Getting a buffer into a stringstream in hex representation:

Look at the stream modifiers: std::setw and std::setfill. It will help you.

#include <sstream>
#include <iomanip>

std::stringstream ss;
ss << std::hex << std::setfill('0');
for (int i = 0; i < 32; ++i)
{
    ss << std::setw(2) << static_cast<unsigned>(buffer[i]);
}

You can do this with C++20 std::format:

std::stringstream ss;
for (int i = 0; i < 32; ++i) {
  ss << std::format("{:02}", buffer[i]);
}

Until std::format is widely available you can use the {fmt} library, std::format is based on. {fmt} also provides the join function that makes this even easier (godbolt):

std::string s = fmt::format("{:02}", fmt::join(buffer, ""));

Disclaimer: I'm the author of {fmt} and C++20 std::format.