Encryption by transposition involves taking the message you wish to send, and switching around the letters. For example, I want to send the message
The lecturer has got his flies undone
to all my friends in the lecture. If the lecturer should notice us passing the message around, and should confiscate it, I want to be sure he can't read what I have written, so that I will not get in trouble. Using one method of transposition, I remove the spaces and take every third letter and write them in order. So it starts with the letters marked here in bold: thelecturerhasgothisfliesundone and then loops round to the 'h' of 'the'.
text: thelecturerhasgothisfliesundone code: ECRHGHFENNHEURSTSIUOTLTEAOILSDE
Knowing my method, to read the message back, all my friend need do is split the letters into three, read up each column from left to right, and sort the spacing out again:
E C R H G H F E N N H E U R S T S I U O T L T E A O I L S D E the lec tur erh asg oth isf lie sun don e the lecturer has got his flies undone
which is basically reversing the method of encryption I used.
The lecturer, on the other hand, seeing the message, and not knowing the encryption method, has difficulty in re-ordering the letters to make sense of the message. Even if he knows that the code is just an anagram (as it is) of the original phrase, the 31 letters in the original message have XXX possible combinations, and if he sits there all week trying one possibility every ten seconds for 24 hours a day he will have less than a 1 in XXX chance of having worked it out.