Safelocks - Safecracking for the computer scientist

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Safecracking for the computer scientist∗

Matt Blaze

Department of Computer and Information Science

University of Pennsylvania

blaze@cis.upenn.edu

DRAFT – 7 December 2004 (Revised 20 December 2004) – DRAFT

The latest version of this document can be found at http://www.crypto.com/papers/safelocks.pdf

This document contains medium resolution photographs and should be printed in color.

Abstract

This paper is a general survey of safe and vault security from a computer science perspective, with

emphasis on the metrics used to evaluate these systems and the weaknesses that cause them to fail.

We examine security against forced, covert and surreptitious safe opening, focusing on the mechanical

combination locks most commonly used on commercial safes in the US. Our analysis contrasts the

philosophy and tools of physical security with those of information security, especially where techniques

might be proﬁtably applied across these disciplines.

1 Safe and vault security: a computer science perspective

There is an undeniable mystique surrounding safes and vaults. Containers to safeguard valuables and secrets

from theft and prying eyes have existed almost as long as the concepts of valuables and secrets themselves,

and yet in spite of the “Internet age,” details of safes and the methods used to defeat them remain shrouded in

obscurity and even a certain amount of mystery. Safe security is a delicate, almost perilous subject, protected

by a near reverence that extends, in our imaginations at least, across both sides of the law. Safecrackers

are perhaps the most romantic and “professional” of thieves, conjuring images of meticulously planned

and executed exploits straight out of Hollywood screenplays. And among the law-abiding, safe and vault

technicians (safe men in the traditional parlance) are perceived as an elite, upper echelon of the locksmithing

community whose formidable trade is surely passed on only to the most trustworthy and dedicated.

Reverence for safe work can even be found in the trade’s own internal literature, with an almost un-

avoidable, if subtle, swagger accompanying mastery of safe opening technique. The title of a venerable

locksmithing treatise on the subject – The Art of Manipulation[LK55] — signals a discipline that demands

artistry, not mere craft. Its text begins with a warning to faithfully guard the material in its pages, as well

as the suggestion that the book be destroyed completely after its techniques are learned. (Fortunately, some

readers have ignored that advice, and a few copies remain available through interlibrary loan). The ambigu-

ity in the term manipulation itself seems oddly appropriate here, evoking perhaps a “lock whisperer,” with

the safe somehow persuaded to open against its better judgment, only to regret it later.

∗All text and images c

non-commercial purposes, is prohibited.

“Security-by-obscurity,” if viewed rather dismissively by those in information security, remains a cen-

tral tenet of the safe and vault trade. It isn’t easy to learn how safes work or what makes one better than

another, and while the basic techniques and designs are available to those who search persistently enough,

few professionals (on either side of the law) openly discuss the details of safe opening with the unindoctri-

nated. Consequently, it can be difﬁcult for a potential user to judge independently whether a given container

is sufﬁciently secure for its intended application; that role is left primarily to the safe industry itself (although

standards bodies and the insurance industry have some inﬂuence here as well).

For all the reticence surrounding the subject, however, safes and safe locks (and how they are defeated)

are worthy topics of study for students not only of locksmithing but of information security. An unfortunate

side effect of the obscurity of safe and vault technology is the obscurity of tools and techniques that deserve

to be better known and more widely applied to other disciplines. The attack models against which safes

are evaluated, for example, are far more sophisticated than their counterparts in computer science. Many

of the attacks, too, will remind us of similar vulnerabilities in computer systems, in spite of having been

discovered (and countermeasures developed against them) decades earlier.

The mechanical combination locks used to control access to safes and vaults are among the most

interesting and elegant examples of security engineering and design available today. The basic internal

structure of (and user interface to) the modern safe lock long predates computers and networks, and yet a

careful study of these devices reveals a rich history of threats and countermeasures that mimic the familiar

cycles of attacks and patches that irk practitioners of computer and network security.

One of the most striking differences between the physical and information security worlds is the rel-

ative sophistication of the threat models against which mechanical security systems are measured. Perhaps

owing to its long history and relatively stable technological base, the physical security community – and

especially the safe and vault community – generally seeks remarkable precision in deﬁning the expected

capabilities of the adversary and the resources required for a successful attack to occur. Far more than in

computers or networks, security here is recognized to be a tradeoff, and a quantiﬁable one at that. The

essence of the compromise is time.

