Heisenbug Demo

heisenbug.cpp

#include <iostream>
#include <vector>
#include <string>
#include <iomanip>

// example sbox, typical in ciphers
const uint8_t S_BOX[16] = { 
    0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 
    0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76 
};

// this ensures a bad "seed" for demonstration purposes
void poison() {
    volatile uint8_t dirty_memory[64];
    for (int i = 0; i < 64; ++i) {
        dirty_memory[i] = 0xAA;
    }
}

std::string decrypt(const std::vector<uint8_t>& ciphertext) {
    std::string plaintext = "";
    uint32_t rolling_state = 0xDEADBEEF; // seed state
    
    // iv_idx is uninitialized
    // in a correct run, this should be 0
    // if it inherits '0xAA' from poison, we start at the WRONG S-Box index.
    int iv_idx;

    for (uint8_t byte : ciphertext) {
        // ensure iv_idx doesn't segfault by masking it (& 0x0F), 
        // but logic is now dependent on the garbage value.
        uint8_t key_byte = S_BOX[(iv_idx & 0x0F)]; 

        //  rolling hash function
        rolling_state = (rolling_state << 5) ^ (rolling_state >> 3) ^ key_byte;
        
        // decrypt
        char out = byte ^ key_byte ^ (rolling_state & 0xFF);
        plaintext += out;

        // std::cout << "0x" 
        //   << std::setw(2) << std::setfill('0') << std::hex 
        //   << (int)(uint8_t)out << " ";

        iv_idx++; 
    }
    return plaintext;
}

int main() {
    // assume this data was encrypted with iv_idx = 0
    std::vector<uint8_t> encrypted_data = { 0x75, 0xce, 0x72, 0xa5, 0xdd };

    // dirty the stack
    poison();

    // the ghost fix (*sometimes* will fix it)
    // std::cout uses stack buffer for formatting integers/strings.
    // often zeroes out its working buffer "cleaning" 
    // the spot where 'iv_idx' will land next.
    // -----------------------------------------------------------
    // std::cout << "got here" << std::endl;
    // -----------------------------------------------------------

    // keep in mind...the real issue is the uninitialized iv_idx
    // the std::cout just hides it
	
    std::string result = decrypt(encrypted_data);

    std::cout << "decrypted: " << result << std::endl;
    
    // check if it matches "HELLO"
    // "HELLO" is the target if iv_idx starts at 0
    return 0;
}