Implement a meet-in-the-middle attack for this double
encryption scheme. Use hash-tables (Python; dictionaries) to store
the table(s) for look-up. As problem a) showed us, using just a
single (pt,ct) pair will not be sufficient, so as in the previous
problems, implement the program so that it can process a list of
plaintext/ciphertext pairs (the list can be of arbitrary length).
The program should then run the meet-in-the-middle attack just on
the first pair, and then check all the resulting key candidates on
the remaining plaintext/ciphertext pairs. Below is a test-run of my
implementation.
- Write In Python Implement A Meet In The Middle Attack For This Double Encryption Scheme Use Hash Tables Python Dictio 1 (396.64 KiB) Viewed 60 times
from time import time
#Sboxes (the first 2 Sboxes from DES)
Sdata = [[[ 14, 4 , 13, 1 , 2 , 15 , 11 , 8 , 3 , 10 , 6 , 12 , 5 ,
9 , 0 , 7],
[0 , 15 , 7 , 4 , 14 , 2 , 13 , 1 , 10 , 6 , 12 , 11 , 9 , 5 , 3 ,
8],
[4 , 1 , 14 , 8 , 13 , 6 , 2 , 11 , 15 , 12 , 9 , 7 , 3 , 10 , 5 ,
0],
[15 , 12 , 8 , 2 , 4 , 9 , 1 , 7 , 5 , 11 , 3 , 14 , 10 , 0 , 6 ,
13]] ,
[[15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10],
[3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5],
[0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15],
[13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9]] ]
#Expansion function spec:
#1st bit of output is 8th bit of input,
#2nd bit is first, etc.
Elst = [8, 1, 2, 3, 4, 3, 4, 5, 6, 7, 8, 1]
#calculate S-box
def S(k,x):
'S-box Sk on input x'
x = x & 0x3F #(keep 6 bits)
row = ((x>>5)<<1) + (x&1)
col = (x>>1)& 0xF
#print(row, col)
return Sdata[k-1][row][col]
#calculate round key (12 bits) for ith round of simplified
DES
def roundkey(k,i):
'key for i-th round from key k'
#rotate key by i positions, and then take 12
bits
return (((k<<16)+k)>>i) & 0xFFF
#expand 8bits to 12 bits
def E(x):
'expand from 8 to 12 bits'
n = len(Elst) # should be 12
return sum(
[((x>>(8-Elst))&1)<<(n-i) for i in
range(n)])
#calculate f for quarter DES on input x, with round key rk
def f(x,rk):
'function f, with input x and roundkey rk'
f1 = E(x)
f2 = f1^rk
inp = [0,0] #inputs to S-boxes
oup = [0,0] #output of S-boxes
out = 0
for i in range(1,-1,-1):
inp = f2 & 0x3F #get last 6
bits
out = out << 4
oup = S(i,inp)
out = out + oup
f2 = f2>>6
return out
#calculate one round of quarter DES on input x, with round key
rk
def round(x,rk):
'one round on input x with roundkey rk'
Lx = x >> 8 #left quarter of x
Rx = x & 0xFF #right quarter of x
return (Rx<<8) + (Lx ^ f(Rx,rk))
#quarter DES encryption: input 16-bit plaintext x, and 16-bit
key k
def qtE(x,k):
'encrypt x with key k using quarterDES'
x = x & 0xFFFF #restrict to 16 bit blocks
k = k & 0xFFFF #and 16 bit keys
for i in range(6):
x = round(x,roundkey(k,i))
return ((x & 0xFF)<<8) + (x>>8)
#quarter DES decryption: input 16-bit plaintext x, and 16-bit
key k
def qtD(x,k):
'decrypt x with key k using quarterDES'
x = x & 0xFFFF #restrict to 16 bit blocks
k = k & 0xFFFF #and 16 bit keys
for i in range(6):
x = round(x,roundkey(k,5-i))
return ((x & 0xFF)<<8) + (x>>8)
= = = >>> kl = randrange (2**16) >>> k2 = randrange (2**16) ( >>> pt1 = 32121 >>> pt2 = 52783 >>> ctl = qtE (qte (pt1, kl), k2) >>> ct2 = qte (qtE (pt2, kl), k2) >>> mithm ( [(pti,ctl), (pt2,ct2)]) (k2, k1) = (5343, {5556, 4279}) , (k2, kl) = (57503, {7556, 61389}) { (5343, 5556), (57503, 7556) } >>> pt3 = 17172 >>> ct3 = qte (qtE (pt3, kl), k2) >>> mithm ([ (pti, ctl), (pt2,ct2), (pt3,ct3)]) (k2, k1) = (57503, {7556, 61389}) { (57503, 7556) } >>> [(pti,ctl), (pt2, ct2), (pt3,ct3)] [ (32121, 61424), (52783, 987), (17172, 23571)] >>> (kl, k2) (7556, 57503) = =