1.1 Safe and vault construction

For the purposes of this discussion, a safe or vault is a container designed to resist (or leave evidence of)

unauthorized entry by force. (That is, we are discussing burglary safes. Many consumer products marketed

as “safes” do not actually meet this deﬁnition, being intended to resist only very casual pilfering or to protect

contents from ﬁre damage; we do not consider such safes here). The difference between a safe and a vault

is scale; safes are small containers designed to store objects, while vaults are essentially room-sized safes

with features (such as lighting and ventilation) that support human activity.

Many different safe and vault designs are in use, including stand-alone “box like” containers, in-ﬂoor

safes, in-wall safes, prefabricated vaults and custom made containers; even a superﬁcial survey would be

beyond the scope of this document. All share certain common characteristics, however.

Normal access to a safe or vault is via a door, which is usually hinged to the container walls. The

door is locked shut by one or more door bolts (comprising the boltwork), which generally are extended

or retracted by an external opening lever, which can only be operated if a lock bolt has been retracted by

the locking mechanism (e.g., after dialing the correct combination). Most modern burglary safes accept a

standard lock package (with an externally-mounted dial), consisting of an internally-mounted lock module

with a small retracting lock bolt designed to mate with the door bolts and handle. See Figure 1. (We will

discuss these locks in more detail later). Some older safes (as well as certain contemporary low security

Figure 1: Standard lock package (in this case, a Sargent & Greenleaf model R6730), shown mounted on a

display stand. The dial (left image) is accessible on the outside of the container. The internal lock module

(right image), in a standard form factor, contains the lock mechanism and retractable lock bolt (the brass

tab at the far right). Note the change key hole on the back of the lock case, into which the user can insert a

tool to change the combination when the container is open. The dial is connected to the lock module via a

spindle running through a small hole in the container wall.

safes) incorporate a customized lock as an integral component of the boltwork and use the lock bolt directly

as the door latch.

The main function of the safe or vault container is to resist opening by force and to protect the lock

package from tampering. Container walls and doors usually consist of several layers of material. The outer

layer is typically of conventional mild steel, intended to resist blunt force and prying. Resistance to more

specialized attacks is provided by barrier layers, which are fabricated from materials that resist penetration

by various kinds of tools. Barrier materials intended to thwart drilling, called hardplate, protect the parts of

the safe (such as the lock package) that might be proﬁtably drilled in an opening.

Barrier materials may protect all six sides of a container or, more often, only one (typically the door

itself). In-wall and in-ﬂoor safes are often protected at the door only, under the assumption that the sur-

rounding environment will prevent access from other directions. To prevent the container itself from being

stolen as a whole, stand-alone safes (especially less heavy models) are often designed to be bolted to a ﬂoor

or wall.

Many safes and vaults (including most burglary safes, but, interestingly, not GSA containers intended

for storage of classiﬁed materials) include one or more internal relockers (also known as relock devices) that

trigger when certain conditions consistent with an attack are detected. Once triggered, the relockers prevent

the door bolts from moving even after the lock bolt is retracted. Several kinds of relockers are in common

use. The most common detect punching attacks, in which the back of the door is damaged (e.g., dislodging

the internal lock package by applying force to the external dial). Thermal links, used in some safes, melt

and trigger a relock under the high temperatures that might be induced by cutting torches. Some of the

highest-end safes include tempered glass plates that trigger relock devices when breached by a drill. Lock

packages themselves often have internal relock triggers that prevent retraction of the lock bolt if the lock

case is forced open.

Any attack that aims to open the container door must therefore avoid triggering relock devices. The

chief value of many relockers seems to be thwarting novice burglars unaware of their existence. Especially

on mass-produced safes (the majority of the market), the types and locations of relockers can be predicted

and triggering them thereby avoided. On higher-end safes and vaults, however, especially those incorporat-

ing tempered glass plates, relockers might be randomly placed as a unique parameter of each instance of

the container. Here the relockers force the attacker to employ a more conservative opening technique (e.g.,

one that involves drilling through more hardplate), making the best-case penetration time slower (and more

predictable), even against the expert.

1.2 Container security metrics

Even the best safes and vaults are not absolutely impenetrable, of course; their strength is constrained by both

physics and economics. Safes are distinguished from one another not by whether they can be penetrated,

but by how long it would be expected to take, the resources required, and the evidence it produces.

