Cryptography Reference
In-Depth Information
I'd appreciate your letting me know). For, in contrast to the usual textbook
style, we will first deal with the practice and sum up a few important things
in Chapter 3. At that point, you'll have a pretty good idea what is behind all
these terms.
Let me tell you at this point: everything described in this chapter is yester-
day's bread. None of the algorithms presented here is secure today (with one
exception). They all originate from prehistoric times when computers weren't
around. Back then, people looked at plaintext as a sequence of characters (while
almost always today we look at it as a sequence of bits). Nevertheless, you can
learn a lot about cryptography and cryptanalysis from the methods discussed
here. This knowledge will come in handy in the later chapters, because it is
more or less the basis of modern cryptology. Moreover, it's simply fitting for
a cryptologist to know about the Enigma. And actually it is a very thrilling
matter — a pure cryptological adventure.
2.1 The Caesar Method and its Relatives
Even the old Romans wanted to send encrypted messages. Caesar used one
of the simplest encryption methods, known as the
Caesar cipher
or Caesar
addition. In this method, each letter is substituted by the one three places
further behind in the alphabet. We think of the alphabet as if it were written
on a ring so that A follows Z. The encryption rule will then look as follows:
A->D
B->E
C->F
...
W->Z
X->A
Y->B
Z->C
Blanks are omitted and no difference is made between lowercase and uppercase
letters. This can result in ambiguities, and the code writer must pay attention.
A quick help can be the use of an agreed division sign (which will always
create a vulnerability). By the way, the Romans didn't use 26 letters, but that
doesn't change the method.
This method may have represented an insurmountable hurdle for the Roman
army and its adversaries. Augustus, Caesar's successor, who was thought to
