Programming

Encrypting Shellcode using SystemFunction032/033

After a while, I’m publishing a blog post which made me interested. With the recent tweets about the undocumented SystemFunction032 Win32 API function, I decided to quickly have a look at it. The first thing I noted after Googling this function was the source code from ReactOS. Seems like other SystemFunctions from 001 got other cryptographic functions and hash functions. The SystemFunction032 is an RC4 implementation. This API is in the export table of Advapi32.dll

The export table entry points to the DLL Cryptsp.dll which actually has the function implemented and exported.

Inside the Cryptsp.dll as you can see the SystemFunction032 and SystemFunction033 point to the same offset, which means loading either of these functions will do the same RC4 encryption.

This is the disassembly of the function which does the RC4 encryption. It takes in the data and key structs as parameters.

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Random Compiler Experiments on Arrays

One day a guy asked me how to print a 2d string array in C. So I coded an example for him. But just for curiosity, I examined the assembly code. In C both string[0][1] and *(*string + 1) are the same. But in reality, the compiler writes the assembly code in 2 different ways. If we use string[0][1] it will directly move the value from the stack. When we dereference a pointer *(*string + 1) it will actually dereference the address pointed inside the register. This happens only in the MinGW GCC compiler. I compiled this using the latest on Windows which is 8.2.0-3 by the time I am writing this.

The assembly code in the left is this one.
[code language=”C”]
#include <stdio.h>

int main() {
char *string[][2] = {
{"Osanda","Malith"},
{"ABC","JKL"},
{"DEF","MNO"},
};

printf("%s %s\n", string[0][0], string[0][1]);
}
[/code]

The assembly code on the right is this.
[code language=”C”]
#include <stdio.h>

int main() {
char *string[][2] = {
{"Osanda","Malith"},
{"ABC","JKL"},
{"DEF","MNO"},
};

printf("%s %s\n", **string, *(*string + 1));
}
[/code]
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A Simple API Monitor

This is a simple Windbg script to monitor common Win32 API calls and display the strings, IPs, Ports, Registry keys passed to the APIs. The Win32 API is huge and I have used common APIs used by programs and malware. I coded this for fun 🙂

[code]
Usage: ApiMon.wds run; g;
[/code]

You can remove APIs as you wish to minimize the output or you can add any API you desire. For example
[code]
bp DLLName!APIName @"$$>a<${$arg0} APIName FileNamePtr

bp kernelbase!CreateFileA @"$$>a<${$arg0} CreateFileA 1";
[/code]

This is a sample output that uses CreateProcess API.

This is from running netcat.

Download: https://github.com/OsandaMalith/ApiMon
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