Original: 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 .
Simplified: The DES is using a block size of 64 bits ( 8 bytes ) while the AES is using a block size of 128 bits ( 16 bytes ) .