# NCtfU - 5/31 (Crypto? Onebone) (Online)
###### tags: `新手場` `nctfu2021`

## 會參加
- Onebone
- idisused
## 講者
- Onebone

## 講者筆記
### RSA
$n = p * q$
$phi = (p-1)*(q-1)$
$ed \equiv 1 (mod\ phi)$
pt: plain text
ct: cipher text
$ct \equiv pt^e mod\ n$
$pt \equiv ct^d mod\ n$
$encrypt(m1) * encrypt(m2) = ((m1^e) * (m2^e)) mod\ n = (m1 * m2)^e mod\ n = encrypt(m1 * m2)$

### when e is small

import gmpy2
from Crypto.Util.number import long_to_bytes
for i in range(a_big_enough_integer):
    m, true_root = gmpy2.iroot(i*n + ct, e)
    if true_root:
        print(long_to_bytes(m))
        break

### when n is small
- 直接找 p q

### Wiener's attack
- The attack uses the continued fraction method to expose the private key **d** when **d** is small

import owiener
d = owiener.attack(e, n)

### python3 library
- pip3 install pycryptodome
- apt install python3-gmpy2
- pip3 install owiener
- binascii

### 也許有幫助的網站
- [RsaCtfTool](https://github.com/Ganapati/RsaCtfTool)
- [factordb](http://factordb.com/)
- [字頻轉換](https://www.quipqiup.com/)
- [rot online](http://theblob.org/rot.cgi)
- [base64 online](https://www.base64decode.org/)
- [Vigenère online](https://www.guballa.de/vigenere-solver)
- [encode/decode 大雜燴](https://www.asciitohex.com/)

### blind sql injection
- substr

substr(select something, start_index, end_index) = 'a';
substr((select col_name from db_name.tbl_name limit 1, 1)1, 1) < '0'

### lab
- picoGym
- Mind your Ps and Qs
- Mini RSA
- Dachshund Attacks
- No Padding, No Problem
- triple-secure
- rsa-pop-quiz

## 共筆
- https://cryptohack.org/
- https://www.sagemath.org/