Internet Security Professional Reference:Encryption Overview

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Transposition

In transposition, the same characters that make up the message are still used, but their order is jumbled in a way that makes it difficult to read the message. Suppose, for example, that you want to encrypt the message: Trucks and vehicles with trailers use right lane. The message can be written in two columns in the following manner:

T	r
u	c
k	s
	a
n	d
	v
e	h
i	c
l	e
s
w	i
t	h
	t
r	a
i	l
e	r
s
u	s
e
r	i
g	h
t
l	a
n	e

If you print the first column of the message, followed by the second column, it becomes: Tuk n vhce wt talr uergtlnrcsadvhce ihtalr s ih ae. This is much more difficult to read and takes some time to break the code.

The code becomes more difficult if you add additional columns and stagger their order. By using five columns and staggering the order in which they are presented, the code becomes:

5	3	1	4	2
T	r	u	c	k
s		a	n	d
	v	e	h	i
c	l	e	s
w	i	t	h
t	r	a	i	l
e	r	s		u
s	e		r	i
g	h	t		l
a	n	e

This translates into: uaeetas tekdi luilr vlirrehncnhshi r Ts cwtesga. Because the exact same message you are trying to send is still here, the only key to deciphering it is figuring out the number of columns used to create it and the order in which they are being presented.

Deciphering

For practice, assume that transposition was used, and the message you find is:

E NYIN BHA RWD OE T AU NFNTEERW EINFWA ALMTPDOSHR TAAR

You now must ascertain two things: the number of columns that were used and the order in which the columns were placed. The length of the string is 55 characters, and if you assume that five columns were used, each column will consist of 11 characters:

1	2	3	4	5
E	R		E	P
	W		I	D
N	D	N	N	O
Y		F	F	S
I	O	N	W	H
N	E	T	A	R
		E
B	T	E	A	T
H		R	L	A
A	A	W	M	A
	U		T	R

Regardless of the order in which you arrange the columns, it fails to be intelligible—an indication that five is not the correct number of columns. Increasing the number to eight renders the following:

1	2	3	4	5	6	7	8
E		D	A	T	I	L	H
	B		U	E	N	M	R
N	H	O		E	F	T
Y	A	E		R	W	P	T
I			N	W	A	D	A
N	R	T	F			O	A
	W		N	E	A	S	R

By mixing the order of the columns about and concentrating on making a legible word in the first row, it is possible to produce the following:

3	6	4	7	2	5	8	1
D	I	A	L		T	H	E
	N	U	M	B	E	R
O	F		T	H	E		N
E	W		P	A	R	T	Y
	A	N	D		W	A	I
T		F	O	R		A	N
	A	N	S	W	E	R

In other words, dial the number of the new party and wait for an answer. The ENCRYPT utility is hard-coded to use the same transposition method utilized in the previous example, where the column ordering becomes: 8-5-1-3-6-2-4-7. This means that the first column of the encrypted message was the eighth column of the original message; the second column of the encrypted message was the fifth column of the original message; and so on.

Figure 10.6 shown previously illustrates how to experiment with transposition. You choose 1 on the menu, type a phrase in the Phrase field, then choose E to encrypt it. The results are shown in the Results field. To decrypt the phrase, choose D. The results are pictured in figure 10.7.

Figure 10.7 Decrypting a message with transcription.

Substitution

Substitution differs from transposition in one key way: with transposition, all characters from the original message are still there; with substitution, none of them are. Substitution replaces each character of the original message with another. Breaking the code is based upon ascertaining which character the one you are seeing is replacing.

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