@LexManos

Wow, I am surprised. Your vocabulary ("screw you") is so non-professional and childish that I am really surprised why you consider yourself a serious developer or member of some team here. I'm not going to report you, but I hope you realize that, when you grow up, you will need to reconsider your tone when you talk to people.

Also, deleting comments is not nice. What are you trying to achieve?

But anyway let me respond to your nonsense from your last comment.

Your use case is assine

I have no idea what "assine" means. What are you trying to say is not clear to me.

and screw you for trying to remove credit to those who have spent countless hours to create the system you enjoy

Not everyone agree with copyright laws. There are some people who think that copyright is nonsense. I am one of them. Basically, only physical object can be sold (house, vehicle, house gadgets, etc). But, information cannot be sold. It means books, songs, videos, games, softwares, etc are free by default, independently of the choice of the author. Therefore, "stealing" such goods is not against my moral laws, and I will keep doing it. But, since the copyright is nonsense, I don't want to see copyright notification in the pirated softwares (games) I'm using.

They spent countless hours, that's true. And they spent those countless hours developing something I enjoy, that's also true. But, that is still not a reason for me to respect them. I will keep "stealing" their software and sharing for free, as long as their software is protected by copyright laws.

You will find no help here

I realized it. But I found help myself and I want to share for other people who may face the same problem. Don't delete my comment. In my comment, I explained how to do it for other people who may have the same problem in the future. There is no reason to delete my comment, it doesn't violate Github terms of use and didn't break any copyright laws. I didn't share serial keys, or cracked minecraft launcher, or premium email and password, etc. I just shared a simple command line arguments. And I'll repeat it here.

Basically, the solution is to put the following string in your JVM argument in the minecraft launcher (any patched launcher from the pirate bay works fine):

-Dfml.ignorePatchDiscrepancies=true -Dfml.ignoreInvalidMinecraftCertificates=true
Then launch minecraft and it should work even if you modify the blr.class file (don't forget to delete meta inf forlder).

and again a giant screw you for trying to remove credit to the authors of this software.

Why so many anger? How much did Notch pay you to talk like that?

Some additional info for future visitors who may face the same problem
If you don't like the copyright notice, then follow these steps in order to remove it:

Extract blr.class from 1.12.2.jar (using winrar for example)
Open it using some hex editor (I would recomment HxD, it is free)
Find string Copyright Mojang AB. Do not distribute!
Replace the whole copyright message with bytes 0x20 (it represents ascii space)
Save the file
Put the file back to 1.12.2.jar
Delete META-INF folder from 1.12.2.jar
In JVM arguments put -Dfml.ignorePatchDiscrepancies=true -Dfml.ignoreInvalidMinecraftCertificates=true
No copyright message will appear. Enjoy your pirated and free software, don't let them convince you that you need to pay before you can play