REVERSE-PRACTICE-BUUCTF-32

• • [第四章 CTF之APK章]數字殼的傳說
  • [第五章 CTF之RE章]Hello, RE
  • [第五章 CTF之RE章]BabyAlgorithm
  • [第五章 CTF之RE章]BabyConst
  • [第五章 CTF之RE章]BabyLib
  • [第五章 CTF之RE章]easy_rust
  • [第五章 CTF之RE章]easy_go
  • [第五章 CTF之RE章]easy_mfc

[第四章 CTF之APK章]數字殼的傳說

查殼，360加固

用frida腳本脫殼后，jadx-gui分析得到的dex文件
輸入作為明文，輸入的前6個字符作為密鑰，字符串"123456"作為iv
對輸入進行AES.CBC加密，得到的密文經base64編碼后與已知的base64字符串比較

分析AESUtils這個類
第19行把密鑰擴展成長度為32的字符串，不足的補空格" "
第37行把iv擴展成長度為16的字符串，不足的補空格" "

密鑰為輸入的前6個字符，猜測是"N1BOOK"或者"n1book"
python解AES.CBC，當密鑰為"n1book"時，解出flag

from Crypto.Cipher import AES import base64 cipher="u0uYYmh4yRpPIT/zSP7EL/MOCliVoVLt3gHcrXDymLc=" cipher_str=base64.b64decode(cipher) key_str="n1book" key_str=key_str.ljust(32," ") iv_str="123456" iv_str=iv_str.ljust(16," ") aes=AES.new(key_str,AES.MODE_CBC,iv_str) print(aes.decrypt(cipher_str)) #n1book{h3ckF0rfun}

[第五章 CTF之RE章]Hello, RE

exe程序，ida打開后，在main函數中找到flag

[第五章 CTF之RE章]BabyAlgorithm

elf文件，ida打開
main函數中，讀取輸入，驗證輸入長度是否為45，然后設置密鑰，對輸入進行RC4加密，得到的密文與已知的v8比較

已知密鑰為"Nu1Lctf233"，密文為v8，解RC4即可得到flag

#include<stdio.h> void rc4_init(unsigned char* s, unsigned char* key, unsigned long Len_k) {int i = 0, j = 0;char k[256] = { 0 };unsigned char tmp = 0;for (i = 0; i < 256; i++) {s[i] = i;k[i] = key[i % Len_k];}for (i = 0; i < 256; i++) {j = (j + s[i] + k[i]) % 256;tmp = s[i];s[i] = s[j];s[j] = tmp;} }/* RC4加解密函數 unsigned char* Data 加解密的數據 unsigned long Len_D 加解密數據的長度 unsigned char* key 密鑰 unsigned long Len_k 密鑰長度 */ void rc4_crypt(unsigned char* Data, unsigned long Len_D, unsigned char* key, unsigned long Len_k) //加解密 {unsigned char s[256];rc4_init(s, key, Len_k);int i = 0, j = 0, t = 0;unsigned long k = 0;unsigned char tmp;for (k = 0; k < Len_D; k++) {i = (i + 1) % 256;j = (j + s[i]) % 256;tmp = s[i];s[i] = s[j];s[j] = tmp;t = (s[i] + s[j]) % 256;Data[k] = Data[k] ^ s[t];} } void main() {//密鑰unsigned char key[] = "Nu1Lctf233";unsigned long key_len = sizeof(key) - 1;//密鑰//unsigned char key[] = {};//unsigned long key_len = sizeof(key);//密文unsigned char data[] = { 0xC6, 0x21, 0xCA, 0xBF, 0x51, 0x43, 0x37, 0x31, 0x75, 0xE4,0x8E, 0xC0, 0x54, 0x6F, 0x8F, 0xEE, 0xF8, 0x5A, 0xA2, 0xC1,0xEB, 0xA5, 0x34, 0x6D, 0x71, 0x55, 0x08, 0x07, 0xB2, 0xA8,0x2F, 0xF4, 0x51, 0x8E, 0x0C, 0xCC, 0x33, 0x53, 0x31, 0x00,0x40, 0xD6, 0xCA, 0xEC, 0xD4 };//解密rc4_crypt(data, sizeof(data), key, key_len);for (int i = 0; i < sizeof(data); i++){printf("%c", data[i]);}printf("\n");return; } //n1book{us1nG_f3atur3s_7o_de7erm1n3_4lg0ri7hm}

