Another day, another neural CTF to solve! This challenge is pretty straight-forward: a keras model is given as only file of the challenge. The first I thought was:

Ehi, it’s just another gradient descent! Let’s try it!… and I failed hard. The network doesn’t compute anything, it just takes in input a greyscale 68 x 218 image and it outputs a number between 0 and 1. Nothing special. If you try to apply some gradient descent on the network, the result will be a useless image.

What to do now? Having a look to a dump of the file containing the model was somehow enlighting:

00003330  b4 8a 32 b8 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..2...28..28..28|
00003340  b4 8a 32 38 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..28..2...2...2.|
00003350  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
00003690  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 38 b4 8a 32 38  |..2...2...28..28|
000036a0  b4 8a 32 38 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..28..2...2...2.|
000036b0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
000037e0  b4 8a 32 38 b4 8a 32 38  b4 8a 32 38 b4 8a 32 b8  |..28..28..28..2.|
000037f0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
000038e0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 38  |..2...2...2...28|
000038f0  b4 8a 32 38 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..28..28..28..28|
*
00003910  b4 8a 32 38 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..28..2...2...2.|
00003920  b4 8a 32 b8 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..2...28..28..28|
00003930  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
00003940  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 38  |..2...2...2...28|
00003950  b4 8a 32 38 b4 8a 32 38  b4 8a 32 b8 b4 8a 32 b8  |..28..28..2...2.|
00003960  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
00003970  b4 8a 32 b8 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..2...28..28..28|
00003980  b4 8a 32 38 b4 8a 32 38  b4 8a 32 38 b4 8a 32 b8  |..28..28..28..2.|
00003990  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
000039b0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 38 b4 8a 32 38  |..2...2...28..28|
000039c0  b4 8a 32 38 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..28..2...2...2.|
000039d0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
000039f0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 38  |..2...2...2...28|
00003a00  b4 8a 32 38 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..28..2...2...2.|
00003a10  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
00003a30  b4 8a 32 b8 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..2...28..28..28|
00003a40  b4 8a 32 38 b4 8a 32 38  b4 8a 32 b8 b4 8a 32 b8  |..28..28..2...2.|
00003a50  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
00003aa0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 38  |..2...2...2...28|
00003ab0  b4 8a 32 38 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..28..28..28..28|
00003ac0  b4 8a 32 38 b4 8a 32 38  b4 8a 32 b8 b4 8a 32 b8  |..28..28..2...2.|
00003ad0  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
00003b40  b4 8a 32 38 b4 8a 32 38  b4 8a 32 38 b4 8a 32 38  |..28..28..28..28|
00003b50  b4 8a 32 38 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..28..2...2...2.|
00003b60  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|
*
00003b80  b4 8a 32 38 b4 8a 32 38  b4 8a 32 b8 b4 8a 32 b8  |..28..28..2...2.|
00003b90  b4 8a 32 b8 b4 8a 32 b8  b4 8a 32 b8 b4 8a 32 b8  |..2...2...2...2.|

… they are kinda all the same, except some values. This made me realize that maybe those weights are fake.

import numpy as np
import skimage
import matplotlib.pyplot as plt
import keras
from keras.models import load_model
from keras import backend as K

SHAPE = (68, 218)
adv_image = np.array([np.ones(SHAPE)])
epsilon = 1

model = load_model('model')
print(model.summary())
weights = model.layers[1].get_weights()[0]
weights = weights.reshape(SHAPE)
print(weights, model.predict(np.array([weights])))
skimage.io.imshow(weights)
plt.show()

Some skimage magic for showing the weights and here it is the flag! just HERE You can find all the files of tis challenge HERE.