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Berd's Red Envelope 2021 WriteUp
Berd's Red Envelope 2021 WriteUp

摸鱼的 2020 最后几小时

Web 手的 Misc/Crypto 修行

在结束之前一直保持密码保护 已经结束力(

开始

由于 <a> 设置了 pointer-events: none; 因此无法点击。复制链接进入第一题

一只方熊猫

下载后发现图片无法打开:

使用 010Editor 打开后提示 CRC 错误:

随便找了个爆破图片宽高的脚本:

# -*- coding: utf8 -*-

import os
import binascii
import struct
misc = open("panda.png","rb").read()

# 爆破宽
for i in range(1024):
    data = misc[12:16] + struct.pack('>i',i)+ misc[20:29]  #IHDR数据
    crc32 = binascii.crc32(data) & 0xffffffff
    if crc32 == 0x4A920BA4: #IHDR块的crc32值
        print("w")
        print(i)
        print("hex:"+hex(i))

# 爆破高		
for i in range(1024):
    data = misc[12:20] + struct.pack('>i',i)+ misc[24:29]
    crc32 = binascii.crc32(data) & 0xffffffff
    if crc32 == 0x4A920BA4:
        print("h")
        print(i)
        print("hex:"+hex(i)) 

拿脚本跑一下,得到图片的真正 height

修改原文件,得到 flag

根据 UUID 进入下一题

🤝

给出的链接是 http://arealexistingdomain/flag.html,第一想法就是在 /etc/hosts 里加个域名(

然后 curl 一下就出来了(

根据 UUID 进入下一题

粗心的小明

题中给出的代码如下:

const fs = require('fs');
const uuid = require('uuid');
const crypto = require('crypto');

let red_envelope_2021 = uuid.v4();

let key = crypto.scryptSync('xiaomingSecureKey2021', 'xiaomingSuperSalt', 32);
let cipher = crypto.createCipheriv('aes-256-cfb', key, crypto.randomBytes(16));

let json = JSON.stringify({
    red_envelope_2021
});

fs.writeFile('red_envelope_encrypted.hex', cipher.update(json, 'utf8', 'hex') + cipher.final('hex'), function (err) {
    if (err) {
        console.error(err);
    } else {
        console.info('Red envelope stored successfully!');
    }
});

给出的 hex 如下:

d365895fbcdbd3a29b1bf00307429fd07d53ba3c0553b8789867d4aee3b8c3bbb0e5a8fd582a9696aabbdc1e373f97efac2529d588320800449553f6

加密使用的是 aes-256-cfb,以 IV 作为初始文本,加密(encrypt_block)得到一个块(block_encrypted),再将 block_encrypted 和明文异或(⊕)得到密文(prev_ciphertext)。之后,再以 prev_ciphertext 为初始文本,加密得到下一个 block_encrypted,再与下一段明文异或……以此类推,如下图所示[1]:

而文本的前半段是确定的,为 {"red_envelope_2021":,这已经超过了一个块的大小(16),因此可以通过第一个块得到 IV。简单步骤如下:

  1. 将原文转换为 hex7b227265645f656e76656c6f70655f32
  2. 从密文中摘取第一块:d365895fbcdbd3a29b1bf00307429fd0
  3. 异或得到 AES 加密后的内容:a847fb3ad884b6cced7e9c6c7727c0e2
  4. Node 的运行结果中得到 32 位密钥:03da3479601ff722a6472329347a161bf8a712ead1d04b5560414be1dead3566
  5. 解密即可

解密使用了 boppreh/aesAES 实现[2],修改后的脚本如下:

