As a field of mechanical engineering, lock design has its own unique challenges, including the fact that you are designing something in the full expectation that someone is going to deliberately attempt to defeat it.
The range of attacks that a good lock must withstand is unique. It becomes a battle of wits between the designer and the would-be challenger.
The challenger has the luxury of time, the ability to purchase the product and disassemble it at leisure, and endless YouTube tutorials.
The designer has to rely on good old-fashioned deviousness whilst working within a budget.
If you get it right you’ll keep your job and perhaps even attain some limited kudos from people who understand what you’ve achieved. Get it wrong and you’ll end up with your very own YouTube video, showing the world just how silly you’ve been…
Disclaimer: Whilst it is certainly not the intent of this article to provide information to any would-be criminal, it is necessary to outline various forms of attack herein. To the best knowledge of the author, all such information is already within the public domain.
About 40 years ago, a large international lock company developed a wonderfully complicated and expensive lock mechanism. Secure in the knowledge that it was ‘un-pickable’ and could withstand all forms of forcible attack, they challenged locksmiths from all over the world to defeat their lock. They even offered a prize. They overlooked just one tiny detail…
Someone stepped up to their display lock with one of their keys. It wasn’t the correct key, but it had the right profile to fit into the keyway. He filed a bit of metal off the key so it could enter the keyway further than it should, inserted the key, whacked it with a hammer so that it sheared off the retaining pin at the back and pulled the entire cylinder out with the wrong key, leaving the lock capable of being opened by the twist of a screwdriver.
Apparently, the phrase ‘performance bonus’ was noticeably absent in the Company memos for a while.
This article is intended to provide some insight into the often conflicting design challenges faced in this field of mechanical design, how they can be overcome (or at least mitigated), and to provide a basic understanding of how one type of lock mechanism works.
Lock Design Fundamentals – What is a lock?
Essentially, it’s a device that selectively allows and denies entry depending on the entrant being in possession of the correct key or code.
In order to do this, a key-operated lock has to have various elements within it moved either sequentially or synchronously by the key. The key also has to either conform to or bypass certain elements of the mechanism, and the key may also have to restrain certain elements within the lock from moving further than a determined distance.
Historically, the first locks were nothing more than a large boulder placed over the entrance to a cave that contained something of value, with the mass of the boulder being such that it required a collaborative effort to move it. This was in the very early stages of the hunter gatherer phase of human existence, but eventually locks developed to the extent that they were accessed with a key. The first known example of this was found in the ruins of the Phoenician palace of Khorsabad, Nineveh, which is now part of Iraq. It’s estimated to be about 4000 years old, which just goes to show how some things don’t change.
As time progressed, technology and manufacturing methods improved and one of the first examples of miniaturization occurred when Mr. Linus Yale Jr. re-discovered the same principle that was first utilized by the Phoenicians.
There are, of course, different types of lock, but this article will focus on this particular type of lock mechanism – the pin cylinder (or if you’re American, pin tumbler) type, otherwise known as the eponymous Yale lock.
We’ll take a look at the locking mechanism (and the design challenges that it presents). To be precise, that is the part of the lock that is directly acted upon by the key.
To understand the mechanism and what follows, it’s necessary to firstly understand the terminology and basic principles of operation. Fig.1 shows a representative type of pin cylinder that is secured by two screws at the rear, and this is the model that will be the subject of analysis.
The material used for the majority of the components is brass, as it has a combination of easy machinability and a low co-efficient of friction.
The exploded view above shows the same mechanism, but in exploded form and with labels that name the fundamental elements of the mechanism.
- The closing pins are fixed elements that have an interference fit with the chambers of the body (cylinder).
- The springs are of a single length and type
- The drivers (top pins) are of two different heights
- The pins are available in ten different heights, but only three different heights are shown in this model for clarity
Anatomy of the key
To understand lock design it’s also equally important to become familiar with the key, as this is vital to the correct functioning and security of the lock, so Fig.3 below shows the important features thereof.
- The shoulder of this key is the feature that locates (stops) the key within the keyway of the cylinder at the correct depth. Other cylinders may also make use of a ‘tip stop’ as an alternative
- The blade of the key has five cuts in it, and the spacing of these cuts corresponds to the pitch of the pin chambers within the cylinder plug. The depth of these cuts correspond to the height of the individual pin within each chamber
The operation of a pin cylinder mechanism is very simple in principle, and the following images illustrate how it works: In the Fig.4, the cylinder is in the locked condition (the different pins and drivers are now shown as colour coded relative to their different lengths). You can see that these locking elements are interfering with the rotation of the plug.
