A transposition isn’t really a cipher in its own right, but can
form a vital part of more complex systems (similar to the polybius
chequerboard and the monoalphabetic substitution).

Transposition is simply moving the relative positions of letters
within a message. I’ll describe a columnar transposition
below, so called because the text is arranged into columns and the
columns are transposed.

When performing a columnar transposition we first need a keyword.
I’ll use the keyword murkydotorg. The first step is to remove duplicate letters from the keyword, this yields murkydotg.

The message is then
written into rows beneath the keyword. The example message which
I’ll use will be you can read murky dot org via the convenient RSS feeds

MURKYDOTG
476381572
---------
youcanrea
dmurkydot
orgviathe
convenien
trssfeeds

You will notice that I’ve added some numbers beneath the keyword.
The numbers refer to the relative positions of the keyword letters
in the alphabet.

Having formed the table we can read back the message in the order
of the keyword letters.

This message becomes NYANE ATENS CRVVS YDOCT RDTIE UUGNS OMROT AKIEF. I have included the spaces for clarity, although in practice these won’t be highlighted!

One should also note that there are many ways of systematically
anagramming the letters of the message. I am sure that you can
come up with some methods yourself!

One well known method is ‘railfence’, where the letters are written in a zigzag, and read off in rows. The height of the zigzag, as well as starting direction and starting row can be adjusted.

     n     u     t     a     n     n     e
y   a r   m r   o o   i t   o v   e t   f e
 o c   e d   k d   r v   h c   e i   r s   d
  u     a     y     g     e     n     s     s

This becomes: nutanneyarmrooitovetfeocedkdrvhceirsduaygenss

Transpositions are often used as part of a more complex system.
If a transposition is used in conjunction with a monoalphabetic
substitution then we may solve the transposition as above, after
having first worked out the plaintext letters of the substitution
by looking at letter frequencies. Quite often though, a transposition is solved by much trial and error!

When solving transpositions, one might have fingers crossed that letters like ‘q’ appear, as this is (usually) followed by ‘u’ - some exceptions exist, like qwerty and qed.

Similarly, sometimes ‘chunks’ of text are transposed, so a large message may be transposed in blocks of 25 letters, and solving one block will solve them all.

It should also be seen that, as with many cryptographic systems
the more ciphertext we have, the easier the cipher is to solve.

Transposition add some security, however as with all
cryptography, one should be aware that sometimes the solution
is easier than we might lead ourselves to believe. Personally, I don’t like trying to solve transpositions - but this is a matter of personal taste.

Author: Murk