#!/usr/bin/env python3

s_box = (
    0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76,
    0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0,
    0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15,
    0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75,
    0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84,
    0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF,
    0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8,
    0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2,
    0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73,
    0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB,
    0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79,
    0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08,
    0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A,
    0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E,
    0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
    0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16,
)

inv_s_box = (
    0x52, 0x09, 0x6A, 0xD5, 0x30, 0x36, 0xA5, 0x38, 0xBF, 0x40, 0xA3, 0x9E, 0x81, 0xF3, 0xD7, 0xFB,
    0x7C, 0xE3, 0x39, 0x82, 0x9B, 0x2F, 0xFF, 0x87, 0x34, 0x8E, 0x43, 0x44, 0xC4, 0xDE, 0xE9, 0xCB,
    0x54, 0x7B, 0x94, 0x32, 0xA6, 0xC2, 0x23, 0x3D, 0xEE, 0x4C, 0x95, 0x0B, 0x42, 0xFA, 0xC3, 0x4E,
    0x08, 0x2E, 0xA1, 0x66, 0x28, 0xD9, 0x24, 0xB2, 0x76, 0x5B, 0xA2, 0x49, 0x6D, 0x8B, 0xD1, 0x25,
    0x72, 0xF8, 0xF6, 0x64, 0x86, 0x68, 0x98, 0x16, 0xD4, 0xA4, 0x5C, 0xCC, 0x5D, 0x65, 0xB6, 0x92,
    0x6C, 0x70, 0x48, 0x50, 0xFD, 0xED, 0xB9, 0xDA, 0x5E, 0x15, 0x46, 0x57, 0xA7, 0x8D, 0x9D, 0x84,
    0x90, 0xD8, 0xAB, 0x00, 0x8C, 0xBC, 0xD3, 0x0A, 0xF7, 0xE4, 0x58, 0x05, 0xB8, 0xB3, 0x45, 0x06,
    0xD0, 0x2C, 0x1E, 0x8F, 0xCA, 0x3F, 0x0F, 0x02, 0xC1, 0xAF, 0xBD, 0x03, 0x01, 0x13, 0x8A, 0x6B,
    0x3A, 0x91, 0x11, 0x41, 0x4F, 0x67, 0xDC, 0xEA, 0x97, 0xF2, 0xCF, 0xCE, 0xF0, 0xB4, 0xE6, 0x73,
    0x96, 0xAC, 0x74, 0x22, 0xE7, 0xAD, 0x35, 0x85, 0xE2, 0xF9, 0x37, 0xE8, 0x1C, 0x75, 0xDF, 0x6E,
    0x47, 0xF1, 0x1A, 0x71, 0x1D, 0x29, 0xC5, 0x89, 0x6F, 0xB7, 0x62, 0x0E, 0xAA, 0x18, 0xBE, 0x1B,
    0xFC, 0x56, 0x3E, 0x4B, 0xC6, 0xD2, 0x79, 0x20, 0x9A, 0xDB, 0xC0, 0xFE, 0x78, 0xCD, 0x5A, 0xF4,
    0x1F, 0xDD, 0xA8, 0x33, 0x88, 0x07, 0xC7, 0x31, 0xB1, 0x12, 0x10, 0x59, 0x27, 0x80, 0xEC, 0x5F,
    0x60, 0x51, 0x7F, 0xA9, 0x19, 0xB5, 0x4A, 0x0D, 0x2D, 0xE5, 0x7A, 0x9F, 0x93, 0xC9, 0x9C, 0xEF,
    0xA0, 0xE0, 0x3B, 0x4D, 0xAE, 0x2A, 0xF5, 0xB0, 0xC8, 0xEB, 0xBB, 0x3C, 0x83, 0x53, 0x99, 0x61,
    0x17, 0x2B, 0x04, 0x7E, 0xBA, 0x77, 0xD6, 0x26, 0xE1, 0x69, 0x14, 0x63, 0x55, 0x21, 0x0C, 0x7D,
)


def sub_bytes(s):
    for i in range(4):
        for j in range(4):
            s[i][j] = s_box[s[i][j]]


def inv_sub_bytes(s):
    for i in range(4):
        for j in range(4):
            s[i][j] = inv_s_box[s[i][j]]


