#
**mayday-mayday**

An RSA challenge with leakage of MSB from the CRT exponents of the private key. The parameters allow an attack described in a paper by May (hinted by the title), Nowakowski and Sarkar leveraging the Coppersmith method to recover the key.

## Challenge Description

The challenge setting is quite simple. We have a `Crypto`

class defining some parameters:

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self.bits = bits
self.alpha = 1/9
self.delta = 1/4
self.known = int(self.bits*self.delta)

The class then generates an RSA instance using CRT exponents:

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while True:
p, q = [getPrime(self.bits//2) for _ in '__']
self.e = getPrime(int(self.bits*self.alpha))
print(f'{self.e.bit_length() = }')
φ = (p-1)*(q-1)
try:
dp = pow(self.e, -1, p-1)
dq = pow(self.e, -1, q-1)
self.n = p*q
break
except:
pass

The challenge is run with `bits = 2048`

(which means `p`

and `q`

are both `1024`

bits). We are given the public key together with a leak of the MSB of the CRT exponents:

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dp = f'0x{(dp >> (rsa.bits//2 - rsa.known)):x}'
dq = f'0x{(dq >> (rsa.bits//2 - rsa.known)):x}'

## Solution

The solution is explained in great detail here. This attack works in general, but is even more efficient for \(e \sim N^{1/12}\) (in our case is `self.bits*self.alpha`

so roughly \(N^{1/9}\); this unusual choice is already extremely suspicious). First, we write

and the same for \(d_q\). We know \(d_p^M\) from the leak, and in our case \(i = 512\). Section 3.1 gives us a way to compute \(k\) and \(l\), where \(ed_p = k(p - 1) + 1\). First, we need to compute \(A\):

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i = 512
dpM = dp
dqM = dq
A = (pow(2, 2*i) * pow(e, 2) * dpM * dqM)//N + 1

Then, notice that in our case (with the paper notation)

\[\delta = \frac{1}{4} < \frac{1}{2} - \frac{2}{9} \sim 0.27\]so we are just inside the bounds. Then we can recover \(k\) and \(l\) as roots of the appropriate polynomial:

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x = PolynomialRing(RationalField(), 'x').gen()
C = (1 - A*(N-1)) % e
f = x**2 - C*x + A
roots = f.roots()
if roots == []:
f = x**2 - (C + e)*x + A
roots = f.roots()
k = roots[0][0]
l = roots[1][0]
assert k*l == A

Finally, given \(k\), we can go to Section 3.3. Here a bit of trial and error is required, since we do not know if \(k\) is actually associated to \(d_p\) or \(d_q\), but this is not a big effort. Notice that we do our computation modulo \(kN\) (the paper mention \(kp\), which may be somewhat confusing). Here applying Coppersmith method for small roots we can find a factor for \(N\).

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x = PolynomialRing(Zmod(k*N), 'x', implementation='NTL').gen()
# Assume k is the coefficient of dp
a_small = (e * dpM * (2**i) + k - 1) * inverse_mod(e, k*N)
a_small = int(a_small)
f = x + a_small
my_dpL = f.small_roots(X=2**i-1, beta=0.5)
# Is actually with dq
if my_dpL == []:
a_small = (e * dqM * (2**i) + k - 1) * inverse_mod(e, k*N)
a_small = int(a_small)
f = x + a_small
my_dqL = f.small_roots(X=2**i-1, beta=0.5)[0]
p = gcd(f.subs(x=my_dqL), N)
print(f'{p = }')
else:
my_dpL = my_dpL[0]
print(my_dpL == dpL)
print(f'{p = }')

Finally, with the factorization we can decrypt the flag: `HTB{f4ct0r1ng_w1th_just_4_f3w_b1ts_0f_th3_CRT_3xp0n3nts!https://eprint.iacr.org/2022/271.pdf}`

. With no surprise, it points again at the paper we’ve been using through all the challenge!

Solve Script

The full solution script can be found here