Tags: crypto 

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Good crypto challenge for beginners (like us), unfortunately we solved it after the end of the event. We're given a piece of Python code and the result of the encryption of the flag:
from Crypto.Util.number import *
from key import FLAG

size = 2048
rand_state = getRandomInteger(size//2)

def keygen(size):
q = getPrime(size)
k = 2
while True:
p = q * k + 1
if isPrime(p):
k += 1
g = 2
while True:
if pow(g, q, p) == 1:
g += 1
A = getRandomInteger(size) % q
B = getRandomInteger(size) % q
x = getRandomInteger(size) % q
h = pow(g, x, p)
return (g, h, A, B, p, q), (x, )

def rand(A, B, M):
global rand_state
rand_state, ret = (A * rand_state + B) % M, rand_state
return ret

def encrypt(pubkey, m):
g, h, A, B, p, q = pubkey
assert 0 < m <= p
r = rand(A, B, q)
c1 = pow(g, r, p)
c2 = (m * pow(h, r, p)) % p
return (c1, c2)

# pubkey, privkey = keygen(size)

m = bytes_to_long(FLAG)
c1, c2 = encrypt(pubkey, m)
c1_, c2_ = encrypt(pubkey, m)

print pubkey
print (c1, c2)
print (c1_, c2_)

Analyzing the code we have:
* $q$, $p=qk+1$ random primes
* $g$ such that $g^q \equiv 1 \pmod{p}$
* $A,B,x \in \\{0,...,q-1\\}$ random values
* $h=g^x \pmod{p}$
* $r$ random
* $r'=Ar+B$
* $m$ that is the flag
* $C_1=g^r \pmod{p}$, $C_2=mh^r \pmod{p}$, $C_1'=g^{r'} \pmod{p}$, $C_2'=mh^{r'}\pmod{p}$ that are the encryption results

We're given $g,h,A,B,p,q$ as a public key, so basically we have to find $x$ or $r$. The problem with this encryption is that $r$ and $r'$ are correlated and are used to encrypt the same message $m$, so let's try to work on $r$. Because we are working modulo a prime $p$ we know that the modular inverse of $h^r$ exists, so $m = (h^r)^{-1}C_2$. Working on $C_2'$ we find $$C_2'=mh^{r'}=mh^{Ar}h^B$$
$$\Rightarrow C_2'(h^B)^{-1}=mh^{Ar}$$
$$\Rightarrow C_2'(h^B)^{-1}C_2^{-1}=h^{(A-1)r}$$
Now recalling Fermat's little theorem: if $p$ is a prime and $a \in \mathbb{Z}_p$ we have $a^{p-1}\equiv 1 \pmod{p}$.
Noticing that $gcd(A-1,p-1)=1$ we know that there exists the multiplicative inverse of $A-1$ modulo $p-1$ $(A-1)^{-1}$, so $(h^{(A-1)r})^{(A-1)^{-1}}=(h^r)^{(A-1)(A-1)^{-1}}=h^r \pmod{p}$

$$\Rightarrow m = ((C_2'(h^B)^{-1}C_2^{-1})^{(A-1)^{-1}})^{-1}C_2$$