[第五章 CTF之RE章]BabyConst

elf文件，ida打開
main函數中，輸入的長度為48，每四個一組，共12組，經過變換后與已知比較
可以看到每組的四個字符經變換后變成了一組包含16字節的數據，即16*8==128bit的數據

往下看sub_40085B函數，a1[2]~a1[5]分別設置成0x67452301，0xEFCDAB89，0x98BADCFE，0x10325476
于是可以猜到是md5算法，這四個值是標準的幻數
參考：MD5算法原理及其實現

最后在md5這個網站解md5，每組解完拼接起來就是flag

arr=[0x39, 0x6D, 0xF0, 0xD2, 0x42, 0x41, 0xF8, 0x02, 0x16, 0x7A,0xDC, 0xC0, 0xD6, 0x10, 0x6E, 0xD3, 0xFE, 0x68, 0x6F, 0x27,0xE9, 0x00, 0x81, 0x6B, 0xB2, 0xB8, 0xBC, 0x63, 0xD7, 0x0D,0x40, 0x08, 0xD2, 0xCD, 0xB7, 0x10, 0x60, 0x4D, 0x69, 0x64,0xA8, 0x36, 0x25, 0x54, 0xF7, 0xDE, 0xF0, 0x96, 0xDF, 0xC4,0xF8, 0x47, 0xAC, 0xDE, 0x92, 0xAC, 0x23, 0xC4, 0x69, 0x4B,0xEF, 0x32, 0x50, 0x28, 0x46, 0x82, 0xA2, 0x4D, 0x59, 0xA9,0x8E, 0x7A, 0x66, 0xD8, 0xE0, 0xD3, 0x1F, 0xD9, 0xBD, 0x68,0x46, 0x04, 0xF6, 0x12, 0xD8, 0xA7, 0x1D, 0xEB, 0x0D, 0xFA,0x03, 0x5E, 0x3F, 0x06, 0xA5, 0x9B, 0x99, 0xAA, 0x58, 0x66,0x28, 0xD9, 0x70, 0x72, 0x73, 0x28, 0x6B, 0x4C, 0x87, 0x32,0xDC, 0xFF, 0xA6, 0xC9, 0xFE, 0x07, 0x74, 0x20, 0xE1, 0xDE,0xFD, 0xD7, 0xC8, 0x7B, 0x59, 0xCD, 0x1D, 0x73, 0xCB, 0x7F,0x25, 0x22, 0xCC, 0x3D, 0xF2, 0x0F, 0xF2, 0x98, 0x7E, 0x98,0x42, 0x7F, 0x05, 0xD2, 0xBE, 0x57, 0x03, 0xDD, 0x22, 0xDD,0xC3, 0x18, 0xEA, 0x07, 0x24, 0x80, 0x44, 0xD7, 0xEC, 0xCC,0xFF, 0xF5, 0x6B, 0xC4, 0x60, 0x70, 0xDA, 0xE9, 0xF9, 0x46,0x6C, 0x11, 0x76, 0x5D, 0x0C, 0x21, 0x0E, 0x49, 0x91, 0xA0,0x2A, 0x04, 0xE4, 0x21, 0x84, 0x9F, 0x2A, 0xB8, 0x9B, 0x7D,0x9E, 0xEA] for i in range(0,len(arr),16):s=""for j in range(16):s+=hex(arr[i+j]).zfill(4).replace("0x","")print(s) # 396df0d24241f802167adcc0d6106ed3 -->n1bo # fe686f27e900816bb2b8bc63d70d4008 -->ok{U # d2cdb710604d6964a8362554f7def096 -->5in9 # dfc4f847acde92ac23c4694bef325028 -->__c0 # 4682a24d59a98e7a66d8e0d31fd9bd68 -->n5ts # 4604f612d8a71deb0dfa035e3f06a59b -->_7o_ # 99aa586628d9707273286b4c8732dcff -->1d3n # a6c9fe077420e1defdd7c87b59cd1d73 -->t1fy # cb7f2522cc3df20ff2987e98427f05d2 -->_mD5 # be5703dd22ddc318ea07248044d7eccc -->__41 # fff56bc46070dae9f9466c11765d0c21 -->gor1 # 0e4991a02a04e421849f2ab89b7d9eea -->thm}#n1book{U5in9__c0n5ts_7o_1d3nt1fy_mD5__41gor1thm}

