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: Rijndael , supports block sizes of 128 , 192 , and 256 bits , although the extra block sizes were not adopted by the AES standard .