Shulou(Shulou.com)06/01 Report--

0x0 first let's install HT with the following command

$brew install ht

0x1 wrote our first macOS program: crackme created a crackme folder in Project to hold the source code and compilation results.

$cd Project

$mkdir crackme

$cd crackme

Use the vim that comes with the system to create the source code file cm01.c for crackme

$vim cm01.c

At this point you can see the editing interface of vim

Press an on the keyboard to enter edit mode and enter the following code

# include int main () {int secret = 0; printf ("Please enter the secret num:"); scanf ("% d", & secret); if (secret! = 123) {printf ("Incorrect secret num.\ n"); return 0;} printf ("Hello world!\ n"); return 0;}

When finished, press the ESC key to exit edit mode, enter: wq to save and exit vim

Then compile with clang

$clang cm01.c

If there is no error, the cm01 executable file will be generated. If there is an error, you should check whether the code is typed incorrectly.

Test whether crackme is running properly.

Next, let's crack it so that it will output "Hello world!" when we enter any value.

Start HT on the terminal, as shown in the figure

Press fn+f3 to select the file, tab to move the cursor to cm01 and press enter

The hexadecimal interface will appear. We press fn+f6 to switch to the disassembly interface and select Mach-0/image.

Start looking for the key jump to determine whether the number is correct, and modify it. A simple way is that we enter a wrong number and there will be Incorrect secret num. We use this sentence as a breakthrough to press fn+f7 to start searching for Incorrect. Here I have a pit that is the case of Incorrect, which will prompt me not to find it. To prevent it from falling, I will change the uppercase I to uppercase I.

Mode We choose "display:regex" to enter Incorrect enter, and the cursor will be located to the logical point. You can see the string "Incorrect secret num", so the code on it is the jump of judgment. The code is as follows

100000ef0! Jz loc_100000f13

To start modifying the key hop, press ctrl+a to change jz to jnz. Enter HT will list the machine code of the assembly code written. HT will help us choose the most appropriate one, and you can enter directly. You can see the "85" turning red.

Press fn+f2 to save and start testing whether crackme is working properly. Entering a wrong number will output "Hello,world" to prove that we have cracked it successfully, as shown in the following figure

But this kind of cracking is not perfect. If you want to output Hello World in any case, just change je to jmp. Test it again.

0x3 everything we enter will indicate success, and here we have cracked the first crackme successfully.

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