I trying to write function, for compile-time hide (obfuscate) text strings in the binary code. Purpose: to disable easy search and modify text messages with binary editor.
Algorithm is simple: convert chars within string to deltas, i. e. string abcd must be saved in the binary file as a\001\001\001.
I wrote the following code, it works:
#include <string>
#include <stdio.h>
#include <string.h>
constexpr std::string str_encrypt(const char *str) {
std::string rc;
unsigned char x = 0, c;
while(c = *str++) {
rc.push_back(c - x);
x = c;
}
return rc;
}
std::string str_decrypt(const std::string &str) {
std::string rc;
unsigned char x = 0;
for(unsigned char c : str)
rc.push_back(x += c);
return rc;
}
const std::string hello(str_encrypt("Hello, world!"));
int main(int argc, char **argv) {
for(unsigned char c : hello)
printf("%d ", c);
putchar('\n');
printf("S=[%s]\n", str_decrypt(hello).c_str());
return 0;
}
But string still is not "encrypted" within binary file:
$ c++ -std=c++2b -O2 constexpr.cpp && strings a.out | grep Hello
Hello, world!
When I change constexpr to consteval, I see compilation error:
$ c++ -std=c++2b -O2 constexpr.cpp
constexpr.cpp:23:36: error: ‘std::string{std::__cxx11::basic_string<char>::_Alloc_hider{((char*)(&<anonymous>.std::__cxx11::basic_string<char>::<anonymous>.std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf))}, 13, std::__cxx11::basic_string<char>::<unnamed union>{char [16]{'H', '\035', '\007', '\000', '\003', '\37777777675', '\37777777764', 'W', '\37777777770', '\003', '\37777777772', '\37777777770', '\37777777675', 0}}}’ is not a constant expression
23 | const std::string hello(str_encrypt("Hello, world!"));
My question: Is possible to write such compile-time encryption? If so, please suggest, how to do.
My compiler: g++ (GCC) 12.2.1 20230201
The problem is that str_encrypt is not evaluated at compile time.
When I change constexpr to consteval, I see compilation error
This is a hint, when we switch to consteval, which requires str_encrypt to be an immediate / compile time function, the compiler gives an error.
Since C++20, std::string can be used in compile time contexts, if it is a transient allocation, i.e. the string must be destroyed before exiting the compile time context.
In this example this is not the case, so str_encrypt cannot be a compile time function.
If we had put:
constexpr std::string hello(str_encrypt("Hello, world!"));
Then we would see a similar error to using consteval on str_encrypt.
However, arrays or std::arrays can be constexpr.
So if instead of returning a std::string we return std::array then the example can work as expected.
This modified example works with g++ / clang++ and requires C++17.
Using consteval would be advisable, to force compile time evaluation, but of course requires C++20.
#include <string>
#include <stdio.h>
#include <string.h>
#include <array>
template <size_t N>
constexpr std::array<char, N> str_encrypt(const char (&str)[N]) {
std::array<char, N> output{};
unsigned char x = 0, c = 0;
for (int i = 0; i < N; ++i) {
c = str[i];
output[i] = (c - x);
x = c;
}
return output;
}
template <size_t N>
std::string str_decrypt(const std::array<char, N> &str) {
std::string rc;
unsigned char x = 0;
for(unsigned char c : str)
rc.push_back(x += c);
return rc;
}
constexpr auto hello = str_encrypt("Hello, world!");
int main(int argc, char **argv) {
for(unsigned char c : hello)
printf("%d ", c);
putchar('\n');
printf("S=[%s]\n", str_decrypt(hello).c_str());
return 0;
}
Output:
$ ./example
72 29 7 0 3 189 244 87 248 3 250 248 189 223
S=[Hello, world!]
The string "Hello, world!" is not in the binary, nor is the str_encrypt function.
$ strings example | grep Hello
$ objdump -t -C example | grep str_
0000000000001302 w F .text 00000000000000a6 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > str_decrypt<14ul>(std::array<char, 14ul> const&)
Godbolt: https://godbolt.org/z/vPj3bW6Yz