def shift_rows(s):
    s[0][1], s[1][1], s[2][1], s[3][1] = s[1][1], s[2][1], s[3][1], s[0][1]
    s[0][2], s[1][2], s[2][2], s[3][2] = s[2][2], s[3][2], s[0][2], s[1][2]
    s[0][3], s[1][3], s[2][3], s[3][3] = s[3][3], s[0][3], s[1][3], s[2][3]


def inv_shift_rows(s):
    s[0][1], s[1][1], s[2][1], s[3][1] = s[3][1], s[0][1], s[1][1], s[2][1]
    s[0][2], s[1][2], s[2][2], s[3][2] = s[2][2], s[3][2], s[0][2], s[1][2]
    s[0][3], s[1][3], s[2][3], s[3][3] = s[1][3], s[2][3], s[3][3], s[0][3]

def add_round_key(s, k):
    for i in range(4):
        for j in range(4):
            s[i][j] ^= k[i][j]


# learned from http://cs.ucsb.edu/~koc/cs178/projects/JT/aes.c
xtime = lambda a: (((a << 1) ^ 0x1B) & 0xFF) if (a & 0x80) else (a << 1)


def mix_single_column(a):
    # see Sec 4.1.2 in The Design of Rijndael
    t = a[0] ^ a[1] ^ a[2] ^ a[3]
    u = a[0]
    a[0] ^= t ^ xtime(a[0] ^ a[1])
    a[1] ^= t ^ xtime(a[1] ^ a[2])
    a[2] ^= t ^ xtime(a[2] ^ a[3])
    a[3] ^= t ^ xtime(a[3] ^ u)


def mix_columns(s):
    for i in range(4):
        mix_single_column(s[i])


def inv_mix_columns(s):
    # see Sec 4.1.3 in The Design of Rijndael
    for i in range(4):
        u = xtime(xtime(s[i][0] ^ s[i][2]))
        v = xtime(xtime(s[i][1] ^ s[i][3]))
        s[i][0] ^= u
        s[i][1] ^= v
        s[i][2] ^= u
        s[i][3] ^= v

    mix_columns(s)


r_con = (
    0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40,
    0x80, 0x1B, 0x36, 0x6C, 0xD8, 0xAB, 0x4D, 0x9A,
    0x2F, 0x5E, 0xBC, 0x63, 0xC6, 0x97, 0x35, 0x6A,
    0xD4, 0xB3, 0x7D, 0xFA, 0xEF, 0xC5, 0x91, 0x39,
)


def bytes2matrix(text):
    """ Converts a 16-byte array into a 4x4 matrix.  """
    return [list(text[i:i+4]) for i in range(0, len(text), 4)]

def matrix2bytes(matrix):
    """ Converts a 4x4 matrix into a 16-byte array.  """
    return bytes(sum(matrix, []))

def xor_bytes(a, b):
    """ Returns a new byte array with the elements xor'ed. """
    return bytes(i^j for i, j in zip(a, b))

def inc_bytes(a):
    """ Returns a new byte array with the value increment by 1 """
    out = list(a)
    for i in reversed(range(len(out))):
        if out[i] == 0xFF:
            out[i] = 0
        else:
            out[i] += 1
            break
    return bytes(out)

def pad(plaintext):
    """
    Pads the given plaintext with PKCS#7 padding to a multiple of 16 bytes.
    Note that if the plaintext size is a multiple of 16,
    a whole block will be added.
    """
    padding_len = 16 - (len(plaintext) % 16)
    padding = bytes([padding_len] * padding_len)
    return plaintext + padding

def unpad(plaintext):
    """
    Removes a PKCS#7 padding, returning the unpadded text and ensuring the
    padding was correct.
    """
    padding_len = plaintext[-1]
    assert padding_len > 0
    message, padding = plaintext[:-padding_len], plaintext[-padding_len:]
    assert all(p == padding_len for p in padding)
    return message

def split_blocks(message, block_size=16, require_padding=True):
        assert len(message) % block_size == 0 or not require_padding
        return [message[i:i+16] for i in range(0, len(message), block_size)]


class AES:
    rounds_by_key_size = {16: 10, 24: 12, 32: 14}
    def __init__(self, master_key):
        """
        Initializes the object with a given key.
        """
        assert len(master_key) in AES.rounds_by_key_size
        self.n_rounds = AES.rounds_by_key_size[len(master_key)]
        self._key_matrices = self._expand_key(master_key)

    def _expand_key(self, master_key):
        """
        Expands and returns a list of key matrices for the given master_key.
        """
        # Initialize round keys with raw key material.
        key_columns = bytes2matrix(master_key)
        iteration_size = len(master_key) // 4

