

Pin 2 – Don’t care

Pin 3 – Don’t care

Pin 4 – Transmit

Pin 5 – Receive

Pin 6 – Don’t care

We then need to connect transmit to receive, receive to transmit, and common ground. This terminology might be at odds with alarm equipment – RS485 buses often label one wire “T” and it means transmit on the master, receive on the slave. I suspect this simplifies wiring as you just connect all “T” wires.

So, to connect the two:

Texecom – FTDI

Pin 3 GND – Pin 1 GND

Pin 4 Receive – Pin 4 Transmit

Pin 5 Transmit – Pin 5 Receive

Just be cautious of the 12V on pin 1 of the alarm board – sending this up the chuff of your PC will result in damage.

Using jumper cables, you could make up a proper cable

Using jumper cables, you could make up a proper cable

Find out which COM port the FTDI cable is using (generally go into Device Manager, and it will be listed there).

COM6 is my FTDI cable

COM6 is my FTDI cable

Go into Wintex and change the PC-COM port to this COM port:

Change Wintex to use COM6

Change Wintex to use COM6

Connect, receive settings, change settings, and monitor Ricochet devices to your heart’s content!

And start setting things up

And start setting things up



We need an antidote to the anti-code

In the last post, I briefly went over the process of reverse engineering the algorithm behind an anti-code generator for an alarm system.

It turned out that the algorithm was very simple indeed. For a given 5-digit numeric quote code, we can derive a 5-digit reset code using a “secret” 8-bit (256 possibilities) version number as a key. This has a lot in common with a keyed hash function or a message authentication code.

There are some pretty serious security implications with this mechanism.

5 digit numeric codes are never going to be strong

Even if I had to enter a pin at random, a 5-digit numeric code only has 100,000 options – I have a 1/100,000 chance of getting it right.

If we made this into a 5-digit hexadecimal code, we would now have a 1/1,048,576 chance – a factor of over 10 times less likely.

Up this to a 6-digit alphanumeric code, and it is now 1/2,176,782,336 – a factor of over 20,000 times less likely we could guess the code.

It doesn’t take many alterations to the limits on codes to make them much more secure.

For this reason it surprises me that alarms are still using 4-digit pins, but most internet forums insist on 8-character passwords with alphanumeric characters and punctuation.

The algorithm isn’t going to stay secret

There is no way to reliably protect a computer application from reverse engineering. If you can run it, at all, it is highly likely the operation can be observed and reversed. Relying on the secrecy of an algorithm or a key hidden within the software is not going to afford any level of security.

Once we know the algorithm, the odds massively improve for an attacker

The algorithm takes a version number from 0-255. For a given quote code, I can try each version number, giving me a list of up to 256 potentially valid reset codes (sometimes, two version numbers will generate the same reset code).

If I enter a code from this list, I now have a 1/256 chance of getting it right. Not great compared to 1/100,000 for a purely random guess.

This is entirely due to the short version number used.

Given a quote/reset code, most of the time we can infer the version

It quickly became apparent that for most quote/reset pairs, there was only a single version number than could produce this pair. I’m awful at probability and decision maths, so I thought running a simulation would be better.

I like running simulations – generally when the number of simulations becomes large enough, the results tend towards the correct value. So I tried the following:

1. Generate a genuine quote/reset pair using a random quote.

2. Use a brute force method to see which version numbers can produce this pair

3. Record if more than one version number can produce this quote/reset pair.

I started doing this exhaustively. This would take a long time though… someone on the Crypto stack exchange answered my question with a neater, random simulation.


I ran this test over 20 million times. From this it turns out that 99.75% of quote/reset code pairs will directly tell me the version number. Most of the remaining 0.25% require yield two version numbers. A tiny number (<0.001%) yield more than four version numbers. You are almost certain to know the version number after two quote/reset pairs as a result.

What does this mean in the real world?

The version number is treated as the secret, and I am informed that this secret is often constant across an entire alarm company. All ADT alarms or all Modern Security Systems alarms may use the same version number to generate reset codes.

This means I could get hold of any quote/reset pair, infer the version number, and then use that later to generate my own anti-codes for any ADT alarm. I could get hold of these quote/reset pairs by going to an accomplice’s house with a ADT alarm system, or by eavesdropping on communications.

