CISCN2024 Qual writeup

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简单题是很水，没做出来的题也很让人崩溃。

asm_re

读ARM64汇编，不用F5，纯靠基本功。（初赛rev签到题该有的样子

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#include <stdint.h>
#include <stdio.h>

uint8_t secret[] = {0xD7,0x1F,0,0,0xB7,0x21,0,0,0x47,0x1E,0,0,0x27,0x20,0,0,0xE7,0x26,0,0,0xD7,0x10,0,0,0x27,0x11,0,0,7,0x20,0,0,0xC7,0x11,0,0,0x47,0x1E,0,0,0x17,0x10,0,0,0x17,0x10,0,0,0xF7,0x11,0,0,7,0x20,0,0,0x37,0x10,0,0,7,0x11,0,0,0x17,0x1F,0,0,0xD7,0x10,0,0,0x17,0x10,0,0,0x17,0x10,0,0,0x67,0x1F,0,0,0x17,0x10,0,0,0xC7,0x11,0,0,0xC7,0x11,0,0,0x17,0x10,0,0,0xD7,0x1F,0,0,0x17,0x1F,0,0,7,0x11,0,0,0x47,0xF,0,0,0x27,0x11,0,0,0x37,0x10,0,0,0x47,0x1E,0,0,0x37,0x10,0,0,0xD7,0x1F,0,0,7,0x11,0,0,0xD7,0x1F,0,0,7,0x11,0,0,0x87,0x27,0,0,};

int main()
{
   uint32_t *ss = secret;
   char c;

   for (int i = 0; i < 38; i++)
   {
      c = (((ss[i] - 0x1e) ^ 0x4d) - 0x14) / 0x50;
      printf("%c", c);
   }
   return 0;
}

android_re

先用Frida拿到Key和IV，然后解密即可。

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import frida

ss = """
Java.perform(function () {
    console.log("Java.perform is called"); // 打印日志
    var jniClass = Java.use("com.example.re11113.jni"); // 获取类引用
    var getkey = jniClass.getkey;
    getkey.implementation = function () { // 替换函数的实现
        send("getkey begin");
        var res = this.getkey();
        send("key: [" + res + "]");
        send("getkey end");
        return res;
    };

    var getiv = jniClass.getiv;
    getiv.implementation = function () { // 替换函数的实现
        send("getiv begin");
        var res = this.getiv();
        send("iv: [" + res + "]");
        send("getiv end");
        return res;
    };
});

"""
device = frida.get_usb_device()
pid = device.spawn(["com.example.re11113"])
session = device.attach(pid)
script = session.create_script(ss)

def on_message(msg, _data):
    if msg["type"] == "send":
        payload = msg["payload"]
        print(f"[*] {payload}")
    else:
        print(msg)


script.on("message", on_message)
script.load()
input("==========")
print("ok")

device.resume(pid)

input("==========")

输出：

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key: A8UdWaeq
iv: Wf3DLups

进行解密：

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public class SymmetricEncryUtils {

    //AES ,DES
    //  算法/模式/补码方式
    //与给定的密钥内容相关联的 密钥算法的名称
    //    private static String ALGRITHM = "AES/CBC/PKCS5PADDING";
    private static String ALGRITHM = "DES/CBC/PKCS5Padding";


    //    private static String TRANSFORMATION = "AES";
    private static String TRANSFORMATION = "AES";


    public static byte[] generateRandomString() {
        Random random = new Random();
        //AES秘钥规定是16位秘钥
        //DES 8bytes
        byte[] rand = new byte[8];
        random.nextBytes(rand);
        return rand;
    }

    private static final byte[] ALIAS;
    private static final byte[] IV;

    static {
        try {
            ALIAS = "A8UdWaeqA8UdWaeq".getBytes("UTF-8");
            IV = "Wf3DLups".getBytes("UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e);
        }
    }


    public static String encrypt(String content) {
        try {
            // 创建秘钥
            SecretKeySpec keySpec = new SecretKeySpec(ALIAS, ALGRITHM);

            // 创建密码器
            Cipher cipher = Cipher.getInstance(TRANSFORMATION);
            // 初始化加密器
            cipher.init(Cipher.ENCRYPT_MODE, keySpec);
            // 加密
            return Base64.encodeToString(cipher.doFinal(content.getBytes("UTF-8")), Base64.NO_WRAP);
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
        } catch (InvalidKeyException e) {
            e.printStackTrace();
        } catch (IllegalBlockSizeException e) {
            e.printStackTrace();
        } catch (BadPaddingException e) {
            e.printStackTrace();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        return "";
    }

    public static String decrypt(String content) {
        try {
            byte[] keyBytes = ALIAS;
            Arrays.copyOf(keyBytes, 8);
            // 创建秘钥
            SecretKeySpec keySpec = new SecretKeySpec(keyBytes, ALGRITHM);
            IvParameterSpec iv = new IvParameterSpec(IV);

