I came across this question:
You are given a permutation cipher defined by the bijection t: N -> N where,
t(i) = i + 10, for i < 10, t(i) = i - 10, for 10 <= i <= 20, and t(i) = i, for i > 20.
Assume that the first index is 0, given a message m = 'HELLO WORLD AND THIS IS A SECRET MESSAGE', what is the ciphertext c?
Where the answer was the following:
D AND THISHELLO WORL IS A SECRET MESSAGE
but i don't get how. That's now what we get if the message is spelled out diagonally down and up over a number of rows and then read off row-by-row.