Cryptography Reference
In-Depth Information
k e y s k e y s k
a f e s t a i m s
K J C K D E G E C
Notice that DLCK is a block that occurs twice, at the beginning and end of the
first table, and the distance between the occurrence of the first D and the second
( in the second block ) is 8 . Also, the diagram CK occurs at the end of the first
table and again 4 units away in the second table. Hence, since gcd(8 , 4) = 4 , this
is the probable keylength by the Kasiski examination, which is indeed correct.
Toward the end of the nineteenth century, another topic, which may be seen
as taking the torch passed by Kasiski, was published. In 1883, La Cryptographie
militaire was published by Jean-Guillaume-Hubert-Victor-Francois-Alexandre-
Auguste Kerckhoffs von Nieuwenhof (but we will just call him Kerckhoffs).
Kerckhoffs was born on January 19, 1835, of Flemish descent, in Holland. His
education involved almost two years of study in England plus degrees obtained
at the university in Liege. After some teaching positions and some travelling,
Kerckhoffs married and settled down in a town outside Paris. He taught lan-
guages there for a number of years. By 1876, he had earned his Ph.D. and
by 1881 became a professor of German in Paris. While there, he wrote the
aforementioned topic, which many consider to be the most succinct text on
cryptography ever written.
In his topic, Kerckhoffs elucidated several basic tenets. In modern times,
one of these has come to be known as Kerckhoffs' Principle and has been incor-
porated into modern cryptographic methodology.
Kerckhoffs' Principle
In assessing the security of a cryptosystem, one should always assume
the enemy knows the method being used.
The telegraph had made possible the introduction to cryptology of a new
device, the field cipher , a rapid means for the military to send secure, secret
messages in a theater of war. Kerckhoffs also instituted several tenets for field
ciphers (from which his above principle has been gleaned).
Kerckhoffs' Principles for Field Ciphers
1. The cryptosystem should be practically unbreakable (breakable in theory,
perhaps, but not in practice).
2. A compromised cryptosystem should not inconvenience the correspondents.
(This is the one from which his aforementioned principle seems to be
derived since it says that the enemy may know the cryptosystem, but one
should still be able to send messages since the enemy cannot cryptanalyze
with this knowledge and without the key.)
3. The key should be easy to both remember, and change at will.
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