Cryptography Reference
In-Depth Information
programming tricks, the computation time can be cut down dramatically. We
will see in Section 3.6.4 that all of this works in the real world.
Compression combined with homophone substitution is a trifle cleverer. Cre-
ating a program that breaks this combination is certainly not easy, but it is
possible. You'd have to do the work only once — then the method is for ever
worthless.
2.2.4 Transposition
While substitution ciphers preserve the order of the plaintext symbols but dis-
guise them, transposition ciphers, in contrast, reorder the letters but do not
disguise them. The easiest transposition method is the 'cube'. Using it, you
write the message line by line in a rectangle:
DELIVE
RTHERA
NSOMTO
MORROW
ASAGRE
EDJOHN
and read it column by column:
DRNMAEETSOSDLHORAJIEMRGOVRTORHEAOWEN
Naturally this method offers no security whatsoever — it uses only the edge
length of the square to serve as key. In practice, the code writer would have
to use more ingenious transpositions, which depend mainly on keys with a
large number of conceivable values. It is, therefore, recommended to transpose
the columns of the rectangle once the plaintext is written and only then start
reading.
The method is very old. Back in the 5th century BCE, the Spartans had
already created this type of transposition (without columnar transposition).
They wrapped a parchment strip on which was written a message around a
rod, and the receiver would then unwrap the paper. The method was called
Skytale
.
The rod, or more exactly its thickness profile, served as a key [BauerMM, 6.3].
