A History of Cryptography - Introduction to Cryptography with Java Applets - page 18

lyst can guess the key if it is a potentially infinite sequence of random numbers. It is math-
ematically provable that plaintext message could map to some particular ciphertext

message if random numbers are used; thus, the ciphertext provides absolutely no informa-
tion to the analyst at all.
Of course, the reason one-time pads are not used today is because they are simply imprac-
tical. The distribution and protection of the pads is a logistical nightmare. For example, if
all the sheets in a pad were used up, it would have to be replaced with a new pad consist-
ing of entirely different random numbers. However, one-time pads have been used; in par-
ticular, certain embassies have used them for highly sensitive communications with their
governments.

any

1.14

TRANSPOSITION CIPHERS
Transposition ciphers were simply a permutation of the letters in a plaintext message; that
is, they reordered the letters of the message. This reordering was specified for blocks of a
predetermined size, and the reordering would occur within each block. Say we choose a
block size of 5, and for a particular block we specify the following:

The 1 st letter becomes the 4 th letter,
the 2 nd letter becomes the 3 rd letter,
the 3 rd letter becomes the 1 st letter, ( * )

the 4 th letter becomes the 5 th letter, and
the 5 th letter becomes the 2 nd letter.

A short way of denoting this permutation is to use the notation

(1 4 5 2 3),

which becomes meaningful if you just rearrange the statements in ( * ).

The 1 st letter becomes the 4 th letter,
the 4 th letter becomes the 5 th letter,
the 5 th letter becomes the 2 nd letter,
the 2 nd letter becomes the 3 rd letter, and
the 3 rd letter becomes the 1 st letter.

Suppose we have the plaintext message

THE SKY FALLING PLEASE ADVISE

which we split into blocks of length 5:

THESK YFALL INGPL EASEA DVISE

If we use the permutation defined by ( * ), we get the following scrambled blocks, which
comprise the ciphertext.

EKHTS ALFYL GLNIP SAAEE IEVDS