Cryptography Reference
In-Depth Information
change the letters, but rather move them around, transpose them, without in-
troducing any
new
letters. Here is a simple illustration. Suppose that we have
thirteen letters in our plaintext, and the following is a permutation that tells
us how to move the thirteen positions around. The way to read the following
is that the symbol in the position number in the top row gets replaced by the
symbol in the position number below it in the second row.
1234 5 678910111213
1234107895 6 111213
Now, suppose that our plaintext is
they flung hags
. Then the ciphertext will
be
THEY HUNG FLAGS
. Notice that the first four and last three plaintext
letters remain in the same position as dictated by the above permutation, but
the
f
in position 5 gets replaced by the
H
in position 10; the
l
in position 6
gets replaced by the
U
in position 7; the
u
in position 7 gets replaced by the
N
in position 8; the
n
in position 8 gets replaced by the
G
in position 9; the
g
in position 9 gets replaced by the
F
in position 5; and the
h
in position 10
gets replaced by the
L
in position 6. So this is an easy-to-understand method of
depicting transposition ciphers that we will use throughout the topic. We can
see that transposition ciphers depend upon the permutation given, such as the
one above, so often transposition ciphers are called
permutation ciphers
.
Now let us return to the Spartans, the great warriors of the Greek states. The
Spartans used a transposition cipher device called a
skytale
(also spelled
scytale
in some sources). This consisted of a tapered wooden staff around which a strip
of parchment (leather or papyrus were also used) was spirally wrapped, layer
upon layer. The secret message was written on the parchment lengthwise down
the staff. Then the parchment was unwrapped and sent. By themselves, the
letters on the parchment were disconnected and made no sense until rewrapped
around a staff of equal proportions, at which time the letters would realign
to once again make sense. One use of the skytale was documented to have
occurred around 475 BC with the recalling of General Pausanius, who was a
Spartan prince. He was attempting to make alliances with the Persians, an act
the Spartans regarded as treasonous. Over one hundred years later, a skytale
was used to recall General Lysander to face charges of sedition. Thus, the Greeks
have been credited with the first use of a device employing a transposition cipher.
The earliest writings on cryptography, as instructional text, is credited to
the Greeks. In the fourth century BC, Aeneas Tacticus wrote a book on military
science, called
OntheDefenseofFortifications
. In this topic, an entire chapter is
devoted to cryptography. In this chapter, Tacticus also describes several clever
steganographic techniques. One of these techniques is to puncture a tiny hole
above or below letters in a document to spell out a secret message. Almost two
thousand years later, this method was used (with invisible ink and microdots
rather than pin pricks) by the Germans during the world wars in the twentieth
century.
More credit goes to the Greeks in terms of development of some of the first
substitution ciphers. Polybius who lived approximately from 200 to 118 BC was
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