Secret Conversation
附件有两个文件，分别是
main.py
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| import secrets
from Crypto.Cipher import AES
from binascii import hexlify, unhexlify
previousCt = ""
previousPt = ""
rand_iv = secrets.token_bytes(16)
def pad(plaintext):
ext = len(plaintext) % 16
if ext != 0:
plaintext += "0" * (16 - ext)
return plaintext
def encrypt(plaintext):
pt = pad(plaintext)
if previousPt == "":
key = pt[:16]
iv = rand_iv
c = AES.new(key, AES.MODE_CBC, iv)
else:
key = previousPt[:16]
iv = previousCt[-16:]
c = AES.new(key, AES.MODE_CBC, iv)
ct = c.encrypt(pt)
return ct, iv
# if recive():
# save(recive())
# if read():
# previousCt = unhexlify(read())
# previousPt = decrypt(previousCt)
plaintext = input()
ciphertext, st = encrypt(plaintext)
print(hexlify(ciphertext + st))
# send(hexlify(ciphertext + st))
|
output.txt
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| Alice: 73d7db011639016953d686a0e1a35bb41500c979211a1cf354bd54f8b38a37a866da4b221f71b2e02f9c08d24908f957
Bob: 52295dc859c7f99e0b711e438fe82009fc3bb14648437adca2ff350a26cdc23c66da4b221f71b2e02f9c08d24908f957
Alice: 89a103a7825bc8e4f6b10062c8a5ff39d224143fd2bc3e7a67085656c054719e66da4b221f71b2e02f9c08d24908f957
Bob: a3f75a5afb2f2d7dfc0fbfe9d01f17cb8aa192f9da49f009c3d467b908b72b3466da4b221f71b2e02f9c08d24908f957
|
根据题目描述，已知第一句密文的明文为hello, how are you?
从main.py代码中可以得到以下几点内容
1、第一句加密的key为明文的前16个字符
2、后一句加密使用的key是前一句明文的前16个字符
由此可以写出解密代码如下
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| from Crypto.Cipher import AES
from binascii import hexlify, unhexlify
previousCt = "hello, how are you?"
previousPt = "73d7db011639016953d686a0e1a35bb41500c979211a1cf354bd54f8b38a37a866da4b221f71b2e02f9c08d24908f957"
rand_iv = bytes.fromhex("66da4b221f71b2e02f9c08d24908f957")
def pad(plaintext):
ext = len(plaintext) % 16
if ext != 0:
plaintext += "0" * (16 - ext)
return plaintext
def encrypt(plaintext):
pt = pad(plaintext)
if previousPt == "":
key = pt[:16]
iv = rand_iv
c = AES.new(key.encode("utf-8"), AES.MODE_CBC, iv)
else:
key = previousPt[:16]
iv = previousCt[-16:]
c = AES.new(key.encode("utf-8"), AES.MODE_CBC, iv.encode("utf-8"))
ct = c.encrypt(pt.encode("utf-8"))
return ct, iv
#
def decrypt(cipertext):
key = previousPt[:16]
iv = previousCt[-16:]
print(key,iv)
c = AES.new(key, AES.MODE_CBC, iv)
ct = c.decrypt(cipertext[:-16])
return ct
# if recive():
# save(recive())
# if read():
# previousCt = unhexlify(read())
# previousPt = decrypt(previousCt)
plaintext = input()
ciphertext, st = encrypt(plaintext)
print(hexlify(ciphertext + st))
# send(hexlify(ciphertext + st))
previousPt = "hello, how are you?".encode("utf-8")
previousCt = unhexlify("73d7db011639016953d686a0e1a35bb41500c979211a1cf354bd54f8b38a37a866da4b221f71b2e02f9c08d24908f957")
previousPt = decrypt(previousCt)
print(previousPt)
previousCt = unhexlify("52295dc859c7f99e0b711e438fe82009fc3bb14648437adca2ff350a26cdc23c66da4b221f71b2e02f9c08d24908f957")
previousPt = decrypt(previousCt)
print(previousPt)
previousCt = unhexlify("89a103a7825bc8e4f6b10062c8a5ff39d224143fd2bc3e7a67085656c054719e66da4b221f71b2e02f9c08d24908f957")
previousPt = decrypt(previousCt)
print(previousPt)
previousCt = unhexlify("a3f75a5afb2f2d7dfc0fbfe9d01f17cb8aa192f9da49f009c3d467b908b72b3466da4b221f71b2e02f9c08d24908f957")
previousPt = decrypt(previousCt)
print(previousPt)
# send(hexlify(ciphertext + st))
|
最终输出结果
PatienceApp
附件内含一个可执行性文件，在windows下修改后缀后运行，结果如下
拖入IDA，可以看到以下代码，程序在这样的循环体中花费大量时间
此时有多种做法，比如直接修改掉跳转语句。这里不做过多说明，主要想通过这题，演示patch过程
通过IDA定位到长循环的汇编代码如下
其中涉及到两个跳转语句
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| .text:004011F0 jle short loc_4011D5
.text:00401207 jge short loc_4011D0
|
打算将其修改成相关的跳转语句
比如
选中语句，右键选择Keypatch->Patcher
Assembly中将jle修改成jge即可，同理修改另一处，修改后的结果如下
此时通过下图选项将修改保存
此时成功将我们两次修改保存下来，退出IDA，直接运行程序，直接打印flag
It all starts from the beginning
Level: Hard
Category: Reverse Engineering
Point: 200
SB-Coin: 5
Challenge Description
Can you find the beginning!?
