EN | ZH

非反馈 feedback shift register¶

Introduction¶

In order to make the sequence of key stream output as complex as possible, a nonlinear feedback shift register is used. There are three common types.

• Nonlinear combination generator that uses a nonlinear combination function for the output of multiple LFSRs
• Nonlinear filter generator that uses a nonlinear combination function for the contents of an LFSR
• Clock generator that uses the output of one (or more) LFSRs to control the clock of another (or multiple) LFSRs

Nonlinear Combination Generator¶

Introduction¶

The combination generator is generally shown below.

Geffe¶

Here we take Geffe as an example. Geffe contains 3 linear feedback shift registers, and the nonlinear combination function is

$F(x_1,x_2,x_3)=(x_1 \and x_2) \oplus (\urcorner x_1 \and x_3)=(x_1 \and x_2) \oplus ( x_1 \and x_3)\oplus x_3$

2018 Strong Net Cup streamgame3¶

Simply look at the topic

from flag import flag

assert flag.startswith("flag{")

assert flag.endswith("}")

assert len(flag)==24

output = (R << 1) & 0xffffff

lastbit=0

while i!=0:

lastbit^=(i&1)

i=i>>1

output^=lastbit

return (output,lastbit)

return (R1_NEW,R2_NEW,R3_NEW,(x1*x2)^((x2^1)*x3))

R1=int(flag[5:11],16)

R2=int(flag[11:17],16)

R3=int(flag[17:23],16)

assert len (bin (R1) [2:]) == 17
assert len (bin (R2) [2:]) == 19
assert len(bin(R3)[2:])==21

for fi in range(1024):

print fi
tmp1mb = &quot;&quot;
for i in range(1024):

tmp1kb = &quot;&quot;
for j in range(1024):

tmp=0

for k in range(8):

tmp = (tmp << 1) ^ out

tmp1kb+=chr(tmp)

tmp1mb + = tmp1kb
f = open("./output/" + str(fi), "ab")

f.write(tmp1mb)

f.close()


It can be seen that the program is very similar to the Geffe generator. Here we use the related attack method to attack. We can count the output of the last class Geffe generator when the three LFSR outputs are different, as follows.

| $x_1$ | $x_2$ | $x_3$ | $F(x_1,x_2,x_3)$ |

| ----- | ----- | ----- | ---------------- |

| 0 | 0 | 0 | 0 |

| 0 | 0 | 1 | 1 |

| 0 | 1 | 0 | 0 |

| 0 | 1 | 1 | 0 |

| 1 | 0 | 0 | 0 |

| 1 | 0 | 1 | 1 |

| 1 | 1 | 0 | 1 |

| 1 | 1 | 1 | 1 |

it can be discovered

• Geffe's output has the same probability as $x_1$ 0.75
• Geffe's output has the same probability of $x_2$ as 0.5
• Geffe's output has the same probability of $x_3$ as 0.75

This shows that the output is very relevant to the first and third. Therefore, we can violently enumerate the output of the first and third LFSRs to determine the number equal to the output of Geffe-like, and if it is about 75%, it can be considered correct. The second is a direct violent enumeration.

The script is as follows

#for x1 in range(2):

#    for x2 in range(2):

#        for x3 in range(2):

#            print x1,x2,x3,(x1*x2)^((x2^1)*x3)

#n = [17,19,21]

#cycle = 1

#for i in n:

#    cycle = cycle*(pow(2,i)-1)

#print cycle

output = (R << 1) & 0xffffff

i = (R & mask) & 0xffffff

lastbit = 0

while i != 0:

lastbit ^= (i & 1)

i = i >> 1

output ^= lastbit

return (output, lastbit)

return (R1_NEW, R2_NEW, R3_NEW, (x1 * x2) ^ ((x2 ^ 1) * x3))

n3 = 21
n2 = 19
n1 = 17

ansn = range(beg, end)

data = ''.join(bin(256 + ord(c))[3:] for c in data)

now = 0

true = 0
for i in ansn:

r = i

cnt = 0

for j in range(num * 8):

lastbit = str(lastbit)

cnt += (lastbit == data[j])

if cnt > now:

now = cnt

res = i

print now, res

return res

def bruteforce2(x, z):

data = ''.join(bin(256 + ord(c))[3:] for c in data)

for y in range(pow(2, n2 - 1), pow(2, n2)):

R1, R2, R3 = x, y, z

flag = True

for i in range(len(data)):

(R1, R2, R3,

if str(out) != data[i]:

flag = False

break

if y % 10000 == 0:

print 'now: ', x, y, z

if flag:

print 'ans: ', hex(x)[2:], hex(y)[2:], hex(z)[2:]

break

R1 = guess(pow(2, n1 - 1), pow(2, n1), 40, R1_mask)

print R1

R3 = guess(pow(2, n3 - 1), pow(2, n3), 40, R3_mask)

print R3

R1 = 113099
R3 = 1487603

bruteforce2(R1, R3)


The results are as follows

➜  2018-CISCN-start-streamgame3 git:(master) ✗ python exp.py

161 65536

172 65538

189 65545

203 65661

210 109191

242 113099

113099

157 1048576

165 1048578

183 1048580

184 1049136

186 1049436

187 1049964

189 1050869

190 1051389

192 1051836

194 1053573

195 1055799

203 1060961

205 1195773

212 1226461

213 1317459

219 1481465

239 1487603

1487603

now:  113099 270000 1487603

now:  113099 280000 1487603

now:  113099 290000 1487603

now:  113099 300000 1487603

now:  113099 310000 1487603

now:  113099 320000 1487603

now:  113099 330000 1487603

now:  113099 340000 1487603

now:  113099 350000 1487603

now:  113099 360000 1487603

years old: 1b9cb 5979c 16b2f3


Thus flag is flag{01b9cb05979c16b2f3}.

topic¶

• 2017 WHCTF Bornpig

• 2018 Google CTF 2018 Betterzip