RSA-CBC v2

Round: 27 (2022/10)
Category: Crypto
Points: 125
Solves: 8

Description

I fixed the problem of the previous attempt in mixing RSA and block ciphers, again!

Solution

$e=9$, and for each encryption we know the iv in

$$ (\text{iv}+x)^e \stackrel{?}{\equiv} \text{ct} \pmod{n} $$

As we know $x < 2^{8 \times 16}$ so $x$ is small enough to be recovered by applying coppersmith's method. (Because $16 \times 8 \times 9 < 2048$)