Tags: aes-cbc lsb-oracle mersenne crypto rsa
Rating:
## Tokyo Western CTF 2018 (Qualification Round) Hints for some Crypto challenges
- Time: _September 1-2, 2018_
- Place: _Online_
- Website: _<https://score.ctf.westerns.tokyo/>_
### scs7
(_112 pt, 134 solves_)
Base59 (each symbol in a ciphertext represents a number in the range 0-58).
Challenge file: None (black box)
### Revolutional Secure Angou
(_154 pt, 82 solves_)
`e*q = 1 (mod p)` implies that `e*q = k*p + 1` (1). Since `q < p`, we know that `k < e = 65537`. So, just brute-force `k` until we find a solution to the equation `e*n = k * p^2 + p` (2). Notice that (2) is (1) with both side multiplied by `p`.
Challenge file: [revolutional-secure-angou.7z](revolutional-secure-angou.7z).
### mixed cipher
(_233 pt, 39 solves_)
You need to:
- Look for some multiples of the modulus `n`, then calculate the GCD to get `n`.
- Learn how to decrypt arbitrary ciphertexts, given an RSA LSB(s) oracle, to get the AES key.
- Learn how to untwist the Mersenne Twister (the `random` module's core PRNG) to predict the IV used to encrypt the flag.
Challenge file: [mixed_cipher.rar](mixed_cipher.rar).
