Characters positions to encrypt and an inverse

characters' positions to encrypt and an inverse function to decrypt.
The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which
it is encoded. In the rail fence cipher, the plaintext is written downwards on successive "rails" of
an imaginary fence, then moving up when we get to the bottom. The message is then read off in
rows. For example, using three "rails" and a message of 'WE ARE DISCOVERED. FLEE AT
ONCE', the cipherer writes out:
W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N .
Then reads off:
WECRL TEERD SOEEF EAOCA IVDEN
c) i) Define threat and attack.