With that anti-code I could either reset a system presenting a quote code, or impersonate an alarm receiving centre (there are other speech based challenge-response requirements here to prove the caller is genuine, which are easily gamed I would imagine).


A 5-digit reset code using an 8-bit key is never going to be secure.

When computer passwords are 8 characters and 128-bit keys are the norm, this anti-code mechanism seems woefully inadequate.

Reversing an anti-code

A contact in the alarm industry recently asked if I could take a look at a quick reverse engineering job. I’m trying to gain some credibility with these guys, so I naturally accepted the challenge.

Many alarms have the concept of an “anti-code”. Your alarm will go off and you will find it saying something like this on the display:


QUOTE 12345

The idea is then that you call the alarm receiving centre, quote 12345, they will input this into a PC application, get a reset code back, tell the customer, and then they can use this to reset the alarm. This means that you need to communicate with the alarm receiving centre to reset the alarm.

Alarm manufacturers provide their own applications to generate these codes. This particular manufacturer provides a 16-bit MS-DOS command line executable, which will refuse to run on modern PCs. This is a pain – it’s not easy to run (you need to use a DOS emulator like DOS-BOX) and it doesn’t allow for automation (it would be convenient to call a DLL from a web-based system, for example).

So I was asked if I could work out the algorithm for generating the unlock codes. x86 reverse engineering is not my forté, especially older stuff, but I thought i would have a quick go at it.

Turns out it was easier than expected! I find documenting reverse engineering incredibly difficult in a blog format, so I’ll just cover some of the key points.

Step 1: Observe the program

First things first, let’s get the program up and running. DOS-BOX is perfect for this kind of thing.

The program takes a 5 digit input and produces a 5 digit output. There is also a version number which can be input which varies from 0-255.

I spent a while playing around with the inputs. Sometimes things like this are so basic you can infer the operation (say, if it is XORing with a fixed key, flipping the order of some bits or similar). It didn’t look trivial, but it was plain to see that there were only two inputs – the input code and version. There was no concept of time or a sequence counter.

At this stage, I’m thinking it might be easiest to just create a lookup for every single pin and version. It would only be 2,560,000 entries (10,000 * 256). That’s a bit boring though, and I don’t have any idea how to simulate user input with DOS-BOX.

Step 2: Disassemble the program

To disassemble a program is to take the machine code and transform it into assembly language, which is marginally more readable.

There are some very powerful disassemblers out there these days – the most famous being IDA. The free version is a bit dated and limited, but it allowed me to quickly locate a few things.

An area of code that listens out for Q (quit) and V (version), along with limiting input characters from 0-9. Hex values in the normal ASCII range along with getch() calls are a giveaway.

Keyboard input
Another area of code appears to have two nested loops that go from 0-4. That would really strongly indicate that it is looping through the digits of the code.

Other areas of code add and subtract 0x30 from keyboard values – this is nearly always converting ASCII text numbers to integers (0x30 is 0, 0x31 is 1 etc. so 0x34 – 0x30 = 4)


A block of data, 256 items long from 0-9. Links in with the maximum value of the “version” above. Might just be an offset for indexing this data?

IDA’s real power is displaying the structure of the code – this can be a lot more telling than what the code does, especially for initial investigations.

Code structure
It’s still assembly language though, and I’m lazy…

Step 3: Decompile the program

Decompiling is converting machine code into a higher level language like C. It can’t recover things like variable names and data structures, but it does tend to give helpful results.

I used the free decompiler dcc to look at this program. I think because they are both quite old, and because dcc has signatures for the specific compiler used, it actually worked really well.

One procedure stood out – proc2, specifically this area of code:
DCC outputIt’s a bit meaningless at the moment, but it looks like it is two nested while loops, moving through some kind of data structure, summing the results and storing them. This is almost certainly the algorithm to generate the reset code.

Now, again, I could work through this all and find out what all the auto named variables are (i.e. change loc4 to “i” and loc5 to “ptrVector”. Or I could step through the code in a debugger and not have to bother…

Step 4: Run the code in a debugger

A debugger allows you to interrupt execution of code and step through the instructions being carried out. It’s generally of more use when you have the source code, but it is still a helpful tool. DOS-BOX can be run in debug mode and a text file generated containing the sequence of assembly instructions along with the current registers and what is being written and read from them. It’s heavy going, but combined with IDA and the output from DCC, it’s actually quite easy to work out what is going on!

