JuliaPoo

Xor Key

The challenge encrypts a very long plaintext with a short (30 byte) key by simply XOR-ing the key with the plaintext. The key thing to note here is that the plaintext isn't random, and we can use this to recover the key.

Challenge Files

import string
from secrets import choice, os, randbelow
from itertools import cycle

JuliaPoo

Let $m$ be the message (flag), $n$ be the public modulus and $c$ be the encrypted_flag. We are going to perform a Binary Search to compute the value of $t = \frac{n}{m}$ to sufficient precision to recover $m$.

First, let's define a function is_error(x:int)->bool. is_error(x) will ask the server to decrypt $c x^e \text{ mod } n$, and return True if the server encounters an invalid padding. Recall that in RSA, using $c x^e \text{ mod } n$ as the ciphertext will decrypt into the plaintext $m x \text{ mod } n$. Note that the padding check only checks if the first byte of the plaintext is \x00. I.e., if the plaintext is smaller than some fixed threshold, then it passes the padding check.

The hint to this challenge will be the answers to the following questions:

• What do you expect is_error(1) to return?
• What do you expect is_error(x) to return when x is a small integer close to 1?
• As you continue to increase the value of x, you'd expect is_error(x) to start returning True. Eventual

JuliaPoo

JuliaPoo

/**
 * This file implements a circular meta-evaluator for a subset of js used
 * by Source Academy: https://sourceacademy.org/sicpjs/
 * It is very simple evaluator that recursively evaluates the AST
 *
 * This is by no means efficient and serves as a means for me to learn
 * about interpreters.
 */

/*

JuliaPoo

# Obfuscated python script (ski combinators) that prints "Hello World!"
# Written by: JuliaPoo

S = lambda x: lambda y: lambda z: x(z)(y(z))
K = lambda x: lambda y: x
I = lambda x: x
A = lambda n: n+1
Z = lambda f: (lambda g: f(g(g)))(lambda g: f(lambda y: g(g)(y)))

to_str = Z(lambda f: lambda l: "" if S(S(I)(K(1)))(K(0))(S(I)(K(K))(l)) else chr(S(S(I)(K(A)))(K(0))(S(K(S(I)(K(K))))(S(I)(K(K(I))))(l)))+f(S(K(S(I)(K(K(I)))))(S(I)(K(K(I))))(l)))

JuliaPoo

; eng-shell-x86.asm
; nasm -f bin eng-shell-x86.asm -o b.exe

BITS 32

;
; MZ header
;
; The only two fields that matter are e_magic and e_lfanew

JuliaPoo

# Python 3.11.0a0 64-bit

import sys
import opcode
import types
import ctypes

# PyBytesObject.ob_sval
PyBytesObject_ob_sval_offset = 0x20
# _frame.f_localsplus