The basic security metrics for safes attempt to measure resistance to the kinds of tools that attackers of

varying degrees of sophistication might be expected to wield. At the bottom of the attack-tool hierarchy are

ordinary hand tools, against which even a low-end safe might be expected to give at least some resistance,

then portable motorized power tools, then cutting torches, and ﬁnally (presumably for those concerned with

international jewel thieves from Hollywood movies), explosives.

We can also measure attacks according to the obviousness of the evidence left behind. Here the termi-

nology is at its most cloak-and-dagger; an attack is said to be surreptitious if it leaves behind no evidence

at all, covert if it leaves behind evidence that would not be noticed in normal use (although it might be

noticed in an expert inspection), and forced if the evidence is obvious (of course, force might be involved in

surreptitious or covert entry as well, so the term is a bit of a misnomer). These distinctions are mainly of

interest for safes used to store conﬁdential (or classiﬁed) information, where prompt discovery of successful

attacks can be almost as important as preventing them in the ﬁrst place.

Safe and vault rating categories aim to provide a multi-dimensional picture that allows the potential

user to evaluate protection according to the perceived threat: a given safe might be rated for a very long time

against surreptitious entry aided only by the simple tools of the most casual thief, but for shorter times as

the tools used become more sophisticated, heavy, conspicuous, and expensive or as the evidence of attack

becomes more pronounced. (Several organizations publish ratings according to various criteria, including,

in the U.S., Underwriters Laboratories (UL) for commercial safes and the General Services Administration

(GSA) for federal government safes).

Because the materials and mechanical designs from which safes and vaults are manufactured have

rather well understood physical properties, relatively simple procedures are used to estimate time bounds

on resistance to attack. The usual approach is to make rather generous assumptions about the skill and

tools of the attacker and the conditions under which an unauthorized opening might be carried out. For

example, a sample safe might be drilled (under laboratory conditions and with the best commercially avail-

able equipment and techniques), and the time for penetration considered to be the minimum required for a

drilling-based opening by a burglar.

These tests produce safe ratings that may seem disturbingly weak at ﬁrst blush. The best UL rating

categories are for only 15, 30 and 60 minutes, and GSA ratings against forced attack are for either zero(!) or

10 minutes. Yet opening even a zero-minute rated GSA container may require an hour or longer under ﬁeld

conditions (and attract considerable attention in the process).

Observe that safe testing as described here does not produce upper or lower bounds on security in

the sense usually used in information security. They are clearly not lower bounds, since better tools or

techniques not known when a safe was tested might substantially reduce the required penetration time. The

results are not especially meaningful as upper bounds, either, since the conditions are sufﬁciently generous

to the attacker to make it very unlikely that they could be achieved under ﬁeld conditions. Instead they are

less formal “guidelines,” intended mainly for comparison, and useful as approximate lower bounds only

under the (perhaps tenuous) assumption that improved tools and techniques will not become available in the

future.

1.3 Lock security metrics

Time is also the essential metric by which the locks used on safes and vaults are measured. Here, however,

we are less concerned with attacks by force, since the sensitive components of the lock are protected by the

container itself. Instead, the primary attacks involve exploiting poorly-chosen combinations (birthdays are

said to be popular), ﬁnding the working combination through exhaustive search, or interpreting incidental

feedback given through a lock’s user interface to make inferences about its internal state. The latter approach

is usually called manipulation within the safe and vault trade, although, as we will later see, the techniques

involve careful observation more than outright manipulation.

Mechanical combination dial locks are the most common access control devices used on burglary

safes and vaults in the United States, and these locks will be the focus of our attention here. Such locks are

opened by demonstrating knowledge of the combination by rotating a dial, reversing direction at speciﬁc

places on the dial; we will discuss the user interface and dialing procedure in detail in Section 2. Electronic

combination locks (using a keypad or rotary-encoder dial) are becoming increasingly popular at the low-

and high-ends of the safe market, but we will not consider them here; analyzing such locks is essentially a

software and embedded system security problem beyond the scope of this paper. Keyed safe locks (usually

of a lever-tumbler design) are more common in Europe and elsewhere, but again, they are beyond our scope

here.

Nondestructive attacks against the combination itself are usually considered to be in the “surreptitious”

category; they leave little or no forensic evidence. (Electronic and electro-mechanical locks may incorporate

logs and audit trails, but we are considering strictly mechanical locks here). Many lock attacks, including

manipulation, can be performed across several (interrupted) sessions, making them an especially serious

threat in some environments.