Step 5: Write code to emulate the behaviour

Shortly after, I had an idea how the algorithm worked. Rather than work it through by hand, I knocked up a quick Python program to emulate the behaviour.The first cut didn’t quite work, but a few debug statements and a couple of tweaks later, and I was mirroring the operation of the original program.

Overall, it was only a few hours work, and I’m not really up on x86 at all.

I’m not releasing the algorithm or the software, as it could be perceived as a threat. In the next post, I am going to discuss some of my security concerns around the idea of an anti-code and this specific implementation.

What’s inside a WebWayOne SPT?

I managed to find a reasonable resolution image of a WebWayOne SPT (supervised premises transceiver, the device that communicates with the ARC (alarm receiving centre)). Just some quick notes about what is on it.

Annotated PCB

Annotated PCB

The Coldfire processors have a hardware encryption acceleration engine on them, which suggests that some fairly heavy duty encryption is happening.

Tomographic motion detection

Typical alarms use PIR (passive infrared), microwave or ultrasound detectors for motion detection. PIR are by far the most common type of detector – they work by detecting changes in infrared emitted by warm bodies. They are cheap, very reliable, and actually quite hard to beat.

Laser break beams are only really seen in films, though simple active infra-red break beams are often used on scaffolding alarms.

The problem with all of these is that they cannot see through objects. A common method of circumventing PIR detectors is to “mask” them – you either cover them  using paint (or another infrared opaque coating) or simply put something like a box in front of them. Higher security systems have “anti-masking” detectors which use an active element to check that their view has not been masked.

It can mean that complex, cluttered, or continually changing spaces need a lot of PIRs to be adequately covered.

Step in a new type of motion detection – tomographic motion detection. This sounds really clever and innovative. You might have heard of tomography from the medical world – CT scan stands for computerised tomography. It means “imaging by cross section”. Xandem have come to the market with a new detector that uses 2.4GHz radio signals to detector motion in a space.

A group of wireless nodes form a mesh of connections, as shown in this image from the patent:

Mesh network

Mesh network

Each one of those lines represents a radio path. The system uses 2.4GHz signals, the same as with WiFi or Bluetooth. These are heavily attenuated by anything containing water – such as the human body. A human body placed in the radio path of any two nodes will reduce the received signal strength (RSS).

By carefully measuring the RSS from each node to each other and doing some clever processing, you should be able to build up an image of what the area usually looks like. Any significant disturbance would signal an alarm. Hence, motion of a human body can be detected.

This would work through walls, shelves, furniture and so on – as long as the signal strength is attenuated too much.

This is clever stuff. Very easy to fit (though you do need power to each node), and probably very hard to beat. It is expensive though.

For those interested, here is a link to the patent:


And I have pulled a picture of the PCB from the FCC report on it:



The markings on the main IC are not visible, but based on the frequency, size of the package, crystal frequency, crystal connections and antenna connections, this is a TI CC2540 RF SoC – a brother to the CC1110 RF SoC, using an 8051 core connected to a RF transceiver.

Interestingly there is a micro-USB and debugging connector on the board as well!

Why am I hacking your alarm?

Since I’ve started posting about alarm systems, a number of people have questioned by motives. I can understand why – these are security products and I can see how many people would think poking around inside them is “dodgy”.

I’d say I have three main drivers:
  1. I love taking electronics apart and working out how they work. It’s much more challenging and interesting when someone has actively tried to stop you doing this – alarms are an ideal target because of this. I initially bought an alarm hoping it would contain a rolling code system for me to reverse, but it turned out to be far too basic. In the end I found a massive string of vulnerabilities anyway.
  2. I find security as a concept fascinating, from locks through access systems through human factors. I love how the perception of security is so often different to the reality. One of my current drivers is that I think the security economics around alarms is totally broken – it’s driven by outdated , rigid standards and insurance rather than actual security.
  3. I used to watch Bugs on BBC1 each week religiously so I spent my teenage years watching people break into high-tech buildings using fancy electronics. Whilst I’m not doing the actual breaking in, having the means to disable alarm systems and bypass access controls is fun and something I never thought would be possible to do.

It swings both ways, especially for RF comms

In a few of the previous posts, I’ve discussed some principles used in the radio communications in alarms. I’ve mentioned that some features are harder to implement well using one-way radios. What is the difference between one-way and two-way? What practical difference will it make?

