Launcher/compat/auth/uuid/ReversibleUUIDs.java

public static UUID toUUID(String username) {
	ByteBuffer buffer = ByteBuffer.wrap(Arrays.copyOf(username.getBytes(StandardCharsets.US_ASCII), 16));
	return new UUID(buffer.getLong(), buffer.getLong()); // MOST, LEAST
}

public static String toUsername(UUID uuid) {
	byte[] bytes = ByteBuffer.allocate(16).
		putLong(uuid.getMostSignificantBits()).
		putLong(uuid.getLeastSignificantBits()).array();

	// Find username end
	int length = 0;
	while(length < bytes.length && bytes[length] != 0) {
		length++;
	}

	// Decode and verify
	return VerifyHelper.verifyUsername(new String(bytes, 0, length, StandardCharsets.US_ASCII));
}