Why do I need to delete the META-INF folder when installing mods?

Question

In Minecraft, nearly every tutorial on how to install a MOD tells you to remove the META-INF folder. However none of the ones I found tell you what it is. I don't like the idea of deleting a folder without knowing what it is or what it does.

So what is the META-INF folder, and why should I delete it?

Answer 1

Because Minecraft is digitally signed. META-INF contains the manifest, but also a signature file (MOJANG_C.SF) that accounts for all the files within the JAR and their checksums, and Mojang's actual signature (MOJANG_C.DSA).

When that signature checks out, you know it wasn't tampered with and comes from a trusted source (in this case, Mojang). When you do tamper with it (by installing mods), the signature is invalid and Minecraft won't start at all.

By deleting the META-INF directory, you remove the signature completely. Depending on your security settings, you might get an "unknown publisher" warning, but you'll be able to run it.

More detailed info: JAR File Specification