Use Minecraft protocol's method to get UUID from username

[joshtch]

The Minecraft protocol specifies how servers obtain the offline UUID given the username. Portablemc should use that method to get the UUID in offline mode when a username is provided, so switching from offline mode in another launcher to offline mode in portablemc will appear the same to a remote server, and player data will carry over.

[joshtch]

From the Minecraft protocol:

For offline-mode servers (the ones with authentication disabled), encryption is optional, and part of the process can be skipped. In that case Login Start is directly followed by Login Success. The Notchian server only uses UUID v3 for offline player UUIDs, deriving it from the string OfflinePlayer:<player's name> For example, Notch’s offline UUID would be chosen from the string OfflinePlayer:Notch.

There are various online services that will calculate the UUID for you, but we can also compute it ourselves.

[joshtch]

In Java, the command would be: (thanks to Tomasan7 on SpigotMC forums)

UUID.nameUUIDFromBytes("OfflinePlayer:<player_name>".getBytes(StandardCharsets.UTF_8));

Ironically, there's no mention of a hashed namespace, and indeed, the Java function doesn't accept one, so Python's uuid.uuid3 won't work. Here's a working reimplementation of Java's UUID.nameUUIDFromBytes in Python:

import hashlib
import uuid

def nameUUIDFromBytes(name: bytes) -> uuid.UUID:
    md5_hash = hashlib.md5(name, usedforsecurity=False)
    md5_bytes = bytearray(md5_hash.digest())
    md5_bytes[6] &= 0x0f   # clear version
    md5_bytes[6] |= 0x30   # set to version 3
    md5_bytes[8] &= 0x3f   # clear variant
    md5_bytes[8] |= 0x80   # set to IETF variant
    return uuid.UUID(bytes=bytes(md5_bytes))

The offlineUUID function is then:

def offlineUUID(player_name: str) -> str:
    return nameUUIDFromBytes(b'OfflinePlayer:' + player_name.encode()).hex

As one function, with nameUUIDFromBytes inlined and uuid.UUID no longer necessary:

import hashlib

def offlineUUID(player_name: str) -> str:
    bytestring = b'OfflinePlayer:' + player_name.encode()
    md5_hash = hashlib.md5(bytestring, usedforsecurity=False)
    md5_bytes = bytearray(md5_hash.digest())
    md5_bytes[6] = md5_bytes[6] & 0x0f | 0x30
    md5_bytes[8] = md5_bytes[8] & 0x3f | 0x80
    return md5_bytes.hex()

[mindstorm38]

I'm aware of this issue, and this is already the default offline mode in the upcomming Rust rewrite!
Thanks for the patch! I'll look into this in the coming days.

I'm concerned that the UUID computation will not be the same anymore and maybe cause issues with players who doesn't expect this? At least, this should not impact singleplayer because UUID is not used, as I'm aware, and for offline-mode servers the computation is done server-side anyway. Do you know of any other cases that would be detrimental to this change?