6
[NPC2CTF 2025]cccc
ccccapp.exe最先我還運行不了，需要先安裝.NET 6.0.0 (x64) ，
運行之后是這樣，但是exe在ida里面沒有看到這樣的明文信息，再查看ccccapp.dll
很多亂碼
開了很多種保護，用de4dot去一部分
看一下對比，還是很有用的
密文和加密
RC4特征，多異或了100
還差key，說是要動調，但是老是出問題，就嘗試用CE看，結果還真有，屬于運氣問題吧
def KSA(key):
S = list(range(256))
j = 0
for i in range(256):
j = (j + S[i] + key[i % len(key)]) % 256
S[i], S[j] = S[j], S[i]
return S
def PRGA(S):
i, j = 0, 0
while True:
i = (i + 1) % 256
j = (j + S[i]) % 256
S[i], S[j] = S[j], S[i]
K = S[(S[i] + S[j]) % 256]
yield K
def RC4Decrypt(key, text):
S = KSA(key)
keystream = PRGA(S)
res = []
for char in text:
res.append(((char ^ next(keystream)) & 0xff) ^ 100)
return bytes(res)
#示例
key = b'doyouknowcsharp'
plaintext = [0xf8,0xb1,0x1e,0xe8,0x7f,0x99,0xde,0x4e,0x84,0xb4,0x15,0x21,0xb6,0x8e,0xd1,0x2a,0x8b,0x9a,0x4d,0xe7,0x8f,0xda,0x23,0xc0,0xa9,0x62,0x63,0xe7,0x2,0x41,0x90,0x88,0x77,0x75,0xba,0x32,0xc6,0xb0,0x84,0x24,0xa6,0xa8,0x45,0xf6,0xcd,0x9c,0x8a,0x32]
Rc4encrypt = RC4Decrypt(key, plaintext)
print(Rc4encrypt.decode())
//flag{y0u_r34lly_kn0w_m@ny_pr0gr@mm1ng_l@ngu@g3$}
[justctf2023] Rustberry
arm架構的匯編題
通過字符串找到主要邏輯，看到有一張表
字符集引用
看匯編，進行賦值，賦值大小均小于表的長度
這是一個索引-字符映射驗證
r6 = 7
r1= 28
r0 = 0xff
[r7 + 0x94] = 7 -----> v7[37] = 7
r6 = 3
[r7 + 0xa4] = 28 ------> v7[41] = 28
r1 = 17
[r7 + 0xa8] = 0xff ----> v7[42] = 0xff
r0 = 0x1a
r3 = 0x21
[r7 + 0x60] = 3 -----> v7[24] = 3
r6 = 0x64
[r7 + 0x98] = 0x21 ----> v7[38] = 0x21
r3 = 0x13
r8 = 0x14
[r7 + 0x64] = 0x1a , 17 , 0x14 --->v7[25] = 0x1a ,v7[26] = 17, v7[27] = 0x14
r2= 0xb
lr = 0x20
[r7 + 0x80] = 17 ------>v7[32] = 17
r4 =1
[r7 + 0x84] = 17 ------->v7[33] = 17
就是像這樣賦值的，每個值占四位，r7對應的數組能夠還原，映射還原flag
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
int main()
{
int v7[44]={0};
v7[37] = 7;
v7[41] = 28;
v7[42] = 255;
v7[24] = 3;
v7[38] = 33;
v7[25] = 26;
v7[26] = 17;
v7[27] = 20;
v7[32] = 17;
v7[33] = 17;
v7[29] = 19;
v7[30] = 1;
v7[31] = 32;
v7[19] = 8;
v7[40] = 11;
v7[39] = 11;
v7[17] = 11;
v7[23] = 11;
v7[8] = 21;
v7[9] = 51;
v7[34] = 24;
v7[16] = 15;
v7[10] = 26;
v7[11] = 9;
v7[12] = 20;
v7[13] = 18;
v7[3] = 5;
v7[28] = 34;
v7[4] = 27;
v7[5] = 13;
v7[6] = 29;
v7[35] = 26;
v7[21] = 26;
v7[15] = 26;
v7[7] = 26;
v7[36] = 2;
v7[18] = 0;
v7[20] = 13;
v7[22] = 29;
v7[14] = 19;
v7[0] = 9;
v7[1] = 2;
v7[2] = 19;
char table[]="abcdefghijklmnopqrstuvwxyz_{}0123456789ABCDEFGHIJKLMNOPQRSTUVWXY";
for(int i=0;i<44;i++){
printf("%c",table[v7[i]]);
}
return 0;
}
雖然題目名稱寫了rust，但是反匯編看不到這里的加密過程，反而匯編看起來更容易
[NewStar2024]Lock
只有一個exe和pyd，輸入20個0會有1個正確的，且不是第一位，check.pyd里面有一個check.check()函數是主要的檢驗函數
直接爆破，用ret的值來判斷是否正確，但是第一位不好弄，所以我是直接將后面的爆出來再來嘗試第一位
import check
charset = "0123456789abcdef"
password = ["d"] * 20 # 初始化,可以用0，這樣的第一位是0，但是只有19個對，替換出來第一位是d，所以這里就直接寫為d
correct_count = 0
for i in range(20):
for c in charset:
attempt = password.copy()
attempt[i] = c
ret = check.check("".join(attempt))
if ret > correct_count: # 這一位猜對了
password[i] = c
correct_count = ret
# print(f"[+] Found position {i}: {c} ({correct_count}/20)")
break
print(f"flag{{{''.join(password)}}}")
