[QCTF2018]picture
目录
题目链接
解题过程
总结
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 reimport 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_list def _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_listdef _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): pass def 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 / 16 trun_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_r num += 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 = [''] * 16 real_len = (key_len / 16) * 16 + 16 if key_len % 16 != 0 else key_len b = [''] * int(real_len / 16) for i in range(int(real_len / 16)): b[i] = a[:] num = 0 trun_len = 4 * key_len for 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_r key_y = self._functionKeySecondChange(run_key) b[num][j] = key_y[:] num += 1 return 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P def _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2 def _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 % 16 for key in code: code_ord = int(key, 16) return_code += self._functionTos(code_ord, 4) if lens != 0: return_code += '0' * (16 - lens) * 4 return 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_code return 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 False if string_len < 1 or key_len < 1: return False key_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。