Cryptography Reference
In-Depth Information
chapter
2
CIPHER SYSTEMS
Cryptography, as defined previously, is the science of
transforming information into a form that is impos-
sible or infeasible to duplicate or undo without knowledge
of a secret key. The easiest way to describe the techniques
on which cryptography depends is first to examine some
simple cipher systems and then abstract from these exam-
ples features that apply to more complex systems. There are
two basic kinds of mathematical operations used in cipher
systems: transpositions and substitutions. Transpositions
rearrange the symbols in the plaintext without changing
the symbols themselves. Substitutions replace plaintext
elements (symbols, pairs of symbols, etc.) with other sym-
bols or groups of symbols without changing the sequence
in which they occur.
TransposiTion ciphers
In manual systems transpositions are generally carried
out with the aid of an easily remembered mnemonic. For
example, a popular schoolboy cipher is the “rail fence,”
in which letters of the plaintext are written alternating
between rows and the rows are then read sequentially to
give the cipher. In a depth-two rail fence (two rows) the
message WE ARE DISCOVERED SAVE YOURSELF
would be written
