[Detect cipher block size] #blockcipher #crypto #oracle #detection

detect_block_size.py

def detect_block_size(cipher_box):
    last_size = len(cipher_box('A'))
    counter = 0
    while True:
        counter += 1
        new_size = len(cipher_box('A' * counter))
        if new_size > last_size:
            return new_size - last_size
        last_size = new_size
        
        
# cipher_box is a function that accepts plain-text and returns cipher-text