# Simple - Crypto 100 problem
In this challenge, we have the ability to log into a website with a usernameand password. These are stored in a json blob, along with the dictionaryelement `"db":"hitcon-ctf"`. This json blob is then encrypted with AES in CFBmode and saved as a cookie. Our goal is to get the json blob to also encode`"admin":true`.
To do this, we feed in a message of length 81, just over 5 blocks. This has afinal 17 bytes of plaintext as `db":hitcon-ctf"}`, but we want it to be`":1,"admin":true}`. We simply use xor to convert the last full block of text,leaving just the last byte corrupted due to propogation in CFB. To solve thiswe simply guess random characters for the last byte of ciphertext until weget our flag: `hitcon{WoW_CFB_m0dE_5o_eAsY}`