Security Through Usability

By Seth Ross

In 1883, Flemish linguist Auguste Kerckhoffs published a groundbreaking
article on military cryptography that is still widely cited for
its admonishments against what is now known as "security by
obscurity". In reviewing the historical use of cryptography,
Kerckhoffs showed how, time and again, cryptographers developed
complex and obscure cryptosystems that relied on the principle that
the enemy wouldn't find out how they worked. Just as often, enemy
cryptanalysts were able to break the systems, usually without detection.
Even when generals realized that their crypto was broken, there
was little they could do in a timely way, since the only solution
was to replace entire cryptosystems.

Historically, cryptography had only been used by the elite, typically
royalty, courtiers, diplomats, and top military leaders. By the
late 19th century, the complexity of modern warfare and new communications
technologies like the telegraph created new security needs. In the
wake of France's defeat in the Franco-Prussian War, Kerckhoffs and
other military thinkers realized that the need for secure communications
was no longer restricted to top military commanders: the communications
between generals and officers, and between officers and field units,
needed protection as well. The idea of pervasive encryption was
born.

Given the rise of field cryptography, Kerckhoffs realized that
cryptographers could no longer assume that the enemy would not figure
out how the underlying cryptosystem works -- the deployment of cryptography
on the front lines vastly increased the risk that the enemy could
steal and analyze the system's operations. He advocated what we
might now call "open systems": instead of relying on the
obscurity of the cryptosystem's underlying operation, Kerckhoffs
argued that designers should assume that the enemy will either know
or be able to deduce how the system works. Instead of relying on
obscurity, he argued, security should depend on the strength of
keys. In the event of a breach, only the keying material would need
to be replaced, not the whole system.

Kerckhoffs was the first to publicly identify the weakness of security
by obscurity. The admonition not to rely on obscurity in security
systems is often called "Kerckhoffs' Principle" or "Kerckhoffs'
Law". The principle is taught in many computer science classes
and is frequently cited and discussed by computer security experts.[1,
2, 3, 4]

In his seminal publication, "La cryptographie militaire",
Kerckhoffs actually presents five other laws as well.[5]
In general, these have received a lot less attention. Here are all
six of Kerckhoffs' Laws (thanks to Fabien A. P. Petitcolas for the
translation)[6]:

The system must be substantially, if not mathematically, undecipherable;

The system must not require secrecy and can be stolen by the
enemy without causing trouble;

It must be easy to communicate and remember the keys without
requiring written notes, it must also be easy to change or modify
the keys with different participants;

The system ought to be compatible with telegraph communication;

The system must be portable, and its use must not require more
than one person;

Finally, regarding the circumstances in which such system is
applied, it must be easy to use and must neither require stress
of mind nor the knowledge of a long series of rules.

While security designers and engineers have embraced requirement
No. 2, there's been less focus on No. 6. Over the past 120 years,
"Security Through Usability" has largely been overlooked.
As a consequence, if you look at modern cryptosystems, you'll find
that many rely on what I'll call "security through complexity".

Security through complexity is embodied in the "more is better"
school of software development that achieves greater functionality
and perceived value through "feature creep". It's the
tendency to develop encryption programs and platforms with so many
systems and subsystems, protocols, rules, caveats, and documentation
sets, that regular people cannot use them. Security engineers are
aware of this drift towards complexity and unusability -- Alma Whitten
published "Why Johnny Can't Encrypt" in 1999 [7]
-- but given market and marketing forces, they are often unable
to stop it.

PGP 8.0 embodies this trend. Philip Zimmermann's spare but functional
encryption program has been transmogrified over the years into a
multi-headed crypto hydra. It does AES. It does TripleDES. It does
CAST. It does IDEA. Those are just the symmetric ciphers. It does
Diffie-Hellman, RSA, and RSA Legacy. Do you know which ciphers you
want to use, and why? It encrypts files, clipboard contents, disk
partitions, or email via Outlook, Outlook Express, or Notes plug-ins.
It supports key splitting so that more than one user must "turn
the key" at the same time to decrypt and a Secure Viewer that
is designed to resist TEMPEST snooping. It supports no fewer than
five Certificate Authorities, four smart cards, and dozens of arcane
key server operations.

The list of PGP features goes on and on. The point here is not
to beat up on PGP, but to demonstrate a commercial violation of
Kerckhoffs' "Security Through Usability" rule. Kerckhoffs
was concerned about the "stress of mind" (_tension d'esprit_
in the original French) that complexity creates for crypto users
in the field. Given the requirement for pervasive encryption, Kerckhoffs
knew that complex systems would either break down in the field or
simply be bypassed. While this problem is particularly acute under
battlefield conditions, where fatigue, forgetfulness, and stress
are chronic, it also impacts modern "road warriors", who
are required to deliver timely results from far-flung locations
while managing jetlag, technical problems, and cross-cultural pleasantries.
Few users -- either military or commercial -- have the time or inclination
to study documentation (the "long series of rules" to
which Kerckhoffs refers).

Ironically, PGP was the first piece of crypto software with source
code published on the Internet, and thus has been a model for compliance
with Kerckhoffs' obscurity rule. The idea of publishing code for
review has been almost universally embraced by the academic community
and commercial crypto vendors, and most of the important ciphers
like AES are open and available for anyone to review or use.

Now that transparency of operation and open systems are nearly
universal in cryptography, perhaps it's time for all those who design
and deploy cryptosystems to embrace usability. Kerckhoffs' Security
Through Usability law should be taught in the universities alongside
his admonitions against obscurity. Security buyers should beware
of large scale, complex, expensive, and ultimately unusable systems
like Public Key Infrastructures, which also violate Kerckhoff's
third requirement (easy to remember or change keys)
and fifth requirement ("not more than one person to operate").
Given the active threats posed by espionage and terrorism, the challenges
of pervasive encryption are even more important now than they were
in Kerckhoffs' era. It's time to make, and keep, our crypto simple.