In Fig.5, the correct key is inserted, and there is now a ‘shear line’ between the bottom pins and the top drivers. This shear line coincides with the diameters of the plug and the cylinder body (within given tolerances).
This alignment allows the key to rotate the plug, as in Fig.6 (plug shown as solid for clarity).
If an incorrect key were to be inserted, as in Fig.7, this shear line would not be present. You can see that the first pin has been lifted slightly too high and the second pin slightly too low by this key.
That’s the basic principle of operation. So what makes a lock of this type any better or worse than another one? What considerations does the designer have to bear in mind when attempting to economically produce such a product, whilst also attempting to introduce elements of uniqueness and security that provide a USP?
The starting point to this problem lies in the keyway. This feature of the plug is fundamental to lock design, in that it is the entry point of the key (and, of course a lock pick etc.). For economies of production, as the designer I’d like a straight slot that a slitting saw can handle in a single pass, but realistically that means that just about any key of this type will fit in the keyway, and that will compromise the key differs to an unacceptable extent (more on this later on).
So the designer needs to incorporate some form of ‘corrugation’ into the profile of the keyway, and therefore the key.
Most keyways are broached and you can see that in Fig.8 below, there is a lack of complexity to the keyway that simplifies this process – broaching usually takes place on a motorbike chain type of arrangement, with a series of plugs clamped above a rotating chain, with each segment of the chain having a slightly longer keyway semi-profile than the last one. As the plugs are depressed at a controlled rate relative to the rotational speed of the chain, the keyway is formed.
Fig.8 shows three examples of pin cylinder plug keyways – the image on the left shows a simplistic keyway with two corrugations. This is relatively easy to manufacture but not secure in terms of either key duplication or the insertion of picking tools.
To manufacture a keyway in a brass cylinder plug requires either a sawing or a multi-stage broaching approach; it isn’t suitable for extrusion or die casting, with the extruding process there is no possibility of including features that extend beyond the diameter of the drawn form, and with die casting there are the inevitable draft angles to consider. These are not good for many reasons that will become apparent later on.
In the middle image you can see a slightly more complex keyway, which has the added inclusion of pips on either side of the center line. However, this is basically a cosmetic inclusion, as the pips serve no real purpose either in terms of inserting a lock picking implement, or in the prevention of the unlawful duplication of a key. If they were removed from either the plug keyway or from a fabricated blank key, it would make no difference to the ease that either of these can enter the keyway. For a key, they can be left out of the profile, and for a lock pick, the ease in which it can be vertically manipulated is not affected.
In the right image, you can see what is known as a paracentric keyway – this is a keyway where the wall of each side of the keyway runs over the centerline of the keyway. You can see that we now have a true ‘corrugation’ to the profile of the key, making it more difficult to duplicate. You can also see that it becomes more difficult to vertically manipulate a lock pick through the profile, resulting in the requirement for a thinner and smaller (and thus weaker and less effective) lock pick.
Design for abuse resistance
The first consideration when designing a lock (as with many other areas of design) is that of risk. For locks, what security risks will both the lock and key be exposed to?
Starting with the key, the most obvious risk is unauthorised duplication, either through an ‘impression’ of the key being made in something like clay, through the illicit removal of the key so that a locksmith may duplicate it, or through other methods such as the key being photocopied. So, the more complex the key, the more difficulty it presents.
As for the lock itself, many more challenges present themselves; the lock must be capable of withstanding three categories of attack, although these can vary greatly depending on the security level of a specific lock. These forms of attack are as follows:
Brute force or overt attack, which could be something such as a hammer and screwdriver, or a drill. Such an attack will be obvious to any user of the lock, whether such an attempt is successful or not.
Surreptitious attack can include lock picking, impressioning and other methods. These bypass methods can be uncovered if the lock is investigated, through evidence such as tell-tale marks and scratches on the interior elements of the lock.
Covert attack can include combing, bumping, and copying and reproduction of a key. If the key has been copied to a level of precision that is within the manufacturer’s tolerances, then it will be evidentially impossible to determine that an unauthorised entry has occurred.
