题目链接

https://buuoj.cn/challenges#[QCTF2018]picture

解题过程

附件没有扩展名，用010Editor打开是png图片

保存为png格式：

根据以往经验BUUCTF平台会缺失一些提示，点开题目链接在GitHub里有题目描述：

Just misc ithint：注意文件名（非hash部分）原文件名：bd4408a3-9835-4962-a9af-6acf6cc56b26.wwjkwywq-2a7c8b1b70e00a010c92cab0394c6f93

根据提示判断应该是lsb隐写，密码为：

wwjkwywq

使用lsb.py工具破解：

python2 lsb.py extract attachment.png attachment.txt wwjkwywq

打开attachment.txt，是python脚本。

#_*_ coding:utf-8 _*_
import re
import sysip=  (58, 50, 42, 34, 26, 18, 10, 2,60, 52, 44, 36, 28, 20, 12, 4,62, 54, 46, 38, 30, 22, 14, 6,64, 56, 48, 40, 32, 24, 16, 8,57, 49, 41, 33, 25, 17, 9 , 1,59, 51, 43, 35, 27, 19, 11, 3,61, 53, 45, 37, 29, 21, 13, 5,63, 55, 47, 39, 31, 23, 15, 7)ip_1=(40, 8, 48, 16, 56, 24, 64, 32,39, 7, 47, 15, 55, 23, 63, 31,38, 6, 46, 14, 54, 22, 62, 30,37, 5, 45, 13, 53, 21, 61, 29,36, 4, 44, 12, 52, 20, 60, 28,35, 3, 43, 11, 51, 19, 59, 27,34, 2, 42, 10, 50, 18, 58, 26,33, 1, 41,  9, 49, 17, 57, 25)e  =(32, 1,  2,  3,  4,  5,  4,  5, 6, 7,  8,  9,  8,  9, 10, 11, 12,13, 12, 13, 14, 15, 16, 17,16,17, 18, 19, 20, 21, 20, 21,22, 23, 24, 25,24, 25, 26, 27,28, 29,28, 29, 30, 31, 32,  1)p=(16,  7, 20, 21, 29, 12, 28, 17,1, 15, 23, 26,  5, 18, 31, 10, 2,  8, 24, 14, 32, 27,  3,  9,19, 13, 30, 6, 22, 11,  4,  25)s=[ [[14, 4, 13,  1,  2, 15, 11,  8,  3, 10,  6, 12,  5,  9,  0,  7],[0, 15,  7,  4, 14,  2, 13,  1, 10,  6, 12, 11,  9,  5,  3,  8],[4,  1, 14,  8, 13,  6,  2, 11, 15, 12,  9,  7,  3, 10,  5,  0],    [15, 12,  8,  2,  4,  9,  1,  7,  5, 11,  3, 14, 10,  0,  6, 13]],[[15,  1,  8, 14,  6, 11,  3,  4,  9,  7,  2, 13, 12,  0,  5, 10],     [3, 13,  4,  7, 15,  2,  8, 14, 12,  0,  1, 10,  6,  9, 11,  5],     [0, 14,  7, 11, 10,  4, 13,  1,  5,  8, 12,  6,  9,  3,  2, 15],     [13,  8, 10,  1,  3, 15,  4,  2, 11,  6,  7, 12,  0,  5, 14,  9]],[[10,  0,  9, 14,  6,  3, 15,  5,  1, 13, 12,  7, 11,  4,  2,  8],     [13,  7,  0,  9,  3,  4,  6, 10,  2,  8,  5, 14, 12, 11, 15,  1],   [13,  6,  4,  9,  8, 15,  3,  0, 11,  1,  2, 12,  5, 10, 14,  7],     [1, 10, 13,  0,  6,  9,  8,  7,  4, 15, 14,  3, 11,  5,  2, 12]],[[7, 13, 14,  3,  0,  6,  9, 10,  1,  2,  8,  5, 11,  12,  4, 15],     [13,  8, 11,  5,  6, 15,  0,  3,  4,  7,  2, 12,  1, 10, 14,9],     [10,  6,  9,  0, 12, 11,  7, 13, 15,  1,  3, 14,  5,  2,  8,  4],     [3, 15,  0,  6, 10,  1, 13,  8,  9,  4,  5, 11, 12,  7,  2, 14]],[[2, 12,  4,  1,  7, 10, 11,  6,  8,  5,  3, 15, 13,  0, 14,  9],     [14, 11,  2, 12,  4,  7, 13,  1,  5,  0, 15, 10,  3,  9,  8,  6],     [4,  2,  1, 11, 10, 13,  7,  8, 15,  9, 12,  5,  6,  3,  0, 14],     [11,  8, 12,  7,  1, 14,  2, 13,  6, 15,  0,  9, 10,  4,  5,  3]],[[12,  1, 10, 15,  9,  2,  6,  8,  0, 13,  3,  4, 14,  7,  5, 11],[10, 15,  4,  2,  7, 12,  9,  5,  6,  1, 13, 14,  0, 11,  3,  8],     [9, 14, 15,  5,  2,  8, 12,  3,  7,  0,  4, 10,  1, 13, 11,  6],     [4,  3,  2, 12,  9,  5, 15, 10, 11, 14,  1,  7,  6,  0,  8, 13]],[[4, 11,  2, 14, 15,  0,  8, 13,  3, 12,  9,  7,  5, 10,  6,  1],     [13,  0, 11,  7,  4,  9,  1, 10, 14,  3,  5, 12,  2, 15,  8,  6],     [1,  4, 11, 13, 12,  3,  7, 14, 10, 15,  6,  8,  0,  5,  9,  2],     [6, 11, 13,  8,  1,  4, 10,  7,  9,  5,  0, 15, 14,  2,  3, 12]],[[13,  2,  8,  4,  6, 15, 11,  1, 10,  9,  3, 14,  5,  0, 12,  7],     [1, 15, 13,  8, 10,  3,  7,  4, 12,  5,  6, 11,  0, 14,  9,  2],     [7, 11,  4,  1,  9, 12, 14,  2,  0,  6, 10, 13, 15,  3,  5,  8],     [2,  1, 14,  7,  4, 10,  8, 13, 15, 12,  9,  0,  3,  5,  6, 11]]]pc1=(57, 49, 41, 33, 25, 17,  9,1, 58, 50, 42, 34, 26, 18,10,  2, 59, 51, 43, 35, 27,19, 11,  3, 60, 52, 44, 36,63, 55, 47, 39, 31, 23, 15,7, 62, 54, 46, 38, 30, 22,14,  6, 61, 53, 45, 37, 29,21, 13,  5, 28, 20, 12, 4)pc2= (14, 17, 11, 24,  1,  5,  3, 28,15,  6, 21, 10, 23, 19, 12,  4, 26,  8, 16,  7, 27, 20, 13,  2, 41, 52, 31, 37, 47, 55, 30, 40, 51, 45, 33, 48, 44, 49, 39, 56, 34, 53, 46, 42, 50, 36, 29, 32)d = (1,  1,  2,  2,  2,  2,  2,  2, 1, 2, 2, 2, 2, 2, 2, 1)__all__=['desencode']
