起因
一道ctf题
$flag = "XXXXXXXXXXXXXXXXXXXXXXX"; $secret = "XXXXXXXXXXXXXXX"; // This secret is 15 characters long for security! $username = $_POST["username"]; $password = $_POST["password"]; if (!empty($_COOKIE["getmein"])) { if (urldecode($username) === "admin" && urldecode($password) != "admin") { if ($COOKIE["getmein"] === md5($secret . urldecode($username . $password))) { echo "Congratulations! You are a registered user./n"; die ("The flag is ". $flag); } else { die ("Your cookies don't match up! STOP HACKING THIS SITE."); } } else { die ("You are not an admin! LEAVE."); } } setcookie("sample-hash", md5($secret . urldecode("admin" . "admin")), time() + (60 * 60 * 24 * 7)); if (empty($_COOKIE["source"])) { setcookie("source", 0, time() + (60 * 60 * 24 * 7)); } else { if ($_COOKIE["source"] != 0) { echo ""; // This source code is outputted here } }这里的关键绕过是这一句:
if ($COOKIE["getmein"] === md5($secret . urldecode($username . $password)))
要
cookie['getmein']===$secret . urldecode($username . $password)
的md5加密，而这里的secret是不可知的，但却知道他的长度，这里我们就涉及到hash扩展攻击。
MD5加密原理
MD5会把原数据分成512为一块的许多块，最后一块加上64字节来表示他的长度，一共构成512*n个字节然后再对这N个512数据块进行N次加密计算(因为过程较复杂，此处不做详解，下文称为复杂计算)，虽然此处加密过程很复杂，但是整个加密过程很容易理解，如下:
加密过程
现在我们知道的是
secretusernamepassword这个数据，那么我们怎么进行攻击呢，我们看一下这个数据的16进制
算一下，22个字符,512/8=64,64/16=4，我们需要4排数据然后最后给一个整个数据长度，22=0x14,然后md5计算是小端存储，所以我们修改如下图
secretusernamepassword转16进制
0x736563726574757365726e616d657617373776f7264
然后填充成
0x736563726574757365726e616d6570617373776f726480000000000000000000000000000000000000000000000000000000000000000000b000000000000000
md5('secretusernamepassword')==3105ff5f8723abe628d54387f2de5641
可以倒推出这个时候的ABCD1:
A=5fff0531
B=e6ab2387
C=8743d528
D=4156def2
现在如果我们继续加数据
0x736563726574757365726e616d6570617373776f726480000000000000000000000000000000000000000000000000000000000000000000b00000000000000072747576
现在我们已知前面512位计算出来的ABCD1,现在我们去掉前面直接用运算出来的ABCD1运算后面0x72747576得到的结果应该和加密全部的结果是一样的
0X72747576会被自动填充为
0x72747576800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002002000000000000
'secretusernamepassword'+'/x80'+'/x00'*33+'/xb0'+'/x00'*7+'/x72/x74/x75/x76'
直接md5加密结果为8e847c325fb05c60d437b23dc38ea6da
使用ABCD1手动加密0X72747576
A=327c848e,B=605cb05f,C=3db237d4,D=daa68ec3
md5:8e847c325fb05c60d437b23dc38ea6da
可以看到相同
攻击流程
既然如初，我们只要知道一个hash值，知道原来数据的数据长度，那么我们就可以算出
原数据+填充数据到512+任意内容的hash值
那么我们来看代码
他是直接用secret+username+password输入的是username和password，那么我们直接得出cookie里面的hash值，拿出这个hash值，倒推出这个ABCD1,然后用这个ABCD1对任意值加密，得出来hash值就是这个任意值附加在
secret+username+password+填充字节+任意值的hash
like this：
加密代码(引用现成代码)