Radio communications can be one-way or two-way, depending on how they have been designed.

A one-way system has a transmitter in each of the detectors and a receiver in the panel. This means that the detectors can send signals to the panel, but the panel cannot send signals to the detectors.

In a two-way system, each component has both a transmitter and receiver. This means that the detectors are now capable of receiving a signal from the panel.

It is fairly normal for the two-way systems to use a combined transmitter and receiver called a “transceiver”. Whilst not a strict limitation, most of these transceivers can only transmit or receive at any given moment in time (this is called half duplex). They can switch from receive to transmit very quickly, so from a user perspective they look like they are transmitting and receiving at the same time.

Most older systems use one-way radio. I suspect this is because there were not easy to use, cheap integrated RF transceivers available 10 or 20 years ago. Often they will use a simple AM transmitter built from discrete components or one of the very old remote control ICs that require an 8-bit address (these are common in wirelessly controlled mains sockets still).

A lot of newer systems use two-way radio. They will use one of the modern integrated RF transceivers like the TI CCxxxx, Si4432, or any of the Nordic Semi products. These do all of the hard RF work (and even a lot of the packet handling and encoding, sometimes even encryption) for you, and are controlled using a simple digital serial protocol. They are very cheap and versatile.

What are the practical limitations of one-way radios?

There are an awful lot of them – too many to list really. Let’s cover a few really key ones

Detectors have no idea if the system is armed or not

There is no way for a detector to know if the system is armed or not as it cannot receive any information.

This means that they always have to behave as if the system was armed. This behaviour has to be a balance of responding to alarms quickly vs preserving battery life. This trade-off is often accomplished by holding-off alarm detection for a period of a few minutes after an alarm has been raised.

It also means that they try to send supervisory “OK” status messages as infrequently as possible – and by standards, this can be up to 240 minutes.

This has practical implications for how responsive an alarm system can be.

The panel cannot ping the detectors when it is armed

Two-way panels all actively check the presence and status of detectors at the moment the system is armed. If any are in tamper, contacts open, detectors missing, or batteries low, the user will be warned (and possibly, arming the alarm is not allowed). This is very similar to how a wired system works.

One-way systems need to rely on the last alarm or status message received. They could be from a long time prior and could be out of date.

Jamming detection is much harder

Jamming detection in a two-way system is easy. Panel sends out a ping, detector responds. If no response is received after several pings, we can assume that communication has been lost for some reason.

Also in a two-way system, when the alarm is actually triggered, the detector will keep on sending alarm signals until it receives an acknowledgement response from the panel.

In one-way systems we need to wait to see if we miss several supervisory signals to know that signals aren’t getting through. This can take hours.

Some one-way systems have passive jamming detection systems. They listen to the RF channel all of the time, and if the channel is in use a lot of the time, they assume it is being jammed. It doesn’t work very well (I will go into this another time). They have to side with less false alarms and lower sensitivity, and the result of this is that they are easy to jam.

Above all, when the alarm goes off in a one way system, all it does is send the signal for a reasonable period of time and assumes that the panel has received it. There is no way for it to be acknowledged.

Rolling code and encryption is much harder to do well

In a previous post, I discussed how rolling code systems can’t just accept the next code in the sequence – they need to accept codes over a wide window, possibly the next 256 valid codes. This is because the transmission is not guaranteed to be received and the transmitter hops forwards regardless.

With a two way system, this window can be avoided. The keyfob can continue to send the same code in the sequence until the panel sends a message back saying that it has been received (this is a simple explanation of how it could work, pure rolling code is rare in two-way systems).

Alongside this, one-way radio makes exchanging keys in encryption systems difficult. A similar concept to the window of valid codes needs to be used to ensure that transmissions are received correctly after a key changes. For this reason, encryption keys in one-way systems are most often fixed (though they can be exchanged during the initial pairing).

Conceptually, it’s exactly the same as two people trying to communicate reliably with each other, where one of them can only speak and the other only listen. There’s also a 2 year old in the room who won’t shut up (interference), and another guy who is actively trying to make sure everything goes wrong (a malicious attacker).

This raises another interesting aside – alarm systems always need to find a balance between security and reliability of communications. There is little use in ensuring that communications are completely secure if it means alarm messages do not make it through.