class DES():def __init__(self):passdef code(self,from_code,key,code_len,key_len):output=""trun_len=0code_string=self._functionCharToA(from_code,code_len)code_key=self._functionCharToA(key,key_len)if code_len%16!=0:real_len=(code_len/16)*16+16else:real_len=code_lenif key_len%16!=0:key_len=(key_len/16)*16+16key_len*=4trun_len=4*real_lenfor i in range(0,trun_len,64):run_code=code_string[i:i+64]l=i%key_lenrun_key=code_key[l:l+64]run_code= self._codefirstchange(run_code)run_key= self._keyfirstchange(run_key)for j in range(16):code_r=run_code[32:64]code_l=run_code[0:32]run_code=code_rcode_r= self._functionE(code_r)key_l=run_key[0:28]key_r=run_key[28:56]key_l=key_l[d[j]:28]+key_l[0:d[j]]key_r=key_r[d[j]:28]+key_r[0:d[j]]run_key=key_l+key_rkey_y= self._functionKeySecondChange(run_key)code_r= self._codeyihuo(code_r,key_y)code_r= self._functionS(code_r)code_r= self._functionP(code_r)code_r= self._codeyihuo(code_l,code_r)run_code+=code_rcode_r=run_code[32:64]code_l=run_code[0:32]run_code=code_r+code_loutput+=self._functionCodeChange(run_code)return outputdef _codeyihuo(self,code,key):code_len=len(key)return_list=''for i in range(code_len):if code[i]==key[i]:return_list+='0'else:return_list+='1'return return_listdef _codefirstchange(self,code):changed_code=''for i in range(64):changed_code+=code[ip[i]-1]return changed_codedef _keyfirstchange (self,key):changed_key=''for i in range(56):changed_key+=key[pc1[i]-1]return changed_keydef _functionCodeChange(self, code):lens=len(code)/4return_list=''for i in range(lens):list=''for j in range(4):list+=code[ip_1[i*4+j]-1]return_list+="%x" %int(list,2)return return_listdef _functionE(self,code):return_list=''for i in range(48):return_list+=code[e[i]-1]return return_list       def _functionP(self,code):return_list=''for i in range(32):return_list+=code[p[i]-1]return return_listdef _functionS(self, key):return_list=''for i in range(8):row=int( str(key[i*6])+str(key[i*6+5]),2)raw=int(str( key[i*6+1])+str(key[i*6+2])+str(key[i*6+3])+str(key[i*6+4]),2)return_list+=self._functionTos(s[i][row][raw],4)return return_listdef _functionKeySecondChange(self,key):return_list=''for i in range(48):return_list+=key[pc2[i]-1]return return_listdef _functionCharToA(self,code,lens):return_code=''lens=lens%16for key in code:code_ord=int(key,16)return_code+=self._functionTos(code_ord,4)        if lens!=0:return_code+='0'*(16-lens)*4return return_codedef _functionTos(self,o,lens):return_code=''for i in range(lens):return_code=str(o>>i &1)+return_codereturn return_codedef tohex(string):return_string=''for i in string:return_string+="%02x"%ord(i)return return_stringdef tounicode(string):return_string=''string_len=len(string)for i in range(0,string_len,2):return_string+=chr(int(string[i:i+2],16))return return_stringdef desencode(from_code,key):from_code=tohex(from_code)key=tohex(key)des=DES()key_len=len(key)string_len=len(from_code)        if string_len<1 or key_len<1:print 'error input'return Falsekey_code= des.code(from_code,key,string_len,key_len)return key_codeif __name__  == '__main__':if(desencode(sys.argv[1],'mtqVwD4JNRjw3bkT9sQ0RYcZaKShU4sf')=='e3fab29a43a70ca72162a132df6ab532535278834e11e6706c61a1a7cefc402c8ecaf601d00eee72'):print 'correct.'else:print 'try again.'

