Theory
With a key of ‘32415’, what is the AMSCO plaintext for the ciphertext version of: nwrertht bapng kiereguo? In this we have 21 characters so it will be laid out as:
X X X X X 2 1 2 1 2 1 2 1 2 1 2 1 2 1
First we lay out the key, and then populate the first column:
3 2 4 1 5 n wr e
And now the next column:
3 2 4 1 5 r n th wr t e
And next:
3 2 4 1 5 ba r n p th wr ng t e
And next:
3 2 4 1 5 ba r ki n p th e wr ng t re e
And finally:
3 2 4 1 5 ba r ki n gu p th e wr o ng t re e
And the result is “barking up the wrong tree”