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Discrete logarigm of in the multiplicative groups of a square prime.




Points: 300

Category: Crypto

Validations: 5


The problem was simple, we had to solve the discrete logarithm of $2^{flag} \mod n$. We could quickly figure out that $n$ is a square of a prime number.

sage: n = 0xfffffffe00000002fffffffe0000000100000001fffffffe00000001fffffffe00000001fffffffefffffffffffffffffffffffe000000000000000000000001
sage: h = 4275333096397007849067777941008675457617279615546105194323216811550797075772016607215703868747693795339025886078058526791464546941731743196384839138407576
sage: n.is_prime()
sage: n.factor()

It means that the order of the multiplicative group is $\varphi(n) = p(p-1)$. The first idea was to compute the Pohlig-Hellman algorithm to recover the flag:

sage: p = 115792089210356248762697446949407573530086143415290314195533631308867097853951
sage: factor(p-1)
2 * 3 * 5^2 * 17 * 257 * 641 * 1531 * 65537 * 490463 * 6700417 * 835945042244614951780389953367877943453916927241
sage: pari(f"znlog(Mod({h}, {n}), Mod({2}, {n}), [{ord}, {factors_str}])")

However, $p$ and the largest factor of $p-1$ were too big, the computation was not finishing. Then we figure out that there is an isomorhism between the cyclic group:

\[H = \{ x \in (\mathbb{Z}/p^2\mathbb{Z})^*, x^{p-1} = 1 \mod p\}\]

to the additive group $\mathbb{Z}/p\mathbb{Z}$ exactly as in the Okamoto–Uchiyama cryptosystem. This isomorphism is $L(x) = \frac{x-1}{p}$ and gives directly the discrete logarithm in the subgroup of order $p$. However, this was not enougth to recover the flag.

Thus, we run the Pohlig-Hellman algorithm on the remaining factors except $p$ and the second bigger one. The we reconstruct the flag modulus $n$ divided by the missing prime number and it was sufficient to recover the flag completely:

$ sage solve.sage

Here it the full solution.

Written on July 25, 2022