Nuit du Hack 2014 Quals - Another One (crypto 300)
Sun 06 April 2014 — download

We were given a crypted.bmp file of 4.3MB, that can't be opened in a picture-viewer. Since the format is BMP (And thus involves no compression at all: It's just a big matrice), it smells like ECB mode.

I created a bmp picture, trying to get something of equal weight. Here is the script is used to replace headers (with head.bmp being my freshly-created bmp):

all: clean
    cp crypted.bmp out.bmp
    dd if=head.bmp of=out.bmp bs=1 count=54 conv=notrunc

clean:
    rm -f out.bmp

Then we played with the header, trying to guess the correct resolution of the original image. Since we were lazy, once we got something readable, we deciphered the flag.

Here is what we got:

raptor

Flag: AllMyLifeIThoughThatAirWasFreeUntilIBoughtABagOfChips