Game server auth: cookie key generate&check
We have better random generators in C++ than std::time(0); if you're using the latter for its speed, it would really be worth a comment in the code to explain that you're willing to live with such a weak random-number source in exchange for the speed.

Note that embedding the secret in the code will make it visible to anyone who can obtain the source or the binary and read it.  I'd recommend that you read it at runtime from a separate file - it's easier to correctly permission that one file rather than your source and all the build products.

For the case where the id is specified, there's no need to compute a time-based one just to throw it away immediately - I'd turn that conditional around:

std::string gameKey(long long id = -1)
{
  if (id == -1) {
    id = std::time(0) * 1000 + dateCounter++;
  }


Alternatively, write an overloaded function pair:

std::string gameKey()
{
   return gameKey(std::time(0) * 1000 + dateCounter++);
}

std::string gameKey(long long id)


A couple of misspellings: std::size_t and std::uint8_t (both missing namespace prefix in the code - it might work on your platform, but it's not portable).

I think that key can be a const char* or even a string literal:

blake2b(out, in, "", outlen, inString.length(), 0);


General: please give your code a bit more room to breathe - bunching up the source with no whitespace won't make it any more efficient.

Given that outlen is constant, we could declare it constexpr - then we wouldn't need to allocate out, but could declare it in automatic storage.  There's certainly no need to allocate writer and have to delete it - just make it an automatic variable:

Json::Writer writer;
return writer.write(obj);
// look, no delete needed!


If you do need a variable outlen, at least use a smart pointer, so it's cleaned up for you.

Instead of writing to 4 buffers a, b, c and d, it might be simpler to write to a single buffer, and copy substrings from there (it would reduce those repeated loops with similar-but-different ranges).