How to calculate the length of a mpz_class in bytes? - c++

Question

I want to implement RSA with padding but first I have to find out the length in bytes of the message which is a mpz_class item. Which function would be useful in cpp to accomplish this?
const mpz_class m(argv[1])
What is the length of m in bytes?
Thank you!

Answer 1

#Shawn's comment is correct: The bytes occupied in memory by your class are not what you should be concerned about. Not only does the location of the bytes in memory depend on how your compiler decides to pack them, but their order can also depend on the hardware used.
So, instead of doing some awkward and very fragile memcopy'ish thing that are almost guaranteed to break at some point, you should construct the message yourself (google keyword: Serialization). This also has the advantage that your class can contain stuff that you don't want to add to the message (caches with temp results, or other implementation/optimization stuff).
To the best of my knowledge C++ (unlike f.ex. C#) does not come with build in serialization support, but there are likely to exist libraries that can do a lot of it for you. Otherwise you just have to write your "data member to byte array" functions yourself.
Super simple example:
#include <vector>
#include<iostream>
class myClass
{
int32_t a;
public:
myClass(int32_t val) : a(val) {}
// Deserializer
bool initFromBytes(std::vector<uint8_t> msg)
{
if (msg.size() < 4)
return false;
a = 0;
for (int i = 0; i < 4; ++i)
{
a += msg[i] << (i * 8);
}
return true;
}
// Serializer
std::vector<uint8_t> toBytes()
{
std::vector<uint8_t> res;
for (int i = 0; i < 4; ++i)
{
res.push_back(a >> (i*8));
}
return res;
}
void print() { std::cout << "myClass: " << a << std::endl; }
};
int main()
{
myClass myC(123456789);
myC.print();
std::vector<uint8_t> message = myC.toBytes();
myClass recreate(0);
if (recreate.initFromBytes(message))
recreate.print();
else
std::cout << "Error" << std::endl;
return 0;
}