Entropy is NOT a result of std::<RNG>

As in any public key cryptography implementation, everything starts with an entropy, or simply put a large number, but with the very unique characteristics of

1. being randomly generated, and
2. has never been generated before, or will ever be in the future, and
3. being 128-256 bits in length

However, be warned that the RNG = random number generator in your programming language's standard library is NOT random enough to be used as an entropy generator.