#!/usr/bin/env python # -*- coding: utf-8 -*- # @Author：DshtAnger # theory reference: # blog： # http://blog.csdn.net/adidala/article/details/28677393 # http://blog.csdn.net/forgotaboutgirl/article/details/7258109 # http://blog.sina.com.cn/s/blog_6fe0eb1901014cpl.html # RFC1321： # https://www.rfc-editor.org/rfc/pdfrfc/rfc1321.txt.pdf ############################################################################## import sys def genMsgLengthDescriptor(msg_bitsLenth): ''' ---args: msg_bitsLenth : the bits length of raw message --return: 16 hex-encoded string , i.e.64bits,8bytes which used to describe the bits length of raw message added after padding ''' return __import__("struct").pack(">Q", msg_bitsLenth).encode("hex") def reverse_hex_8bytes(hex_str): ''' --args: hex_str: a hex-encoded string with length 16 , i.e.8bytes --return: transform raw message descriptor to little-endian ''' hex_str = "%016x" % int(hex_str, 16) assert len(hex_str) == 16 return __import__("struct").pack("> (32 - n))) & (0xffffffff)) def FF(a, b, c, d, x, s, ac): a = (a + F((b), (c), (d)) + (x) + (ac) & 0xffffffff) & 0xffffffff; a = RL((a), (s)) & 0xffffffff; a = (a + b) & 0xffffffff return a def GG(a, b, c, d, x, s, ac): a = (a + G((b), (c), (d)) + (x) + (ac) & 0xffffffff) & 0xffffffff; a = RL((a), (s)) & 0xffffffff; a = (a + b) & 0xffffffff return a def HH(a, b, c, d, x, s, ac): a = (a + H((b), (c), (d)) + (x) + (ac) & 0xffffffff) & 0xffffffff; a = RL((a), (s)) & 0xffffffff; a = (a + b) & 0xffffffff return a def II(a, b, c, d, x, s, ac): a = (a + I((b), (c), (d)) + (x) + (ac) & 0xffffffff) & 0xffffffff; a = RL((a), (s)) & 0xffffffff; a = (a + b) & 0xffffffff return a def show_md5(A, B, C, D): return "".join(["".join(__import__("re").findall(r"..", "%08x" % i)[::-1]) for i in (A, B, C, D)]) def run_md5(A=0x67452301, B=0xefcdab89, C=0x98badcfe, D=0x10325476, readyMsg=""): a = A b = B c = C d = D for i in xrange(0, len(readyMsg) / 128): M = getM16(readyMsg, i + 1) for i in xrange(16): exec "M" + str(i) + "=M[" + str(i) + "]" # First round a = FF(a, b, c, d, M0, 7, 0xd76aa478L) d = FF(d, a, b, c, M1, 12, 0xe8c7b756L) c = FF(c, d, a, b, M2, 17, 0x242070dbL) b = FF(b, c, d, a, M3, 22, 0xc1bdceeeL) a = FF(a, b, c, d, M4, 7, 0xf57c0fafL) d = FF(d, a, b, c, M5, 12, 0x4787c62aL) c = FF(c, d, a, b, M6, 17, 0xa8304613L) b = FF(b, c, d, a, M7, 22, 0xfd469501L) a = FF(a, b, c, d, M8, 7, 0x698098d8L) d = FF(d, a, b, c, M9, 12, 0x8b44f7afL) c = FF(c, d, a, b, M10, 17, 0xffff5bb1L) b = FF(b, c, d, a, M11, 22, 0x895cd7beL) a = FF(a, b, c, d, M12, 7, 0x6b901122L) d = FF(d, a, b, c, M13, 12, 0xfd987193L) c = FF(c, d, a, b, M14, 17, 0xa679438eL) b = FF(b, c, d, a, M15, 22, 0x49b40821L) # Second round a = GG(a, b, c, d, M1, 5, 