Security devices and product differentiation

An interesting subject has come up on the TSI forumsproduct differentiation in relation to encryption and security in alarm signalling systems.

As with alarms, there are different grades of signalling devices. These go from grade 1 (low risk, doesn’t seem to be used much or at all) to grade 4 (high risk, banks, jewellers). It’s common for the signalling device to be a higher grade than the alarm system, although this is not mandated.

Grade 4 requires encryption, protection from message substitution and replay etc. One provider, WebWayOne has built a system that uses several proven technologies like AES-128 and other widely known cryptographic fundamentals.

One of WebWayOne’s representatives said on the forum:

“Once these techniques are in place they may as well be deployed across all grades if system, it makes no sense not to.”

This is an awesome attitude to have and, to me, signals that these guys have actually understood the challenges in implementing a secure protocol. They are not weakening lower grade systems by weakening the cryptography and protocol.

Why do I think this is sound reasoning? It’s probably easier to argue why weakening the cryptography and protocol is not a good idea – here are some ways I have seen it done in other systems using cryptography (not alarm signalling systems – I am extending my reasoning from other products to apply to them).

Reducing key-length

Some products differentiate different grades of security by reducing key length. This tends to be a bad idea.

Practically all cryptographic techniques are vulnerable to brute-force attacks – it really is just trying every single key, one by one. It’s accepted at the moment that 40, 56 and 64 bit keys are not long enough to protect against brute-force attacks. 112 bit (twice 56, used in keying method 2 in triple DES) and 128 bit are currently long enough to protect against brute-force attacks. This will change in the future, but we are safe for a good few years yet.

Anything above 128 bits is therefore deemed wasteful – your highest grade product could use 128 bits and be secure. You could alter your lower grade product to use 64 bit keys. To the lay person, you might think that this would take half the time to brute force –  but it is actually easier by a factor of 2^64 (18446744073709551616 times easier).

You could offer 127 bit encryption – this would take half the time to crack. But what would be the point? It would be product differentiation for no reason, and implementing a custom key length nearly always means you are “rolling your own” and will make mistakes.

Altering the protocol

Changing the protocol in anyway would also be an odd way to differentiate a lower grade.

Outside of key length, most aspects of a protocol are either a binary secure/not secure. You can’t offer 50% of message authentication. You can’t offer 50% of a secure means of key exchange. They are either present and secure, present and insecure, or not present at all.

If any aspect of a secure protocol is deemed insecure, it’s highly likely that the whole thing will fall apart. This isn’t always the case, but it’s fairly usual to see a theoretical vulnerability against a single part (say, the message authentication) turn into a full blown practical exploit against the whole thing. This means you need to tread carefully when trying to artificially weaken a protocol.

The hardware is there anyway

Signaling systems don’t have the same constraints as wireless detectors. They have plentiful power and space, which affords the use of comparatively powerful hardware.

Most detectors use 8-bit microcontrollers like the PIC, ATmega, or 8051 built into the CC1110. They run using slow clock rates (this lowers power consumption) and have limited RAM and register space. Implementing full blown cryptographic schemes in these is not easy, especially when you move up to something like RSA with 1024 bit keys (RSA is public key cryptography, where you need a much longer key to be secure than with symmetric cryptography like AES).

I have not seen inside any IP signaling devices, but I would wager that they use modern, powerful 32-bit processors like the ARM, with plentiful RAM and fast clocks. There are cryptographic libraries already available on these processors that allow you to easily build a secure protocol.

This hardware is likely the same across all grades. Again, it just makes no sense to build a lower grade system using different hardware to artificially constrain it.


Properly pen testing products, as compared to “test house” testing to standards, is a time consuming, expensive and highly skilled job. Having two distinct products, even if they only different slightly in hardware and software, would really require two distinct pen tests to be performed. This is cost you do not need to bear. Test the grade 4 product, use the same hardware and software for grade 2, and you have just tested both at the same time.

Differentiate on the tangible aspects

When it comes down to it, all of this doesn’t really matter to the customer. They just want something secure. So differentiate on the tangible things – how long the signalling takes to report issues, and the response to alarms.

Encryption is only part of the solution

So, in the previous two posts I have covered:

And talked about how, although they are useful techniques to make an alarm better, they need to be implemented correctly.

Now, I am going to briefly cover encryption, and how it can go wrong.

What is encryption?