File:fina,Main.class
fina内容如下：
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| ~T\7EWE1W;A5ZTdJ0Vag[ZREaHQEK~
|
用java反编译工具打开Main.class文件，获取到源码如下
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| import java.io.FileWriter;
import java.io.IOException;
import java.util.Arrays;
import java.util.Random;
public class Main {
static String passer="a";
static int run=0;
public static void main(String[] args)throws IOException {
System.out.println("you shouldn't start from here :)");
}
public static void juzbYn(String[] args)throws IOException {
String Input;
if (args[0].contains("SBCTF{")){
Input=args[0];
}
else {
run++;
if (run==5){
System.exit(0);
}
Input=passer;
}
char array[]=passer.toCharArray();
int j=0;
for (int i=0;i<array.length;i++){
if (i%5==0){
if (j!=0) {
System.out.println(i + " : " + array[i]);
char a = array[j* 5];
System.out.println(a);
array[j* 5] = array[j-1];
array[j-1] = a;
}
j++;
}
}
StringBuilder stringBuilder=new StringBuilder();
for (char ch:array){
stringBuilder.append(ch);
}
passer = stringBuilder.toString();
Ft7BEg(new String[]{args[0]});
}
public static void b837Lz(String[] args) throws IOException {
String Input;
if (args[0].contains("SBCTF{")){
Input=args[0];
args[0]="aa";
}
else {
run++;
if (run==5){
System.exit(0);
}
Input=passer;
}
char array[]=passer.toCharArray();
for (int i=0;i<array.length;i++){
if (i%5==0){
array[i]=(char)((array[i]+255)-51*5);
}
if (i%2==0){
array[i]=(char)((array[i]+282)-141*2);
}
}
StringBuilder stringBuilder=new StringBuilder();
for (char ch:array){
stringBuilder.append(ch);
}
passer = stringBuilder.toString();
FileWriter myWriter = new FileWriter("fina",true);
myWriter.write(passer+"\n");
myWriter.close();
e66Cbf(new String[]{args[0]});
}
public static void e66Cbf(String[] args)throws IOException {
String Input;
Random rand = new Random();
if (args[0].contains("SBCTF{")){
Input=args[0];
args[0]="aa";
run++;
}
else {
run++;
if (run==5){
System.exit(0);
}
Input=passer;
}
char array[]= Input.toCharArray();
System.out.println(array);
int i=0;
for (char b:array){
array[i]= (char) (b+1);
i++;
}
int randoms[] = new int[6];
for (i=0;i<5;i++){
randoms[i]= rand.nextInt(9);
}
i=0;
for (char b:array) {
array[i] = (char) (b + randoms[i % 6]);
i++;
}
StringBuilder stringBuilder=new StringBuilder();
for (char ch:array){
stringBuilder.append(ch);
}
passer = stringBuilder.toString();
juzbYn(new String[]{args[0]});
}
public static void Ft7BEg(String[] args)throws IOException {
String Input;
if (args[0].contains("SBCTF{")){
Input=args[0];
}
else {
run++;
if (run==5){
System.exit(0);
}
Input=passer;
}
char array[]=passer.toCharArray();
for (int i=0;i<array.length;i++){
if (i%2==0){
array[i]= (char) (array[i]^2);
}
}
StringBuilder stringBuilder=new StringBuilder();
for (char ch:array){
stringBuilder.append(ch);
}
passer = stringBuilder.toString();
b837Lz(new String[]{args[0]});
}
}
|
通过运行可以知道函数的运行顺序是e66Cbf->juzbYn->Ft7BEg->b837Lz
然后代码阅读可以知道一下几点内容：
- Flag开头是SBCTF{
- e66Cbf:随机生成一个长度为6的数组randoms，将用户输入后的Flag，先每个字符都+1，再根据索引i，每个字符加上randoms[i%6]，逆过程可以根据Flag开头6位的变动可以推出实际的randoms
- juzbYn：将上一步的结果的前6位，分别与索引5，10，15，20，25的字符交换位置。逆过程是同样的。
- Ft7BEg：偶数索引的字符与2做异或。逆过程是同样的
- b837Lz：将结果输出到文件中。函数内的循环是一个烟雾弹
基于以上几点内容，写出以下脚本
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| prefix = "SBCTF{"
result = "~T\\7EWE1W;A5ZTdJ0Vag[ZREaHQEK~"
rlist = list(result)
for i in range(len(rlist)):
if i % 2 == 0:
rlist[i] = chr(ord(rlist[i]) ^ 2)
j = 0
for i in range(len(rlist)):
if i % 5 == 0:
if j != 0:
rlist[i], rlist[j - 1] = rlist[j - 1], rlist[i]
j += 1
randoms = []
for i in range(6):
randoms.append(ord(rlist[i]) - ord(prefix[i]))
rlist[i] = prefix[i]
for i in range(6, len(rlist)):
rlist[i]=chr(ord(rlist[i])-randoms[i%6])
print("".join(rlist))
|
运行后的结果如下：