cython
將py文件編譯為c，會生成庫文件pyd供使用
pyx文件就是對應的源碼
逆向中需要恢復符號表和還原結構體
[psxd] cookie
有花指令，去花之后就是tea加密
#include <stdio.h>
#include <stdint.h>
void decrypt(uint32_t* v, uint32_t* key){
uint32_t delta = 0x897354D2;
uint32_t v0 = v[0], v1 = v[1], sum = delta * 32;
for (int i = 0; i < 32; i++) {
v1 -= sum ^ (v0 + sum) ^ ((v0 << 4) + key[2]) ^ ((v0 >> 5) + key[3]);
v0 -= sum ^ (v1 + sum) ^ ((v1 << 4) + key[0]) ^ ((v1 >> 5) + key[1]);
sum -= delta;
}
v[0] = v0;
v[1] = v1;
}
int main()
{
uint32_t key[] = { 2, 0, 2, 2 };
uint32_t EncFlag[] = {
0x569A1C45, 0xEF2C6A10,
0xFB440BD6, 0x5797F41D,
0x523FF2C3, 0x48337CD9,
0x3616AC2D, 0x06B6312D
};
for (int i = 0; i < 8; i += 2) {
decrypt(&EncFlag[i], key);
}
// 輸出結果（32字節，解釋為字符）
char* flag = (char*)EncFlag;
for (int i = 0; i < 32; i++) {
putchar(flag[i]);
}
putchar('\n');
return 0;
}
[psxd] EasyRE
開頭有反調試，將3條跳轉全部nop
魔改rc4
這里是額外的異或處理
ai直接出
# encrypted_data = [
# 0x93, 0xf9, 0x8d, 0x92, 0x52, 0x57, 0xd9, 0x05,
# 0xc6, 0x0a, 0x50, 0xc7, 0xdb, 0x4f, 0xcb, 0xd8,
# 0x5d, 0xa6, 0xb9, 0x40, 0x95, 0x70, 0xe7, 0x9a,
# 0x37, 0x72, 0x4d, 0xef, 0x57
# ]
#
# # 復制數據避免修改原數組
# decrypted = bytearray(encrypted_data)
#
# # 逆向鏈式異或（從后往前）
# for i in range(len(decrypted)-1, 0, -1):
# decrypted[i] ^= decrypted[i-1]
#
# # 最后統一異或 0x42
# for i in range(len(decrypted)):
# decrypted[i] ^= 0x42
# print(decrypted[i])
# # print(hex(decrypted[i])[2:],end=' ')
# #
# # print(hex(ord('f')))
def rol(val, r_bits):
return ((val << r_bits) | (val >> (8 - r_bits))) & 0xFF
def ror(val, r_bits):
return ((val >> r_bits) | (val << (8 - r_bits))) & 0xFF
def decrypt(data: bytearray):
n29 = len(data)
decrypted = bytearray(n29) # 存儲解密結果
# Step 1: reverse the post-processing (XOR with 0x42 and previous byte)
for i in range(n29 - 1, -1, -1):
if i > 0:
data[i] ^= data[i - 1]
data[i] ^= 0x42
# Step 2: RC4 variant reverse (simulate keystream and reverse encrypt)
S = list(range(256))
v8 = 0
for i in range(256):
v8 = (v8 + S[i] - 7 * (i // 7) + i + 4919) % 256
S[i], S[v8] = S[v8], S[i]
v14 = 0
v15 = 0
for i in range(n29):
v14 = (v14 + 1) % 256
if v14 % 3 == 0:
v20 = S[(3 * v14) % 256] + v15
else:
v20 = S[v14] + v15
v15 = v20 % 256
S[v14], S[v15] = S[v15], S[v14]
tmp = (v14 * v15) % 16
key_stream = S[(S[v14] + S[v15]) % 256] # 修正索引計算
cipher_byte = data[i]
plain_byte = ror(cipher_byte, 3) - tmp
plain_byte = (plain_byte ^ key_stream) & 0xFF
decrypted[i] = plain_byte
return decrypted
# 加密數據
encrypted_hex = "93 f9 8d 92 52 57 d9 05 c6 0a 50 c7 db 4f cb d8 5d a6 b9 40 95 70 e7 9a 37 72 4d ef 57"
encrypted_bytes = bytearray.fromhex(encrypted_hex)
# 解密
decrypted_data = decrypt(encrypted_bytes)
# 輸出結果
print("解密結果（hex）：", decrypted_data.hex())
print("解密結果（ASCII）：", decrypted_data.decode('latin1', errors='replace'))
[psxd] MFC
CFF看到有明文信息
調試的時候可能有反調試，直接附加就可以直接調試
字符串看到有base64編碼表，交叉引用發現rc6加密之后進行base64編碼
異或的是iv，WcE4Bbm4kHYQsAcX
看到12位的明文，猜測是key，用4填充至16位
那么密文就是RKCTaz+fty1J2qsz4DI6t9bmMiLBxqFrpI70fU4IMemczIlM+Z1IoVQobIt1MbXF（base64）
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