0xf61e2562L) d = GG(d, a, b, c, M6, 9, 0xc040b340L) c = GG(c, d, a, b, M11, 14, 0x265e5a51L) b = GG(b, c, d, a, M0, 20, 0xe9b6c7aaL) a = GG(a, b, c, d, M5, 5, 0xd62f105dL) d = GG(d, a, b, c, M10, 9, 0x02441453L) c = GG(c, d, a, b, M15, 14, 0xd8a1e681L) b = GG(b, c, d, a, M4, 20, 0xe7d3fbc8L) a = GG(a, b, c, d, M9, 5, 0x21e1cde6L) d = GG(d, a, b, c, M14, 9, 0xc33707d6L) c = GG(c, d, a, b, M3, 14, 0xf4d50d87L) b = GG(b, c, d, a, M8, 20, 0x455a14edL) a = GG(a, b, c, d, M13, 5, 0xa9e3e905L) d = GG(d, a, b, c, M2, 9, 0xfcefa3f8L) c = GG(c, d, a, b, M7, 14, 0x676f02d9L) b = GG(b, c, d, a, M12, 20, 0x8d2a4c8aL) # Third round a = HH(a, b, c, d, M5, 4, 0xfffa3942L) d = HH(d, a, b, c, M8, 11, 0x8771f681L) c = HH(c, d, a, b, M11, 16, 0x6d9d6122L) b = HH(b, c, d, a, M14, 23, 0xfde5380c) a = HH(a, b, c, d, M1, 4, 0xa4beea44L) d = HH(d, a, b, c, M4, 11, 0x4bdecfa9L) c = HH(c, d, a, b, M7, 16, 0xf6bb4b60L) b = HH(b, c, d, a, M10, 23, 0xbebfbc70L) a = HH(a, b, c, d, M13, 4, 0x289b7ec6L) d = HH(d, a, b, c, M0, 11, 0xeaa127faL) c = HH(c, d, a, b, M3, 16, 0xd4ef3085L) b = HH(b, c, d, a, M6, 23, 0x04881d05L) a = HH(a, b, c, d, M9, 4, 0xd9d4d039L) d = HH(d, a, b, c, M12, 11, 0xe6db99e5L) c = HH(c, d, a, b, M15, 16, 0x1fa27cf8L) b = HH(b, c, d, a, M2, 23, 0xc4ac5665L) # Fourth round a = II(a, b, c, d, M0, 6, 0xf4292244L) d = II(d, a, b, c, M7, 10, 0x432aff97L) c = II(c, d, a, b, M14, 15, 0xab9423a7L) b = II(b, c, d, a, M5, 21, 0xfc93a039L) a = II(a, b, c, d, M12, 6, 0x655b59c3L) d = II(d, a, b, c, M3, 10, 0x8f0ccc92L) c = II(c, d, a, b, M10, 15, 0xffeff47dL) b = II(b, c, d, a, M1, 21, 0x85845dd1L) a = II(a, b, c, d, M8, 6, 0x6fa87e4fL) d = II(d, a, b, c, M15, 10, 0xfe2ce6e0L) c = II(c, d, a, b, M6, 15, 0xa3014314L) b = II(b, c, d, a, M13, 21, 0x4e0811a1L) a = II(a, b, c, d, M4, 6, 0xf7537e82L) d = II(d, a, b, c, M11, 10, 0xbd3af235L) c = II(c, d, a, b, M2, 15, 0x2ad7d2bbL) b = II(b, c, d, a, M9, 21, 0xeb86d391L) A += a B += b C += c D += d A = A & 0xffffffff B = B & 0xffffffff C = C & 0xffffffff D = D & 0xffffffff a = A b = B c = C d = D print "%x,%x,%x,%x" % (a, b, c, d) return show_md5(a, b, c, d) samplehash="571580b26c65f306376d4f64e53cb5c7" s1=0x5fff0531 s2=0xe6ab2387 s3=0x8743d528 s4=0x4156def2 secret = 'secretusernamepassword' test=secret+'/x80'+'/x00'*33+'/xb0'+'/x00'*7+'/x72/x74/x75/x76' s = deal_rawInputMsg(test) inp = s[len(s)/2:] print test+'/n' print '----------------------------------------------------------' print s print '----------------------------------------------------------' print inp print '----------------------------------------------------------' print "md5:"+run_md5(s1,s2,s3,s4,inp)