ROR Writeup

zer0pts CTF 2020 - crypto 260

LOL

Observations

Plain RSA cryptosystem given. I was given only set of ciphertexts, which were generated by RORing flag. 2 is factor of public modulus N, so applying modular operation does not change the parity of rotated plaintext. Every bit of plaintext can be recovered by just observing least significant bits of ciphertexts.

Get flag

Concat every LSBs of ciphertext and get flag:

zer0pts{0h_1t_l34ks_th3_l34st_s1gn1f1c4nt_b1t}

Exploit code: solve.py

Original writeup (https://github.com/pcw109550/write-up/tree/master/2020/zer0pts/ROR).