看了半天代码是DES加密的脚本，原脚本在网上能找到：

https://my.oschina.net/liupengs/blog/124230

也有解密的脚本：

# _*_ coding:utf-8 _*_ip = (58, 50, 42, 34, 26, 18, 10, 2,60, 52, 44, 36, 28, 20, 12, 4,62, 54, 46, 38, 30, 22, 14, 6,64, 56, 48, 40, 32, 24, 16, 8,57, 49, 41, 33, 25, 17, 9, 1,59, 51, 43, 35, 27, 19, 11, 3,61, 53, 45, 37, 29, 21, 13, 5,63, 55, 47, 39, 31, 23, 15, 7)ip_1 = (40, 8, 48, 16, 56, 24, 64, 32,39, 7, 47, 15, 55, 23, 63, 31,38, 6, 46, 14, 54, 22, 62, 30,37, 5, 45, 13, 53, 21, 61, 29,36, 4, 44, 12, 52, 20, 60, 28,35, 3, 43, 11, 51, 19, 59, 27,34, 2, 42, 10, 50, 18, 58, 26,33, 1, 41, 9, 49, 17, 57, 25)e = (32, 1, 2, 3, 4, 5, 4, 5,6, 7, 8, 9, 8, 9, 10, 11,12, 13, 12, 13, 14, 15, 16, 17,16, 17, 18, 19, 20, 21, 20, 21,22, 23, 24, 25, 24, 25, 26, 27,28, 29, 28, 29, 30, 31, 32, 1)p = (16, 7, 20, 21, 29, 12, 28, 17,1, 15, 23, 26, 5, 18, 31, 10,2, 8, 24, 14, 32, 27, 3, 9,19, 13, 30, 6, 22, 11, 4, 25)s = [[[14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7],[0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8],[4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0],[15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13]],[[15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10],[3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5],[0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15],[13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9]],[[10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8],[13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1],[13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7],[1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12]],[[7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15],[13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9],[10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4],[3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14]],[[2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9],[14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6],[4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14],[11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3]],[[12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11],[10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8],[9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6],[4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13]],[[4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1],[13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6],[1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2],[6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12]],[[13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7],[1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2],[7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8],[2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11]]]pc1 = (57, 49, 41, 33, 25, 17, 9,1, 58, 50, 42, 34, 26, 18,10, 2, 59, 51, 43, 35, 27,19, 11, 3, 60, 52, 44, 36,63, 55, 47, 39, 31, 23, 15,7, 62, 54, 46, 38, 30, 22,14, 6, 61, 53, 45, 37, 29,21, 13, 5, 28, 20, 12, 4)pc2 = (14, 17, 11, 24, 1, 5, 3, 28,15, 6, 21, 10, 23, 19, 12, 4,26, 8, 16, 7, 27, 20, 13, 2,41, 52, 31, 37, 47, 55, 30, 40,51, 45, 33, 48, 44, 49, 39, 56,34, 53, 46, 42, 50, 36, 29, 32)d = (1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1)__all__ = ['desdecode']class DES:"""解密函数，DES加密与解密的方法相差不大只是在解密的时候所用的子密钥与加密的子密钥相反"""def __init__(self):passdef decode(self, string, key, key_len, string_len):output = ""num = 0# 将密文转换为二进制code_string = self._functionCharToA(string, string_len)# 获取字密钥code_key = self._getkey(key, key_len)# 如果密钥长度不是16的整数倍则以增加0的方式变为16的整数倍real_len = (key_len / 16) + 1 if key_len % 16 != 0 else key_len / 16trun_len = string_len * 4# 