            // 创建密码器
            Cipher cipher = Cipher.getInstance(TRANSFORMATION);
            // 初始化解密器
            cipher.init(Cipher.DECRYPT_MODE, keySpec, iv);
            // 解密
            return new String(cipher.doFinal(Base64.decode(content, Base64.NO_WRAP)), StandardCharsets.UTF_8);
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
        } catch (InvalidKeyException e) {
            e.printStackTrace();
        } catch (IllegalBlockSizeException e) {
            e.printStackTrace();
        } catch (BadPaddingException e) {
            e.printStackTrace();
        } catch (InvalidAlgorithmParameterException e) {
            throw new RuntimeException(e);
        }
        return "";
    }
}

gdb_debug

跟GDB没关系，也不用debug。直接爆破16个seed。

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#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

void do_dec(int seed)
{
    srand(seed);
    char secret[] = "congratulationstoyoucongratulationstoy";
    uint8_t keys[] = {
        0xBF, 0xD7, 0x2E, 0xDA, 0xEE, 0xA8, 0x1A, 0x10, 0x83, 0x73,
        0xAC, 0xF1, 0x06, 0xBE, 0xAD, 0x88, 0x04, 0xD7, 0x12, 0xFE,
        0xB5, 0xE2, 0x61, 0xB7, 0x3D, 0x07, 0x4A, 0xE8, 0x96, 0xA2,
        0x9D, 0x4D, 0xBC, 0x81, 0x8C, 0xE9, 0x88, 0x78, 0x00, 0x00};
    char data[40];
    char data2[40];
    memset(data, 0, sizeof(data));
    for (int i = 0; i < 38; i++)
    {
        data[i] = secret[i] ^ keys[i];
    }
    uint8_t finalXorKeys[40];
    for (int i = 0; i < 38; i++)
        finalXorKeys[i] = rand() & 0xff;

    char SBox[40];
    for (int i = 0; i < 38; i++)
        SBox[i] = i;

    for (int i = 37; i; i--)
    {
        int j = rand() % (i + 1);
        int tmp = SBox[i];
        SBox[i] = SBox[j];
        SBox[j] = tmp;
    }

    for (int i = 0; i < 38; i++)
    {
        data[i] ^= rand();
    }

    for (int i = 0; i < 38; i++)
    {
        data2[SBox[i]] = data[i];
    }

    for (int i = 0; i < 38; i++)
    {
        data2[i] ^= finalXorKeys[i];
    }

    if (data2[0] == 'f' && data2[1] == 'l' && data2[2] == 'a' && data2[3] == 'g' && data2[4] == '{' && data2[37] == '}')
    {
        for (int i = 0; i < 38; i++)
        {
            printf("%c", data2[i]);
        }
        printf("\n");
        exit(0);
    }
}

int main()
{
    for (int i = 0; i < 0x100000000; i += 0x10000000)
    {
        do_dec(i);
    }
}

whereThel1b

有符号，静态就可以差不多看懂。不过远程调试attach python然后给函数下断点也很方便。
相关结构体可以靠官方文档和猜。

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encry = [108, 117, 72, 80, 64, 49, 99, 19, 69, 115, 94, 93, 94, 115, 71, 95, 84, 89, 56, 101, 70, 2, 84, 75, 127, 68, 103, 85, 105, 113, 80, 103, 95, 67, 81, 7, 113, 70, 47, 73, 92, 124, 93, 120, 104, 108, 106, 17, 80, 102, 101, 75, 93, 68, 121, 26]

b = []
import base64
my_enc = [53, 102, 39, 25, 108, 82, 103, 41, 103, 100, 51, 12, 51, 113, 123, 69, 19, 37, 50, 115, 25, 70, 98, 121, 57, 91, 9, 14, 121, 35, 2, 50, 62, 26, 53, 122, 126, 91, 27, 97, 42, 59, 17, 66, 101, 16, 74, 69, 110, 127, 8, 57, 45, 77, 0, 85, 97, 57, 4, 73, 94, 48, 51, 46, 44, 86, 77, 52, 50, 100, 28, 1, 61, 2, 22, 104, 23, 104, 47, 7, 115, 72, 27, 65, 3, 56, 78, 112, 44, 31, 90, 118, 63, 69, 32, 45, 96, 92, 76, 64, 41, 22, 49, 86, 84, 113, 0, 77, 3, 88, 0, 66, 49, 33, 3, 114, 63, 26, 0, 35]
my_enc = encry

import random
random.seed(0)

for i in range(len(my_enc)):
    a = random.randint(0, len(my_enc))
    # print(a, end=' ')
    b += [(a ^ my_enc[i])]

# print()
tmp = (''.join(map(chr, b)))
print(base64.b64decode(tmp.encode()))