In addition to these forms of attack, the lock must also have an acceptable minimum amount of combinations of differently cut keys available to it, only one of which will be capable of opening it (this is known as the amount of key differs which the lock has). For a mechanism of this type, that theoretical number of minimum differs is generally considered to be anything over 5000.
That number is very much theoretical and is arrived at by multiplying the different amount of available pin heights by the available amount of pin chambers, so with 10 different pin heights being available and 5 chambers available -104 = 10,000. However, some combinations of cuts are undesirable, such as a depth combination of 1,2,3,4,5 in that order, because it’s possible that the key could be removed when the lock plug is not at its ‘home’ position.
But by far the biggest limitation to the actual amount of key differs that are available is something known as MACS. This is an industry acronym for Maximum Adjacent Cut Specification, and is directly linked to the pitch between neighbouring pin chambers.
In the image on the right, two keys can be seen, each of which (for clarity) has a single representative pin of identical length positioned at its correct height above the second cut of the key. The uppermost of the two keys is supporting this pin at the correct height to form a shear line, but the bottommost key is incapable of this, as the third cut on this key is so deep (and therefore, wide) that it has removed too much supporting material from the neighbouring pin positions.
So in theory, a much wider pitch between pin chambers would allow for approximately 10,000 key differs, but the reality is that depth constraints in terms of existing locks and doors often mandate the overall length of the cylinder.
Therefore a MACS of at least 5 is just about acceptable, so that if the pin in any given chamber is of (for example) a number 2 length, then the pin in either adjacent chamber may be at most a number 7 length.
This will bring us back down to around the 5,000 key differs range. In theory, this means that we will only have what the industry terms ‘cross-keying’ once in every 5,000 locks, meaning that if 5001 people are carrying a key that is of the same type and key profile, two keys will work the same lock (in theory…)
Protection against brute force attack
The most common method of brute force attack for a lock mechanism is through the use of a drill.
To clarify this, that means that most locksmiths will make use of a drill to forcibly bypass the lock – criminals tend not to use any relative form of finesse and will simply target the weakest point. If this requires a boot on the door then so be it, but that isn’t relevant to this article.
It’s also important to keep in mind that locksmiths are hugely relevant to the perception of any particular lock, as not only do they specify security levels to their clients, they also keep stock of preferred suppliers, and as a lock designer you’d like your brand to be recommended.
In Fig.10 you can see that the sample cylinder would be secured to the lock body by two screws at the rear.
Fig.11 is a wire frame view of the cylinder showing how a drill could be used to remove these screws.
To prevent this type of attack, hardened pins may be inserted at strategic points to both deter and deflect such an attack.
Fig.12 shows two such hardened pins inserted in the cylinder.
For manufacturing, this requires an axial displacement for drilling these pin holes, but this can be achieved on a multi-axis driven tool CNC lathe or machining center.
A bigger production concern is that the diameter of the hardened pins in Fig.12 is greater than the other pin chambers, and this would require a tool change, which takes longer to accomplish.
Another common method of drill attack is to use the drill to create an artificial shear line, as in Fig.13.
This has the effect of cutting through the pins between the plug and the body, as shown in Fig.14, which will allow the plug to turn with the aid of a screwdriver or other such implement when the drill is withdrawn.
To prevent this from occurring, there are several options available, including the use of a hardened steel driver in the first chamber. This would rotate when pressure was applied, and hopefully apply sufficient displacement to the drill bit to deflect it to the extent where the hardened pin would ‘grab’ the bit and snap it.
A cheaper option would be the insertion of two ‘off the shelf’ ball bearings, as in Fig.15 These will freely rotate, with the bottom one remaining at the same height (being restrained by the depth of the plug chamber), deflecting the drill bit, whilst the top one will be acted upon by the spring, repeatedly forcing it down into the flutes of the drill bit, thus damaging the drill and hopefully ‘grabbing’ it to the extent that the bit breaks in the hole; then the attacker would have to use another drill bit to attempt to drill through the remains of his first bit, which is of the same hardness.
The downside to this cheap arrangement is that the cylinder now loses some potential key differs through the radius of the BB’s and also has a second loose shear line between the bottom BB and the pin.
In Lock Design Masterclass – Part 2: Prevention of non-forcible entry attacks, tolerance issues, the problem with springs and how intelligent design can ‘lock in’ a customer – no pun intended.
Text and illustrations © [email protected] 2016