$($c$)[5$ marks$]$ Let $$ represent some $$xed letter of the alphabet. Della encrypts the plaintext

PAWNSHOP with the message key AB$.$ The ciphertext includes some repeated letters in the

form $($that is$,$ the $1$st letter is the same as the $8$th letter, and the $4$th letter is

as same as the $5$th letter$).$ Determine with justi$$cation all possible values of $$ or justify

why none exist.

Avoid an $\backslash$exhaustive attack approach: do not encrypt the full plaintext $26$ times.