# asm3

## Problem

> What does asm3(0xc4bd37e3,0xf516e15e,0xeea4f333) return? Submit the flag as a hexadecimal value (starting with '0x'). NOTE: Your submission for this question will NOT be in the normal flag format. Source located in the directory at /problems/asm3\_4\_c89016e12b8f3cac92a2e637c03f6139.

* [Source](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Reverse%20Engineering/asm3/test.S)

## Solution

1. Let's look at the source:

   ```
    asm3:
        <+0>:    push   ebp
        <+1>:    mov    ebp,esp
        <+3>:    xor    eax,eax
        <+5>:    mov    ah,BYTE PTR [ebp+0x9]
        <+8>:    shl    ax,0x10
        <+12>:    sub    al,BYTE PTR [ebp+0xd]
        <+15>:    add    ah,BYTE PTR [ebp+0xe]
        <+18>:    xor    ax,WORD PTR [ebp+0x10]
        <+22>:    nop
        <+23>:    pop    ebp
        <+24>:    ret
   ```
2. Since this challenge is more complicated than the previous `asm*` challenges, we will compile and run it.
3. We will modify [test.S](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Reverse%20Engineering/asm3/test.S) to [test\_modified.S](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Reverse%20Engineering/asm3/test_modified.S) like so:

   ```
    .intel_syntax noprefix
    .global asm3

    asm3:
        push   ebp
        mov    ebp,esp
        xor    eax,eax
        mov    ah,BYTE PTR [ebp+0x9]
        shl    ax,0x10
        sub    al,BYTE PTR [ebp+0xd]
        add    ah,BYTE PTR [ebp+0xe]
        xor    ax,WORD PTR [ebp+0x10]
        nop
        pop    ebp
        ret
   ```
4. We also create a [solve.c](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Reverse%20Engineering/asm3/solve.c) script:

   ```cpp
    #include <stdio.h>

    int asm3(int, int, int);

    int main(int argc, char* argv[])
    {
        printf("0x%x\n", asm3(0xc4bd37e3,0xf516e15e,0xeea4f333));
        return 0;
    }
   ```
5. Compile:

   ```
    $ gcc -masm=intel -m32 -c test_modified.S -o test_modified.o
    $ gcc -m32 -c solve.c -o solve.o
    $ gcc -m32 test_modified.o solve.o -o solve
   ```
6. Run `./solve` to get flag.

### Flag

`0xe52c`


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://picoctf2019.haydenhousen.com/reverse-engineering/asm3.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.