AIS3 Pre-exam & MyFirstCTF WriteUp

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MyFirstCTF
6st place

alt text

misc

  • Quantum Nim Heist
    發現輸入時按enter好像會沒移除石頭，反正最後使自己移除最後一顆
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  • Three Dimensional Secret
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    追蹤TCP流，並另存為一個檔案
    在 https://ncviewer.com/ 開啟此檔案
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  • Emoji Console
    試了很久發現”:|”後的指令可以執行
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web

  • Evil Calculator
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rev

  • The Long Print
    這本來是一個很大的數字，patch成1，變每秒print一個字元
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    趕在消失不見前趕快截圖
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    最後的”}”大概一輸出就不見了，自己補上

  • 火拳のエース
    先執行，發現flag輸出了一小部份且輸出速度太慢
    IDA，從main進到print_flag()，patch輸出時要等待的時間，改成0
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    輸出正常後回到main繼續分析，找剩下的flag
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    首先是輸入四個字串空白分隔
    然後在 xor_strings() 這四個字串分別跟四個不同的東西做XOR，從IDA我不會看這4個東西是甚麼，用ghidra看到了，把他們的前8 bytes 記下來
    再來四個字串的每個字元經過 complex_function() 轉換
    最後判斷是否4個轉換後的字串都等於對應的字串
    => 於是4個字串分別暴力破解，找出輸入什麼最後會變成對應的字串
    每個字串的暴力破解程式有微調，以第四個字串為例

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    #include <bits/stdc++.h>
    using namespace std;

    int xor_hex0[] = {0x0E, 0x0D, 0x7D, 0x06, 0x0F, 0x17, 0x76, 0x04};
    int xor_hex1[] = {0x6D, 0x00, 0x1B, 0x7C, 0x6C, 0x13, 0x62, 0x11};
    int xor_hex2[] = {0x1E, 0x7E, 0x06, 0x13, 0x07, 0x66, 0x0E, 0x71};
    int xor_hex3[] = {0x17, 0x14, 0x1D, 0x70, 0x79, 0x67, 0x74, 0x33};

    string target0 = "DHLIYJEG";
    string target1 = "MZRERYND";
    string target2 = "RUYODBAH";
    string target3 = "BKEMPBRE";

    void xor_strings(string input, int xor_hex[], int* result_int)
    {
      int i;
      int input_int[9];

      for ( i = 0; i <= 7; ++i )
      {
        input_int[i] = int(input[i]);
        result_int[i] = input_int[i] ^ xor_hex[i];
      }

    }

    int complex_function(int a1, int a2){
      int v2; // eax
      int v4; // [esp-Ch] [ebp-24h]
      int v5; // [esp-8h] [ebp-20h]
      int v6; // [esp-4h] [ebp-1Ch]
      int v7; // [esp+8h] [ebp-10h]
      int v8; // [esp+Ch] [ebp-Ch]

      if ( a1 <= 64 || a1 > 90 )
      {
        return 64;
      }

      v8 = (17 * a2 + a1 - 65) % 26;
      v7 = a2 % 3 + 3;
      v2 = a2 % 3;
      if ( a2 % 3 == 2 )
      {
        v8 = (v8 - v7 + 26) % 26;
      }
      else if ( v2 <= 2 )
      {
        if ( v2 )
        {
          if ( v2 == 1 )
            v8 = (2 * v7 + v8) % 26;
        }
        else
        {
          v8 = (v7 * v8 + 7) % 26;
        }
      }
      return v8 + 65;
    }

    int main(){
        string input = "        ";
        string result = "AAAAAAAA";
        for(int i=0;i<8;i++){
            for(int j=33;j<127;j++){
                input[i] = char(j);
                int tmp[9];
                xor_strings(input, xor_hex3, tmp);
                result[i] = char(complex_function(tmp[i], i+96));
                if(result[i] == target3[i]){
                    cout<<result<<endl;
                    cout<<input<<endl;
                    break;
                }
            }

        }
    }

輸出
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crypto

  • babyRSA
    改編 babyRSA.py
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    import random
    from Crypto.Util.number import getPrime

    def gcd(a, b):
        while b:
            a, b = b, a % b
        return a

    def generate_keypair(keysize):
        p = getPrime(keysize)
        q = getPrime(keysize)
        n = p * q
        phi = (p-1) * (q-1)
        
        e = random.randrange(1, phi)
        g = gcd(e, phi)
        while g != 1:
            e = random.randrange(1, phi)
            g = gcd(e, phi)
        d = pow(e, -1, phi)
        return ((e, n), (d, n))

    def encrypt(pk, plaintext):
        key, n = pk
        cipher = [pow(ord(char), key, n) for char in plaintext]
        return cipher

    e = 64917055846592305247490566318353366999709874684278480849508851204751189365198819392860386504785643859122396657301225094708026391204100352682992979425763157452255909781003406602228716107905797084217189131716198785709124050278116966890968003294485934472496151582084561439957513571043497031319413889856520421733
    n = 115676743153063753482251273007095369919613374531038288437295760314264647231038870203981488393720761532040569270340726478402172283300622527884543078194060647393394510524980830171230330673500741683492143805583694395504141751460090539868114454005046898551218623342425465650881666420408703144859108346202894384649

    public = (e, n)
    enc = [...] // 就是output.txt的Encrypted:[]，太長，略

    flag = ""
    for c in enc:
        for i in range(128):

            encrypted_msg = encrypt(public, chr(i))
            if c in encrypted_msg:

                flag += chr(i)
                print(flag)
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