It's a secret (5)

(Continuation from Saturday)

A simple example of a transposition cipher is what is known as the rail fence cipher - or to be more accurate, the two bar rail fence cipher.  The plain text is written out on two lines - the first letter goes on the top line, the second letter on the bottom line, the third letter on the top line, the fourth letter on the bottom line, and so on.  So our message (put the kettle on back in ten minutes) would look like this -

P    T     H    K     T     L     O     B     C     I     T    N      I     U    E

     U     T     E     E     T     E     N     A      K    N    E     M    N    T    S

And then we write out the top line followed by the bottom line -

PTHKTLOBCITNIUEUTEETENAKNEMNTS

and tradition has it (this goes back to the early days of the telegraph when you paid by the word) that we then divide the cipher text up into five letter groups -

PTHKT LOBCI TNIUE UTEET ENAKN EMNTS

and there you are.  You might think that both the substitution cipher in the last post, and this one, would be pretty impenetrable, but in fact both are relatively easily solved. How?  Watch this space.