At a very basic level, encrypting something is encoding a message in a way which means an eavesdropper cannot determine the contents of the message.

There are many techniques – the one most familiar to people is a substitution cipher, where each letter is substituted for another.

In the real world, encryption uses more advanced techniques than this. Some encryption techniques are, to all intents and purposes, impossible to crack – unless you know the secret key, you are not going to find out what is in the message.

Sounds great! Where do I sign up? Many alarm manufacturers use encryption, and many don’t do it quite right.

Designing a good encryption scheme is hard

A famous saying is:

“Anyone can invent an encryption algorithm they themselves can’t break; it’s much harder to invent one that no one else can break”

Time and time again, custom built encryption algorithms have been broken.

Again, I’m sure there is a more concise saying about this, but I subscribe to this:

For a defender to succeed, he must have a 100% success rate against multiple attacks and attackers.

For an attacker to succeed, he only needs to succeed once.

What’s the solution to this:

  1. Use something off-the-shelf. There are mathematicians and developers who only develop encryption. Use their skills, don’t try to use your own.
  2. Open your code up – no encryption algorithm or implementation should be weakened by an attacker seeing the code. So open it up and let other people tell you the problems!

Encryption is often good in theory and fails in the implementation

Very closely linked to the above point – even if you use something off the shelf, make sure you use it right!

Common mistakes are:

  • Leaking key material by XORing the key with a constant padding value (in a previous post!)
  • Building a really strong encryption scheme but then failing to exchange the key in a secure way (i.e. you show everyone the key!)

Encryption by itself does not stop replay attacks

If the message “disarm the system” becomes “fodst, yjr dudyr,” when encrypted, there may be no easy way for the attacker to decode that message. Indeed, there would be no easy way for him to encode that message unless he had the key.

This may not matter though. You can infer what the message contains based on timing (the homeowner has just arrived, and pressed the disarm button), and replay the encrypted packet alone. You need to combine encryption with other techniques to ensure integrity and protect against replays.

Using a fixed key across all products

Again, this is partly the result of using a one-way radio system. Before the detector and panel can communicate, they need to perform key exchange, so that they both know the secret key. There are a few options here:

  1. You can send the key in the clear when paired and hope no one is listening.
  2. You can use a secure method of exchanging keys (like public key cryptography)
  3. You can program the key in during manufacture

Method 3 is attractive. On paper, your system is “encrypted” – you could even claim AES-128 encryption. But if that key is constant across every detector and every panel you sell, you have a problem. Why is this?

Firstly, you can never really assume that your code is safe. Whilst most modern microprocessors have some means of protecting code, this is by no means fail-safe. Sometimes designers forget to set the lock bits (well, quite often in fact), and the code can just be read out. Some PIC processors are vulnerable to a simple attack where you can recover 75% of the code from one device, and the remaining 25% from another (one manufacturer’s detectors have this issue). If you want to go further, you can decapsulate the microcontroller and read one-time-programmable memory visually – this is possible in the real world.

Secondly, it is sometimes the case that the alarm architecture means that I don’t need to know this key. Some alarms have an RF SoC performing all encryption – the main microcontroller just passes it simple unencrypted data. I can decouple the RF SoC and send my own, unencrypted data, and let the SoC do the hard work for me.

Thirdly, manufacturers offer firmware upgrades for download. These can contain the secret key material. It’s often easy to find – key material should look very random (more random than normal code). I have a simple tool to scan a file and graph entropy. It’s fairly easy to find key material using this, especially if you know how long the key is.

Using too short a key

One method of breaking encryption is to just try every single key and hope for the best – this is called “brute forcing“.

If the key is short, this process can be very quick.

Each time I add a single bit to a key, I double how many possibilities there are. A 1-bit key has 2, a 2-bit key 4, 3-bit has 8. This rapidly hits very large numbers – even at 32-bit we have 4294967296 keys. But modern computers are very, very fast – checking 100,000 keys a second is possible. That 32-bit key would fall in under 12 hours.

So key length has a big impact on how long it takes to perform this process. It’s now pretty much assumed a key length of 64-bits and under is too short to protect against brute forcing. But DES and other encryption methods still support 40-bit and 56-bit keys. If the option is there, people will take it.


Encryption is important in a wireless protocol if you want it to be genuinely secure – but it is only part of picture. If implemented badly, it can add little security and decrease reliability and usability.