1.3.1 The combination keyspace

The most obvious lock security factor is the number of distinct combinations; it provides a bound on the time

required for exhaustive search. Most safe and vault lock dials are divided into 100 graduations (see Figure 2),

with three (or occasionally four) dialed numbers in the combination. This implies 1003(1,000,000) possible

combinations for a three number lock and 1004(100,000,000) possible combinations in a four number lock.

The number of effectively distinct combinations is usually considerably lower, however. Most locks

have a wider dialing tolerance than the dial graduations would suggest, allowing an error of anywhere

between ±.75 and ±1.25 in each dialed number, depending on the lock model. So although there may be

100 marked positions on the dial, there may be as few as 40 mechanically distinct positions. A three number

Figure 2: External dial (user interface) of common Group 2 lock (again, a Sargent & Greenleaf model

R6730). The actual dial, with 100 graduations, rotates; the surrounding dial ring is ﬁxed to the container.

The main index mark at 12 o’clock (shown here at dial position 2) is used for dialing the combination to

open the lock; the smaller index mark at 11 o’clock (at 94) is used only when setting a new combination.

lock would thus have between 403(64,000) and 673(300,763) effective combinations. Other restrictions

reduce the combination keyspace a bit further: the selection of the last number is usually constrained to

about 80% of the dial, depending on the lock design. With 20% of the last number’s space lost, the effective

number of distinct combinations on three wheel locks is in practice between 51,200 (with a tolerance of

±1.25) and 242,406 (with a tolerance of ±.75).1

Clearly, even 51,200 combinations would render manual exhaustive search by an unaided attacker

infeasible. However, commercially available robotic dialers (a servo motor attached to the dial controlled

by a simple microcontroller) can search the effective keyspace of most three-number locks, as well as some

four-number locks, overnight or over a weekend (however, this is still generally longer than the expected

required penetration time for the container itself; the repeated, high speed dialing also introduces signiﬁcant

wear on the lock).

The size of the combination keyspace is one of the most important metrics used in the certiﬁcation of

safe locks by various standards bodies. In the United States, UL rating standards for Group 2 safe locks

(the most common commercial locks) specify that there must be at least 1,000,000 different combinations

and that the dialing tolerance be at most ±1.25. (The standard does not explicitly address the number of

usable combinations that can actually be set by the user, however, and so three-number locks can be certi-

ﬁed even when the last number is constrained). Comparable standards in other countries demand speciﬁc

minimum sizes for the combination keyspace more directly. CEN, the European standards body, requires at

least 80,000 distinct usable combinations for “Class A” locks (roughly equivalent to UL Group 2). VdS, a

similar German standard, has the same requirement for its “Class 1” rating (again, roughly equivalent to UL

Group 2).

Unfortunately – and ironically – a signiﬁcant further reduction in combination keyspace comes from

overly broad “guidelines” concerning the choice of “good” combinations. Presumably to compensate for

1One of the better examples on the market in this regard, the (Group 2) Sargent and Greenleaf R6730 lock, has a dialing tolerance

of ±.75 and allows the use of 94% of the dial for the last combination number, yielding a usable combination keyspace of roughly

282,807 distinct combinations.

the notoriously poor ability of users to select sufﬁciently “random” combinations, many lock manufacturers

recommend avoiding selection of combinations that do not “look random.” A typical example is Sargent and

Greenleaf[Cos01], which recommends for its three-number locks the combination as a whole not consist of

a monotonically increasing or decreasing series, that adjacent numbers differ by at least ten graduations2,

and that 25% of the dial be avoided for the ﬁnal number (although the mechanism itself on S&G locks

requires avoiding only 6% of the dial). Acceptable combinations under these recommendations comprise

less than 50% of the usable combination keyspace. For example, while the S&G R6730 lock has 282,807

distinct usable combinations according to its mechanical speciﬁcations, only 111,139 of them are considered

“good” according to the manufacturer’s recommendations. For locks with the full ±1.25 dialing tolerance

allowed under UL Group 2, these recommendations seem especially misguided, leaving only 22,330 distinct

“good” combinations. Observe that this is less than 2.5% of the apparent keyspace of 1,000,000.

Similar reductions in effective keyspace will be familiar to observers of many computer password

authentication systems.

