"Since DES only encrypts in blocks of 64 bits (8 bytes) then encrypting and keeping the length of blocks means that you actually get a very good idea of the length of the password - that is, anything with only one block is a password length between 1 and 8 characters, with two blocks between 9 and 16 characters etc. In addition a password of "1234567812345678" would encrypt into two identical blocks."
"Since DES only encrypts in blocks of 64 bits (8 bytes) then encrypting and keeping the length of blocks means that you actually get a very good idea of the length of the password - that is, anything with only one block is a password length between 1 and 8 characters, with two blocks between 9 and 16 characters etc. In addition a password of "1234567812345678" would encrypt into two identical blocks."
http://www.explainxkcd.com/wiki/index.php?title=1286