Both the input ( plaintext ) and output ( ciphertext ) are the same length ; the output can not be shorter than the input -- this is logically required by the Pigeonhole principle and the fact that the cipher must be invertible -- and it is simply undesirable for the output to be longer than the input . Both the input ( plaintext ) and output ( ciphertext ) are the same length ; the output can not be shorter than the input -- this follows logically from the Pigeonhole principle and the fact that the cipher must be reversible -- and it is undesirable for the output to be longer than the input .