New auth plugin:

• uses KDF (try to use what openssl provides)
• salted
• uses server- and client-side scrambles
• set as default auth plugin
• ed25519 based? (or try to use what openssl provides)
• support old hashes?

reduce roundtrips:

• set as default
• allow clients to provide the salt (--plugin-salt=XXX)
  • if it's incorrect — connection fails.
  • PASSWORD() returns the salt in a Note

Rough idea

• the KDF function is pbkdf2 (supported by everything, including windows native, Java, javascript, PHP, .NET
• parameters to the pbkdf2 are stored in with authentication plugin data : hash function (SHA512,SHA256), interation count, salt, key_length, together with derived key = PBKDF2(func, password, oalt, iteration_count, key_length)

Login process, packet exchange

1. Server sends ServerPluginParameters message with hash function, interation count, salt, key_length.
This is the only unencrypted message during entire exchange

2 . Client computes derived key from password and parameters:
derived_key= PBKDF2(hash_func, password, salt, iteration_count, key_length)
Client sends ServerVerificationChallenge = AES_ENCRYPT(client_scramble,derived_key) to server

3. server decrypts ServerVerificationChallenge and sends

ServerVerificationResponse = AES_ENCRYPT(concat(server_scramble,client_scramble)), derived_key))

4. client verifies AES_DECRYPT(ServerVerificationResponse, derived_key) =concat(server_scramble,client_scramble).
If they don't match, client could not verify the server, and error is reported.

Client still has to prove it has the password, not just the derived key
So it sends
ClientEncryptedPassword message = AES_ENCRYPT(concat(hash_func(password),server_scramble,client_scramble)),derived_key)

5. Server verifies the client
a) tmp = AES_DECRYPT(ClientEncryptedPassword. derived_key)
b) hashed_password=substr(tmp, hash_length)
c) derived_key2 = PBKDF2(hash_func, hashed_password, salt, iteration_count, key_length)
and compares derived_key and derived_key2 for equality, (due to the HMAC_collisions property of pbkdf2, password and hash_func(password) would produce the same keys)
Server sends OK or ERR packet