1.3.2 Manipulation resistance

Some combination lock designs, including those used on burglary safes, are subject to imperfections that

leak information about their internal state through the external dial user interface. It may be feasible for an

attacker to exploit this information to discover a working combination by manipulation, the systematic entry

of trial combinations and interpretation of state information.

An obvious security metric for a combination lock, therefore, is whether the design (and its fabrication

processes) resists manipulation attacks. Because elaborate equipment is not generally required to perform

these attacks, the most signiﬁcant variable in the threat model is whether the attacker is familiar with and

practiced in the technique. Ratings for these locks distinguish between “expert” and “non-expert” attacks.

There are two classes of commercial (UL-rated) safe locks in the United States. “Group 1” locks

are intended to resist expert manipulation for at least twenty hours; in practice this means the best attack

against such locks should be exhaustive search. (A sub-category, “Group 1R,” also requires resistance to

radiological analysis, perhaps the only lock attack assumed to involve special tools). “Group 2” locks

provide only “moderate” resistance to manipulation, but are considered secure against non-experts. (Locks

in a recently introduced sub-category, “Group 2M,” are said to resist expert manipulation for up to two

hours).

The vast majority of commercial safes use Group 2 (and sometimes even unrated) locks. Even ap-

parently formidable containers that might require signiﬁcant effort to penetrate by force are often equipped

with locks that can be manipulated open with no evidence by anyone familiar with the procedure. Group 1

locks are usually found only on high-end safes and vaults intended speciﬁcally for the storage of high-value

items or classiﬁed materials.

The relative rarity of mechanical locks designed to resist expert manipulation seems somewhat surpris-

ing, especially given that the containers on which they are used are often quite secure against penetration by

experts. Group 1 locks are not substantially more expensive than their Group 2 counterparts, and certainly

would not represent a signiﬁcant increase in the overall cost of a container. The likely explanation is that

mechanical Group 1 locks typically have a more complex user interface, usually requiring an additional step

before unlocking, and are less forgiving of dialing errors. Just as in computing systems, many users are

willing to exchange even a signiﬁcant degree of security for improved usability and convenience.

2This recommendation also may slightly improve resistance to manipulation.

1.4 Using security metrics

Time for successful attack is used as a metric of safe and vault quality not only because it is somewhat

measurable, but because it is exactly the property that the security engineer must know in order to design

and evaluate the system as a whole. If a safe or vault is trusted to resist a particular kind of attack (e.g.,

drilling) for a particular amount of time, (e.g., 30 minutes), we can conclude that the system is secure as

long as the conditions for the attack cannot occur for longer than the period speciﬁed (e.g., guards and

alarm sensors that prevent people with drills from having unsupervised access to the safe for more than 30

minutes).

It is notable that analogous security metrics do not generally exist for information systems. In partic-

ular, while some measures of required attack resources do exist (e.g., for cryptographic work factors), in

practice the resources required to attack most information security mechanisms are either completely un-

known or are known only with low conﬁdence. When such metrics are available, the usual design principles

of computer and communications security consider a system to be secure only when the work factor is so

large to make any possibility of attack completely infeasible (e.g., requiring turning every molecule in the

solar system into a supercomputer). In physical security, perhaps because the security metrics are believed

to be more realistic, much smaller “safety margins” are generally tolerated.

In other words, the tools of information security generally group systems into one of three categories

(completely secure, completely insecure, or, most commonly, unknown security), with few meaningful ways

to compare systems within a given category. The measurement tools of physical security, on the other

hand, recognize ﬁner shades of security, allowing comparisons to be made in which one system might be

considered more secure than another for a given purpose. It is unclear which approach is sounder; attacks

occur in both domains, of course. In any case, the principles of physical security design and evaluation

richly repay careful study by computer scientists, and the development of similar metrics for information

security would represent a signiﬁcant advance in the ﬁeld.

2 Group 2 mechanical combination locks

The modern dial combination lock mechanism is relatively simple, and its basic design has remained essen-

tially unchanged for at least a century. There are relatively few variations from the standard design, although

some models incorporate extra security features (e.g., to meet Group 1 standards).

The most common are the Group 2 locks, and among them, most current products use a “spring loaded

lever-fence, key changeable” design. The “standard” such lock is the Sargent & Greenleaf model R6730.

