Three Ravens Writeup

Crypto CTF 2020 - Crypto 90 - 52 solves

There were three ravens sat on a tree, Downe a downe, hay downe, a downe, They were as black as they might be.

Encryption Logic

Multiprime rsa using 512 bit primes p, q, r, p + q + r, using public exponent e = 0x10001 and public modulus N = p * q * r * (p + q + r). N and p + q + r was given

Exploit

Use public modulus as p + q + r. I can derive private key d since p + q + r is prime: d = inverse_mod(e, p + q + r - 1). Luckily this works because message size is lower than size of p + q + r.

I get flag:

CCTF{tH3_thr3E_r4V3n5_ThRe3_cR0w5}

Exploit code: solve.sage with config.py