Why does Monero use epee in the crypto module?

Question

Is there any obvious gain to using epee in the crypto module?

I don't know why this code is inside crypto instead of cryptonote:

  inline std::ostream &operator <<(std::ostream &o, const crypto::public_key &v) {
    epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
  }
  inline std::ostream &operator <<(std::ostream &o, const crypto::secret_key &v) {
    epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
  }
  inline std::ostream &operator <<(std::ostream &o, const crypto::key_derivation &v) {
    epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
  }
  inline std::ostream &operator <<(std::ostream &o, const crypto::key_image &v) {
    epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
  }
  inline std::ostream &operator <<(std::ostream &o, const crypto::signature &v) {
    epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
  }

It is a way to convert data to hex, so why is this part of the crypto module?

And maybe a template function can do the job:

template <typename T> std::ostream &operator <<(std::ostream &o, T &v) {
  epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
}

As we know public_key and private_key are parts of cryptonote not crypto. So why should this code be put into the crypto module?

To my understanding, the crypto module only cares about hashing. Am I wrong?

Answer 1

It may help to first understand that CryptoNote is a protocol and that the crypto module contains cryptographic primitives.

public_key, secret_key, key_derivation, key_image, signature... these are all cryptographic primitives, hence they are all correctly in the crypto module, as are the hashing functions.

The reason for the output stream functions being in the crypto module and not CryptoNote is that they are not CryptoNote specific. They provide a way to output the primitives to a stream (encoded as hex).