对每64位进行一次加密for i in range(0, trun_len, 64):run_code = code_string[i:i + 64]run_key = code_key[int(num % real_len)]# 64位明文初始置换run_code = self._codefirstchange(run_code)# 16次迭代for j in range(16):code_r = run_code[32:64]code_l = run_code[0:32]# 64左右交换run_code = code_r# 右边32位扩展置换code_r = self._functionE(code_r)# 获取本轮子密钥key_y = run_key[15 - j]# 异或code_r = self._codeyihuo(code_r, key_y)# S盒代替/选择code_r = self._functionS(code_r)# P转换code_r = self._functionP(code_r)# 异或code_r = self._codeyihuo(code_l, code_r)run_code += code_rnum += 1# 32互换code_r = run_code[32:64]code_l = run_code[0:32]run_code = code_r + code_l# 将二进制转换为16进制、逆初始置换output += self._functionCodeChange(run_code)return output# 获取子密钥def _getkey(self, key, key_len):# 将密钥转换为二进制code_key = self._functionCharToA(key, key_len)a = [''] * 16real_len = (key_len / 16) * 16 + 16 if key_len % 16 != 0 else key_lenb = [''] * int(real_len / 16)for i in range(int(real_len / 16)):b[i] = a[:]num = 0trun_len = 4 * key_lenfor i in range(0, trun_len, 64):run_key = code_key[i:i + 64]run_key = self._keyfirstchange(run_key)for j in range(16):key_l = run_key[0:28]key_r = run_key[28:56]key_l = key_l[d[j]:28] + key_l[0:d[j]]key_r = key_r[d[j]:28] + key_r[0:d[j]]run_key = key_l + key_rkey_y = self._functionKeySecondChange(run_key)b[num][j] = key_y[:]num += 1return b# 异或def _codeyihuo(self, code, key):code_len = len(key)return_list = ''for i in range(code_len):if code[i] == key[i]:return_list += '0'else:return_list += '1'return return_list# 密文或明文初始置换def _codefirstchange(self, code):changed_code = ''for i in range(64):changed_code += code[ip[i] - 1]return changed_code# 密钥初始置换def _keyfirstchange(self, key):changed_key = ''for i in range(56):changed_key += key[pc1[i] - 1]return changed_key# 逆初始置换def _functionCodeChange(self, code):return_list = ''for i in range(16):list = ''for j in range(4):list += code[ip_1[i * 4 + j] - 1]return_list += "%x" % int(list, 2)return return_list# 扩展置换def _functionE(self, code):return_list = ''for i in range(48):return_list += code[e[i] - 1]return return_list# 置换Pdef _functionP(self, code):return_list = ''for i in range(32):return_list += code[p[i] - 1]return return_list# S盒代替选择置换def _functionS(self, key):return_list = ''for i in range(8):row = int(str(key[i * 6]) + str(key[i * 6 + 5]), 2)raw = int(str(key[i * 6 + 1]) + str(key[i * 6 + 2]) + str(key[i * 6 + 3]) + str(key[i * 6 + 4]), 2)return_list += self._functionTos(s[i][row][raw], 4)return return_list# 密钥置换选择2def _functionKeySecondChange(self, key):return_list = ''for i in range(48):return_list += key[pc2[i] - 1]return return_list# 将十六进制转换为二进制字符串def _functionCharToA(self, code, lens):return_code = ''lens = lens % 16for key in code:code_ord = int(key, 16)return_code += self._functionTos(code_ord, 4)if lens != 0:return_code += '0' * (16 - lens) * 4return return_code# 二进制转换def _functionTos(self, o, lens):return_code = ''for i in range(lens):return_code = str(o >> i & 1) + return_codereturn return_code# 将unicode字符转换为16进制
def tohex(string):return_string = ''for i in string:return_string += "%02x" % ord(i)return return_stringdef tounicode(string):return_string = ''string_len = len(string)for i in range(0, string_len, 2):return_string += chr(int(string[i:i + 2], 16))return return_string# 入口函数
def desdecode(from_code, key):key = tohex(key)des = DES()key_len = len(key)string_len = len(from_code)if string_len % 16 != 0:return Falseif string_len < 1 or key_len < 1:return Falsekey_code = des.decode(from_code, key, key_len, string_len)return tounicode(key_code)# 测试
if __name__ == '__main__':print(desdecode('e3fab29a43a70ca72162a132df6ab532535278834e11e6706c61a1a7cefc402c8ecaf601d00eee72', 'mtqVwD4JNRjw3bkT9sQ0RYcZaKShU4sf'))

