You’re told that the key is
time. This means that the message was written by rows into an array with four columns; since there are $24$ letters, this took six rows. The key
time was written at the top of the array, so that the
t was above the first column, the
i above the second column, and so on. Finally, the letters were read out one column at a time, taking the columns in alphabetical order of the letters in the keyword. Thus, the fourth column, under the
time, was read out first, and the first column, under the
t, was read out last.
If the message had been
This is my note, the initial setup would have looked like this:
The letters of
time in alphabetical order are
eimt, so I now read off columns $4$, $2$, $3$, and $1$ in that order to produce
SYE HSO IMT TIN.
To decipher the given message, reverse the procedure.
RLAPET is the first column read out, so it must be the
e (fourth) column of the original array:
HWBUIE was the second column read out, so it must be the
i (second) column of the original array:
Continuing in this fashion, we reconstruct the entire initial array:
The original message was filled in by rows, so it must have been
THEREWILLBEASURPRISETEST, and you’ll have no trouble supplying the correct word divisions.