Other current locks employing a virtually identical design as the R6730 include the Kaba-Ilco model 673

and the LaGard model 3330. Because the design is so common, and also because it illustrates the basic

principles of operation (and security pitfalls) of mechanical combination locks well, it will be the focus of

our attention here. (We will discuss variants on the design later).

The standard external user interface is via a roughly 3 inch diameter rotating dial mounted to the door

of the container and graduated into 100 positions. A dial ring, ﬁxed to the door’s wall, has a primary index

mark (usually at 12 o’clock) for dialing the numbers of combination, plus a second index mark (usually at

about 11 o’clock) that is used only when changing the combination. See Figure 2.

The dial is connected to the internal lock module via a spindle running through the container wall that

serves as the dial’s axis and that rotates along with it. The major internal components of (Group 2) lock

modules are shown in Figure 3.

Figure 3: Major components of Group 2 lever-fence lock, as seen from the back (Kaba-Ilco model 673)

Although many of the lock components serve more than one purpose, with complex interactions that

depend on the lock state, the design is simpler than it might ﬁrst seem. Recall that the purpose of the lock is

to retract the lock bolt (and thereby release the door bolts) only after a correct combination has been entered.

It is easier to understand the design as a whole by studying its two basic functions separately – retracting the

lock bolt and enforcing the combination.

2.1 Retracting the lock bolt: the drive cam and lever

The two main internal components involved in retracting the lock bolt are the drive cam and the lever.

Within the lock module, the spindle terminates at a drive cam (also known as the cam wheel or simply

the cam). The cam moves with the external dial, with all rotational movement of the dial transmitted directly

to the cam. (On most locks, including that shown in Figure 3, the cam is the rear-most element, but that is not

essential to the design.) Observe that the cam is circular with a wedge-shaped notch cut in its circumference;

the notch is called the cam gate.

The lock bolt slides partly into or out of the lock within a channel in the side of the module’s housing

(i.e., to the left or the right in the ﬁgures here). The bolt is attached within the lock module to the lever. The

lever is attached to the bolt with a lever screw, which acts as a pivot point for the lever, allowing it to move

upward and downward across a range of a few degrees. The lever is pressed downward by a lever spring,

which is usually wound around around the lever screw.

The lever runs within the module from the lock bolt to near the spindle axis. At the far end of the lever,

the lever nose rests along the edge of the cam, held down by the pressure of the lever spring. Observe that

the lever nose is in the same wedge shape as the cam gate. The bolt is moved by allowing the lever nose to

mate with the cam gate.

Figure 4: Opening a lock. In this cutaway view from the back of the lock, the dial is rotated clockwise

to retract the lock bolt. In (a) through (c), the cam rotates toward the lever, allowing the lever to lower as

the nose mates with the cam gate. In (d), further rotation of the cam has pulled the lever to the left, which

retracted the bolt.

In Figure 4 the dial (and hence the cam) is rotated clockwise3and the cam gate approaches the lever

nose. As the cam gate moves under the lever nose, the nose is pushed downward in to the gate. Continued

clockwise rotation, with the lever nose fully mated with the cam gate, pulls the lever, which in turn retracts

the bolt. Once the bolt is fully retracted, the dial cannot be turned further clockwise; counterclockwise dial

rotation extends the bolt back to the locked position.

2.2 Enforcing the combination: the fence and wheel pack

As described so far, our lock can retract and extend its bolt but does not have any security; it is opened by

simple clockwise dial rotation. Two additional components, the wheel pack and the fence, interact with the

lever and cam to allow the bolt to retract only after a correct combination has been dialed.

The wheel pack is the set of “security tumblers” for the lock. It is mounted behind the cam around the

spindle, but does not make direct contact with the spindle itself. The wheel pack consists of a collection of

disks (called wheels), of larger diameter than the cam and that can rotate independently of the cam and of

one another. In the edge of each wheel is a notched combination gate (or simply a gate). See Figure 5. (On

3In most ﬁgures here the view is from the back of the lock, and so clockwise and counterclockwise rotation appear reversed.

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SafecrackingforthecomputerscientistMattBlazeDepartmentofComputerandInformationScienceUniversityofPennsylvaniablaze@cis.upenn.eduDRAFT7December2004(Revised20December2004)DRAFTThelatestversionofthisdocumentcanbefoundathttp://www.crypto.com/papers/safelocks.pdfThisdocumentcontainsmediumresolutionpho...

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