运行后结果为：

QCTF{eCy0AALMDH9rLoBnWnTigXpYPkgU0sU4}

总结

思路：用lsb.py破解png的隐写，得到了一个DES加密脚本。这个脚本是liupengs编写的，在开源博客和GitHub上能找到源码。再用作者的解密脚本破解。lsb隐写的密码这块儿有些坑，没有题目提示不好猜。

知识点：lsb隐写、DES加解密。

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H.264 picture parameter sets成员值含义学习笔记
-picture parameter sets 1. pic_parameter_set_id pic_parameter_set_id指明了在切片头中对应的某个psp.pic_parameter_s ...
Codeforces Global Round 8 C. Even Picture 题解[巧妙法构造题]
C. Even Picture 题目大意:让你构造一个连通图,图中有k个灰点其中n个点4面都是灰点,(k-n)个点周围2个点是灰点我们设最上面的为(1,1)点就可以了 ps:exactly:只有,其 ...
图像隐藏matlab代码,picture Matlab简单操作实现图像的隐藏加密联合开发网 - pudn.com...
picture 所属分类:其他开发工具:Others 文件大小:15KB 下载次数:1 上传日期:2018-07-18 09:50:48 上传者:至尊宝物语说明: Matlab简单操作实现图 ...
十、图像参数集Picture Paramater Set(PPS)解析
GitHub代码地址:点击这里除了序列参数集SPS之外,H.264中另一重要的参数集合为图像参数集Picture Paramater Set(PPS).通常情况下,PPS类似于SPS,在H.264的 ...
视频压缩编码 gop（Group of Pictures）（I帧间隔）的概念、IDR、I帧（关键帧，intra picture）、P帧、B帧、帧内压缩、帧间压缩、pts（显示时间）、dts（解码时间）
文章目录前言: 关于视频的基础定义 GOP:Group of Pictures 压缩(也就是编码) 帧内压缩:自己本身压缩,不参考其他的帧. 帧间压缩:参考其他视频帧压缩. 解码(解压缩) 为什么好 ...

[QCTF2018]picture

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