        # Each iteration has exactly as many columns as the key material.
        columns_per_iteration = len(key_columns)
        i = 1
        while len(key_columns) < (self.n_rounds + 1) * 4:
            # Copy previous word.
            word = list(key_columns[-1])

            # Perform schedule_core once every "row".
            if len(key_columns) % iteration_size == 0:
                # Circular shift.
                word.append(word.pop(0))
                # Map to S-BOX.
                word = [s_box[b] for b in word]
                # XOR with first byte of R-CON, since the others bytes of R-CON are 0.
                word[0] ^= r_con[i]
                i += 1
            elif len(master_key) == 32 and len(key_columns) % iteration_size == 4:
                # Run word through S-box in the fourth iteration when using a
                # 256-bit key.
                word = [s_box[b] for b in word]

            # XOR with equivalent word from previous iteration.
            word = xor_bytes(word, key_columns[-iteration_size])
            key_columns.append(word)

        # Group key words in 4x4 byte matrices.
        return [key_columns[4*i : 4*(i+1)] for i in range(len(key_columns) // 4)]

    def encrypt_block(self, plaintext):
        """
        Encrypts a single block of 16 byte long plaintext.
        """
        assert len(plaintext) == 16

        plain_state = bytes2matrix(plaintext)

        add_round_key(plain_state, self._key_matrices[0])

        for i in range(1, self.n_rounds):
            sub_bytes(plain_state)
            shift_rows(plain_state)
            mix_columns(plain_state)
            add_round_key(plain_state, self._key_matrices[i])

        sub_bytes(plain_state)
        shift_rows(plain_state)
        add_round_key(plain_state, self._key_matrices[-1])

        return matrix2bytes(plain_state)

    def decrypt_block(self, ciphertext):
        """
        Decrypts a single block of 16 byte long ciphertext.
        """
        assert len(ciphertext) == 16

        cipher_state = bytes2matrix(ciphertext)

        add_round_key(cipher_state, self._key_matrices[-1])
        inv_shift_rows(cipher_state)
        inv_sub_bytes(cipher_state)

        for i in range(self.n_rounds - 1, 0, -1):
            add_round_key(cipher_state, self._key_matrices[i])
            inv_mix_columns(cipher_state)
            inv_shift_rows(cipher_state)
            inv_sub_bytes(cipher_state)

        add_round_key(cipher_state, self._key_matrices[0])

        return matrix2bytes(cipher_state)

if __name__ == '__main__':
    from base64 import b64encode
    from binascii import unhexlify
    ret=AES(unhexlify('03da3479601ff722a6472329347a161bf8a712ead1d04b5560414be1dead3566')).decrypt_block(unhexlify('a847fb3ad884b6cced7e9c6c7727c0e2'))
    print(b64encode(ret))

得到 IVbase64Jle9TPezMnv770Y5b+Wc1g==,最后使用 CyberChef 解密:

得到下一题的地址

太 🆒 啦

找了好久,结果 Esolang Wiki 还真有全名就叫这个的语言[3](

题面如下:

