Pasticciotto is a virtual machine which can be used to obfuscate code. It was developed for the PoliCTF 17 as a reversing challenge.

The key feature is its opcode "shuffling": their actual values are determined by a password. (More in IMPLEMENTATION.md)

I wanted to experiment with VM obfuscation since it was a topic that caught my attention while reversing challenges for various CTFs. So, I decided to write one from scratch in order to understand better how instruction set architectures are implemented!

The design and the implementation behind Pasticciotto are not state-of-the-art but hey, it works! :D

In Italian, "Pasticciotto" has two meanings!

The first one is "little mess" which perfectly describes how I put up this project. The second one is a typical dessert from Southern Italy, Salento! It's filled with cream! Yum!

You can use pasticciotto in your own binary! It's easy to do!

Let's say you want to run this C code into pasticciotto:

void main() {
    uint16_t i, a, b;
    a = 0;
    b = 0x10;

    for (i = 0; i < b; i++) {
        a += b;
    }
    return;
}

It can be translated into this pasticciotto's assembly snippet:

$ cat example.pstc
def main:
movi r0, 0x0  # a
movi r1, 0x10 # b
movi s1, 0x0  # i
loop:
addr r0, r1
addi s1, 1
cmpr s1, r1
jpbi loop
shit

Let's assemble it with key HelloWorld:

$ python3 assembler.py HelloWorld example.pstc example_assembled.pstc

Now we are ready to embed the VM in a .c program:

#include "vm/vm.h"
#include <fstream>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    /*
    In order to create the bytecode for pasticciotto, you can use
    the assembler in the assembler/ directory. You can include it with
    `xxd -i example_assembled.pstc`
    */
    unsigned char example_assembled_pstc[] = {
    0x32, 0x00, 0x00, 0x00, 0x32, 0x01, 0x10, 0x00, 0x32, 0x05, 0x00, 0x00,
    0xaf, 0x01, 0xcf, 0x05, 0x01, 0x00, 0x8b, 0x51, 0xc5, 0x0c, 0x00, 0x0c
    };
    unsigned int example_assembled_pstc_len = 24;
    unsigned char key[] = {
    0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x57, 0x6f, 0x72, 0x6c, 0x64, 0x0a
    };


    puts("I should try to eat a pasticciotto...\n");
    VM vm(key, example_assembled_pstc, example_assembled_pstc_len);
    vm.run();
    return 0;
}

That's it!

The VM data / code / stack sections are represented through the VMAddrSpace object. It is defined here. The registers are in a uint16_t array in the VM object defined here.

void foo() {
    // creating the VM with some code
    VM vm(key, code, codelen);

    // accessing the data section
    printf("First data byte: 0x%x", VM.as.data[0]);
    // accessing the code section
    printf("First code byte: 0x%x", VM.as.code[0]);    
    // accessing the stack section
    printf("First stack byte: 0x%x", VM.as.stack[0]);
    // accessing the IP register
    printf("The IP is: 0x%x", VM.regs[IP]);
    return;
}

You can find the client and the server under the polictf/ directory. I have also written a small writeup. Check it out!

Check out the file IMPLEMENTATION.MD to understand how the VM works and which operations it can do! Watch out for some spoilers if you haven't completed the challenge though!

I wanted to polish the VM even more but I haven't got the time to do it. There are rough edges for sure!

Any contribution is very welcome! Feel free to open issues and pull requests!

Copyright 2017 Giulio De Pasquale

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