ESPIO payload decryption utility

epsiod.cpp

/*
* ESPIO payload decryption utility
* Author: @luc4m
*
* Compile with g++ ./espiod espiod.cpp
* Usage:
*    ./espiod KEYFILE ENCRYPTEDPAYLOADFILE
*     it outputs the decrypted file on "plaintext.bin"
*
* References:
*   - https://github.com/Konis-Bros/Espio/
*/
#include <iostream>
#include <fstream>
#include <string>
#include <vector>


static const std::string base64_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static inline bool is_base64(unsigned char c) {
    return (isalnum(c) || (c == '+') || (c == '/'));
}

const std::string base64_decode(std::string const& encoded_string) {
    int i = 0;
    int j = 0;
    int in_ = 0;
    unsigned char char_array_4[4], char_array_3[3];
    std::string decoded_string;
    size_t in_len = encoded_string.size();

    while (in_len-- && (encoded_string[in_] != '=') && is_base64(encoded_string[in_]))
    {
        char_array_4[i++] = encoded_string[in_]; in_++;
        if (i == 4)
        {
            for (i = 0; i < 4; i++)
            {
                char_array_4[i] = base64_chars.find(char_array_4[i]);
            }

            char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
            char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
            char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

            for (i = 0; (i < 3); i++)
            {
                decoded_string += char_array_3[i];
            }
            i = 0;
        }
    }

    if (i)
    {
        for (j = i; j < 4; j++)
        {
            char_array_4[j] = 0;
        }

        for (j = 0; j < 4; j++)
        {
            char_array_4[j] = base64_chars.find(char_array_4[j]);
        }

        char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
        char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

        for (j = 0; (j < i - 1); j++)
        {
            decoded_string += char_array_3[j];
        }
    }

    return decoded_string;
}

int main(int argc, char *argv[])
{
    if (argc != 3)
    {
        std::cerr << "Usage: " << argv[0] << " key.bin encrypted_payload.bin" << std::endl;
        return 1;
    }

    std::ifstream keyFile(argv[1], std::ios::binary | std::ios::ate);
    std::ifstream encryptedPayloadFile(argv[2], std::ios::binary | std::ios::ate);
    if (!keyFile || !encryptedPayloadFile)
    {
        std::cerr << "Failed to open input files." << std::endl;
        return 1;
    }

    size_t keySize = keyFile.tellg();
    size_t encryptedPayloadSize = encryptedPayloadFile.tellg();
    keyFile.seekg(0, std::ios::beg);
    encryptedPayloadFile.seekg(0, std::ios::beg);

    std::vector<unsigned char> key(keySize);
    std::vector<unsigned char> encryptedPayload(encryptedPayloadSize);
    keyFile.read(reinterpret_cast<char*>(key.data()), keySize);
    encryptedPayloadFile.read(reinterpret_cast<char*>(encryptedPayload.data()), encryptedPayloadSize);
    keyFile.close();
    encryptedPayloadFile.close();
    char* obfuscatedPayload = reinterpret_cast<char*>(encryptedPayload.data());

    const std::string toDecryptPayload = base64_decode(obfuscatedPayload);
    size_t toDecryptSize = toDecryptPayload.size();

    int keyIndex = 0;
    std::string payload = "";
    for (int i = 0; i < toDecryptSize; i += 4)
    {
        std::string currentByte = std::string() + toDecryptPayload[i] + toDecryptPayload[i + 1] + toDecryptPayload[i + 2] + toDecryptPayload[i + 3];
        payload += stol(currentByte, nullptr, 0) ^ key[keyIndex++ % keySize];
    }

    std::ofstream plaintextFile("plaintext.bin", std::ios::binary);
    plaintextFile.write(payload.c_str(), payload.size());
    plaintextFile.close();

    std::cout << "Decryption complete." << std::endl;

    return 0;
}