Page 224 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 14: Network Security

ROT13 is a Caesar cipher, a type of substitution cipher. In ROT13, the alphabet is rotated 13 steps. Notes
Substitution over a single letter—simple substitution—can be demonstrated by writing out the
alphabet in some order to represent the substitution. This is termed a substitution alphabet. The
cipher alphabet may be shifted or reversed (creating the Caesar and Atbash ciphers, respectively)
or scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged
alphabet. Traditionally, mixed alphabets are created by first writing out a keyword, removing
repeated letters in it, then writing all the remaining letters in the alphabet.

Examples

Using this system, the keyword “zebras” gives us the following alphabets:

Plaintext alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Ciphertext alphabet: ZEBRASCDFGHIJKLMNOPQTUVWXY
A message of
flee at once. we are discovered!

enciphers to
SIAA ZQ LKBA. VA ZOA RFPBLUAOAR!
Traditionally, the ciphertext is written out in blocks of fixed length, omitting punctuation and
spaces; this is done to help avoid transmission errors and to disguise word boundaries from the
plaintext. These blocks are called “groups”, and sometimes a “group count” (i.e., the number of
groups) is given as an additional check. Five letter groups are traditional, dating from when
messages used to be transmitted by telegraph:
SIAAZ QLKBA VAZOA RFPBL UAOAR

If the length of the message happens not to be divisible by five, it may be padded at the end with
“nulls”. These can be any characters that decrypt to obvious nonsense, so the receiver can easily
spot them and discard them.
The ciphertext alphabet is sometimes different from the plaintext alphabet; for example, in the
pigpen cipher, the ciphertext consists of a set of symbols derived from a grid. For example:

Such features make little difference to the security of a scheme, however – at the very least, any
set of strange symbols can be transcribed back into an A-Z alphabet and dealt with as normal.
In lists and catalogues for sales people sometimes a very simple encryption is used to replace
numeric digits by letters.
Plain digits: 1234567890
Ciphertext alphabet: MAKEPROFIT

Example: MAT would be used to represent 120.

Security for Simple Substitution Ciphers

A disadvantage of this method of derangement is that the last letters of the alphabet (which are
mostly low frequency) tend to stay at the end. A stronger way of constructing a mixed alphabet

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