The second major family of encryption techniques is called transposition ciphers. I find transposition ciphers to be
rather dull; in their pure form, they’re very simple, and not very difficult
to crack, even without computers. But some of the most sophisticated
modern ciphers can be looked at as a sort of strange combination of
substitution and transposition, so it’s worth looking at.
A transposition cipher doesn’t change the characters in the plain-text when it generates the cipher-text – it just re-arranges them. It applies some kind of permutation function to the text to produce a re-arrangement, which can be reversed if you know the secret to the the permutation.