🆕6️⃣🍿🍔🌭🥪🌮🍆🥑💬🍔💬🌭💬🥪💬🌮💬🍆💬🥑🛑🆕🔟🤔🖇🙃🍖🍟🥔🌽🥩🥕🌶️🌯🥥❓🆓⚖️🆓✖️🆓➗🙃🔢1️⃣8️⃣4️⃣7️⃣🔢🛑4️⃣🛑🔢2️⃣4️⃣8️⃣🔢🛑🆓🍿🌶️🥥🌽🥕🌽🥔🍿🌽🍖🌽🥩🌯🍟🛑🆓💬🔤🚫📥️🔛🔤🛑🛑💬🔤💻🔑🔑🔑🔑🔑🔑🔤🤔🎛️🔤9️⃣❤️9️⃣4️⃣🔤🔤1️⃣9️⃣7️⃣4️⃣🔤🔤4️⃣💜💛7️⃣🔤🔤➖🔤🔤🧡7️⃣9️⃣9️⃣🔤🔤8️⃣❤️4️⃣💛🔤🔤💚6️⃣0️⃣💛🔤🔤2️⃣5️⃣💙1️⃣🔤🔤2️⃣3️⃣7️⃣2️⃣🔤

提示

到一半给出了提示:

❤️ A 🧡 B 💛 C 💚 D 💙 E 💜 F

于是下面分析的内容就将这些红心全都替换成对应的文本了。

🍿

square-cool 仓库[4] 的描述,🆕 是定义函数,后面跟随一个数字 n,表示参数的数量,然后是函数名 emoji,最后的 nemoji 表示参数名,函数以 🛑 结尾。于是开头的一一部分可以翻译成下面的形式:

# 🍿: Print six arguments
🆕6️🍿

🍔🌭🥪🌮🍆🥑

💬🍔
💬🌭
💬🥪
💬🌮
💬🍆
💬🥑

🛑

其中 💬 是内置的函数,表示输出。这个函数的功能也就是连续输出对应的 6 个参数。

🤔

同样是一个函数,翻译如下:

# 🤔: 
🆕🔟🤔
🖇  # modifier
🙃 # input
🍖 # p1
🍟 # p2
🥔 # p3
🌽 # p4
🥩 # p5
🥕 # p6
🌶️ # p7
🌯 # p8
🥥 # p9
❓ # Returns the second argument if the first is true, otherwise returns the third argument.
# input / 1847 * 4 == 2️4️8️
🆓
  ⚖️
  🆓
    ✖️
    🆓
      ➗🙃🔢1️8️4️7️🔢
    🛑
    4️
  🛑
  🔢2️4️8️🔢
🛑

🆓
  # 🍿(p7, p9, -, p6, -, p3)
  🍿🌶️🥥🌽🥕🌽🥔
  # 🍿(-, p1, -, p5, p8, p2)
  🍿🌽🍖🌽🥩🌯🍟
🛑

🆓
  # print('🚫📥️🔛')
  💬🔤🚫📥️🔛🔤
🛑

🛑

其中 🆓 的功能类似作用域开始,并以 🛑 结束。

最后

# main
# print('💻🔑🔑🔑🔑🔑🔑')
💬🔤💻🔑🔑🔑🔑🔑🔑🔤

# 🤔(stdin, '9️A9️4️', '1️9️7️4️', '4️FC7️', '-', 'B7️9️9️', '8️A4️C', 'D6️0️C', '2️5️E1️', '2️3️7️2️')
# d60c2372-8a4c-4fc7-9a94-b79925e11974
🤔🎛️
🔤9️A9️4️🔤
🔤1️9️7️4️🔤
🔤4️FC7️🔤
🔤➖🔤
🔤B7️9️9️🔤
🔤8️A4️C🔤
🔤D6️0️C🔤
🔤2️5️E1️🔤
🔤2️3️7️2️🔤

最后就是调用了,其中 🎛️ 表示从 stdin 读入一个整数,但在实际分析中并没有起到作用。

最后得到 UUIDd60c2372-8a4c-4fc7-9a94-b79925e11974,进入终章

Happy 2021

游戏结束,新年快乐!あけましておめでとうございます!(笑)

参考

  1. https://ctf-wiki.github.io/ctf-wiki/crypto/blockcipher/mode/cfb-zh/
  2. https://github.com/boppreh/aes
  3. https://esolangs.org/wiki/%F0%9F%86%92
  4. https://gitlab.com/fogity/squared-cool

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