[第五章 CTF之RE章]BabyLib

elf文件，ida打開
交叉引用字符串"Input flag:"來到主邏輯函數
可以看到存在非常大的整數，而且65537很令人在意
RSA的題目做多了就會有感覺，65537通常是作為逆向題目中RSA的指數e，其他的大整數就分別是p，q和c

解RSA即可得到flag

import gmpy2 from Crypto.Util.number import long_to_bytes p=9842210544704105105386460208892325341518023212707647450607909247791133264519200579173177912467785842558060968492186761162106356089009194470097546296829163 q=12972360952153818155692381381571252126631475184728971905301445264084096070607651598626783223094292740492828654265391639843199875189333033337169565006624907 n=p*q e=65537 c=110694010334901653238216140152683772418101197298114114481381418739511015861349388028360214495059500357527716613334520805339266807313669925649167175211788624655809951516502907329949137499677877779584898365309802983718066683849944838484002656376845311375573423677826690834875095904482448693671735053088583663382 phin=(p-1)*(q-1) d=gmpy2.invert(e,phin) m=gmpy2.powmod(c,d,n) print(long_to_bytes(m)) # n1book{1d3nt1fy_GMp_l1br4ry}

[第五章 CTF之RE章]easy_rust

rust語言，調試
變表base64，在這個地方找到變表

然后main函數中的第46行的cmp的第二個參數指向的是密文字符串

密文字符串，實際上在上面找到變表的那個位置，在"Input your flag:\n"與"Failed to read lineTrue!\nFalse!\n"之間就是密文字符串

解變表base64，第一個等號"=“隔斷了flag的兩個部分，最后的”=MlW"沒有用

拼接起來，中間加個"_"，最后加上"}"，于是flag為n1book{9o0d_j0b}

[第五章 CTF之RE章]easy_go

go語言，調試
輸入的包絡為"n1book{}"，輸入的總長度為21
比較驗證在第79行的if語句
邏輯為：byte_561538[i] + byte_561518[i] * input[i] == byte_561528[i]

每個參與運算的整型都是8bit，寫腳本時要注意截斷

arr_18=[0xD3, 0x75, 0x9B, 0xF9, 0xA3, 0x87, 0xED, 0x93, 0x8D, 0xDD,0x77, 0xED, 0x67, 0x00, 0x00, 0x00] arr_28=[0xB7, 0x9C, 0x79, 0x43, 0x9B, 0xAF, 0x94, 0xE4, 0x94, 0x71,0xEC, 0xEA, 0x8E, 0x00, 0x00, 0x00] arr_38=[0xDB, 0x9E, 0xB7, 0x9A, 0x91, 0xCA, 0xA1, 0x6B, 0x97, 0xC1,0x74, 0xB3, 0x90, 0x00, 0x00, 0x00] flag="" for i in range(13):for j in range(32,128):if arr_28[i]==(arr_38[i]+(arr_18[i]*j)&0xff)&0xff:flag+=chr(j) print("n1book{"+flag+"}") #n1book{4Ff1n3_C1pH3R}

[第五章 CTF之RE章]easy_mfc

mfc的exe，用xspy找到一個函數
“OnCommand: notifycode=0000 id=0001,func= 0x00C61880(easy_mfc.exe+ 0x001880 )”

ida打開，去找偏移為"0x1880"的函數，讀取輸入，然后驗證

進入sub_401910函數，可以看到，輸入與v7循環異或，結果與v5比較

寫逆異或腳本即可得到flag

v7=[0xBC, 0x9D, 0x8C, 0x92,0x40, 0x47, 0x86, 0x21,0xF5, 0xAC, 0x8F, 0xFD,0x68, 0xE4, 0xE9, 0x3A, 0xC0, 0x66, 0xB3, 0x64,0x7E, 0x79, 0xD3, 0x22,0x31, 0xF8] v5=[0xD2, 0xAC, 0xEE, 0xFD,0x2F, 0x2C, 0xFD, 0x79,0xC5, 0xDE, 0xD0, 0x85,0x58, 0xB6, 0xB6, 0x0B,0xF5, 0x39, 0xC0, 0x54,0x21, 0x1F, 0xE2, 0x54,0x02, 0x85] flag="" for i in range(len(v7)):flag+=chr(v7[i]^v5[i]) print(flag) #n1book{X0r_x0R_15_s0_f1v3} 創作挑戰賽新人創作獎勵來咯，堅持創作打卡瓜分現金大獎

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