WHCTF-babyre

首先执行file命令得到如下信息

ELF 64-bit LSB executable, x86-64

尝试用IDA64打开，定位到关键函数main发现无法F5，尝试了修复无果，于是用gdb动态调试一发。

在scanf处下断点

b *0x0400660

输入12346789之后next

=> 0x40066c <main+102>:	call   0x4004c0 <strlen@plt>
   0x400671 <main+107>:	mov    DWORD PTR [rbp-0x8],eax
   0x400674 <main+110>:	cmp    DWORD PTR [rbp-0x8],0xe

可以看到调用了strlen函数，然后将长度和0xe比较，于是知道flag长度为0xe。

再次运行

Please input flag:abcdefghijklmn

通过长度验证之后可以看到

0x40067a <main+116>:	mov    edx,0x600b00
0x40067f <main+121>:	lea    rax,[rbp-0x20]
0x400683 <main+125>:	mov    rdi,rax
0x400686 <main+128>:	call   rdx

接着调用了0x600b00处的内容，于是

b *0x600b00
c(ontinue)

然后看到一连串的数

   0x600b08 <judge+8>:	mov    BYTE PTR [rbp-0x20],0x66
   0x600b0c <judge+12>:	mov    BYTE PTR [rbp-0x1f],0x6d
   0x600b10 <judge+16>:	mov    BYTE PTR [rbp-0x1e],0x63
=> 0x600b14 <judge+20>:	mov    BYTE PTR [rbp-0x1d],0x64
   0x600b18 <judge+24>:	mov    BYTE PTR [rbp-0x1c],0x7f
   0x600b1c <judge+28>:	mov    BYTE PTR [rbp-0x1b],0x6b
   0x600b20 <judge+32>:	mov    BYTE PTR [rbp-0x1a],0x37
   0x600b24 <judge+36>:	mov    BYTE PTR [rbp-0x19],0x64
   0x600b28 <judge+40>:	mov    BYTE PTR [rbp-0x18],0x3b
   0x600b2c <judge+44>:	mov    BYTE PTR [rbp-0x17],0x56
   0x600b30 <judge+48>:	mov    BYTE PTR [rbp-0x16],0x60
=> 0x600b34 <judge+52>:	mov    BYTE PTR [rbp-0x15],0x3b
   0x600b38 <judge+56>:	mov    BYTE PTR [rbp-0x14],0x6e
   0x600b3c <judge+60>:	mov    BYTE PTR [rbp-0x13],0x70
   0x600b40 <judge+64>:	mov    DWORD PTR [rbp-0x4],0x0
   0x600b47 <judge+71>:	jmp    0x600b71 <judge+113>

这串数字在后面会用到，接着调试

RAX: 0x7fffffffe2c0 ("abcdefghijklmn")
RBX: 0x0 
RCX: 0x0 
RDX: 0x61 ('a')
EFLAGS: 0x206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x600b60 <judge+96>:	add    rdx,rcx
   0x600b63 <judge+99>:	movzx  edx,BYTE PTR [rdx]
   0x600b66 <judge+102>:	mov    ecx,DWORD PTR [rbp-0x4]
=> 0x600b69 <judge+105>:	xor    edx,ecx

可以看到，rdx=0x61('a')，rcx=0x0，然后rdx=rdx^rcx。于是大致知道是异或加密了flag。

接着调试发现在下一次，rdx=0x62('b')，rcx=0x1。

于是知道flag[i] = key[i] ^ i && 0 <= i < strlen(flag)

而这里的key便是上面的一连串数字，脚本输出flag

#!/usr/bin/python
# -*- coding: utf-8 -*-
__Author__ = "LB@10.0.0.55"

key = [0x66,0x6d,0x63,0x64,0x7f,0x6b,0x37,0x64,0x3b,0x56,0x60,0x3b,0x6e,0x70]
flag = ''
for i in range(len(key)):
	flag += chr(key[i]^i)
print flag
#flag{n1c3_j0b}

>###作者： LB919 >###出处：http://www.cnblogs.com/L1B0/ >###如有转载，荣幸之至！请随手标明出处；