CTFLearn - Bite-code

자바 바이트 코드를 풀어내는 문제

바이트코드는 어렵진 않으나... 맞는 답을 찾는 과정이 더 어려운 기이한 문제

public static boolean checkNum(int);     descriptor: (I)Z     flags: ACC_PUBLIC, ACC_STATIC     Code:       stack=2, locals=3, args_size=1          0: iload_0          1: iconst_3          2: ishl          3: istore_1                           //v1 = param <<3          4: iload_0          5: ldc           #2                  // int 525024598          7: ixor          8: istore_2                           //v2 = param ^  525024598           9: iload_1         10: iload_2         11: ixor         12: ldc           #3                  // int -889275714                                                 //v1^ v2== -889275714일때 정답출력         14: if_icmpne     21         17: iconst_1         18: goto          22         21: iconst_0         22: ireturn       LineNumberTable:         line 3: 0         line 4: 4         line 5: 9       StackMapTable: number_of_entries = 2         frame_type = 253 /* append */           offset_delta = 21           locals = [ int, int ]         frame_type = 64 /* same_locals_1_stack_item */           stack = [ int ]

파이썬으로 브루트포스 스크립트를 만들어서 풀었다

계산량이 꽤 많으므로 멀티프로세스를 이용했다

import multiprocessing   base_list = [0x00000000,0x10000000,0x20000000,0x30000000,             0x40000000,0x50000000,0x60000000,0x70000000,             0x80000000,0x90000000,0xa0000000,0xb0000000,             0xc0000000,0xd0000000,0xe0000000,0xf0000000] def solve(base):     for i in range(base,base+0xfffffff):         v1 = (i << 3)&0xffffffff          v2 = i ^ 0x1f4b3d56         if (v1^v2) == 0xcafebabe :             print(f'Answer is {i}')             break   if __name__ == '__main__':     pool = multiprocessing.Pool(processes=8)     pool.map(solve,base_list)     pool.close()     pool.join()

 

(base) D:\pythonprojects>python bitecode.py
Answer is 2942112424

 

이때 2942112424 는 signed int 로는 -1352854872가 된다