hackyou_crypto300.py

from scipy import linalg
import numpy as np
from struct import pack,unpack
import sys

filename = 'flag.wmv' if len(sys.argv)==1 else sys.argv[1]

m_transform = np.frompyfunc(lambda x: int(round(x)),1,1)

header_byte_seq  = [0x30,0x26,0xB2,0x75,0x8E,0x66,0xCF,0x11,0xA6,0xD9,0x00,0xAA,0x00,0x62,0xCE,0x6C]
#turn header_byte_seq into a 4x4 matrix
hbs_matrix = np.array( header_byte_seq ).reshape(4,4)



#get ciphertext
ciphertext_file = open(filename+'.out','rb')
ciphertext = ciphertext_file.read()
ciphertext_file.close()

#ciphertext was packed as a series of shorts

#get length
length = unpack('!I',ciphertext[0:4])[0]
ciphertext = ciphertext[4:]

#convert into list integers
ciphertext = [ unpack('!H',ciphertext[i*2:i*2+2])[0] for i in range(0,length) ]

#first 16 bytes hold the header guid
hbs_ciphertext = ciphertext[0:16]



#RECOVER KEY USED FOR ENCRYPTION

# Let hbs_matrix = B
# Let key = K
# Let hbs_ciphertext = C
# BK = C
# (B^(-1)B)K = B^(-1)C
# K = B^(-1)C

B = hbs_matrix
C = np.array(hbs_ciphertext).reshape(4,4)
B_inverse = linalg.inv(B)
K = m_transform(B_inverse.dot(C))


#RECOVER ORIGINAL DATA GIVEN KEY AND CIPHERTEXT
# BK = C
# BK.K^(1) = C.K^(-1)
# B = C.K^(-1)

K_inverse = linalg.inv(K)

plaintext = []

for i in range(0,length/16):
	C = ciphertext[i*16:i*16+16]
	C = np.array(C).reshape(4,4)
	B = m_transform(C.dot(K_inverse))
	plaintext += [x for x in B.reshape(1,16)[0]]


#WRITE DECRYPTED DATA TO FILE

decrypted_file = open(filename,'wb')
data = ''.join( pack('!B',x) for x in plaintext )
decrypted_file.write(data)
decrypted_file.close()