Doom cheat code encryption

Latest revision as of 11:46, 13 January 2023

Doom cheat code encryption was used in the original 1993 version of Doom to make the cheat codes a little harder for hackers to find, so they didn't appear in the raw binary code as plain ASCII characters. (Of course, the hackers found them anyway.) Twitter user @Foone described it in a 2019 thread.

This low-grade encryption is done by shifting the bits of 8-bit numbers (which can represent single ASCII characters) which reverses the order of the bits except for those representing 4 and 32. The shifted values are stored in a lookup table in the Doom program.

[edit] Details

As described by @Foone, who has allowed these descriptions to be released as CC0 so they can be used here:

So Doom (1993) has a neat bit of encryption in it. It's not very strong encryption, but it's still encryption.

And it's not used in any sort of way you'd normally expect: not copy protection, or multiplayer anti-cheat, or anti-tampering on saves... It's to slow down FAQs.

So here's the code I'm talking about, the macro SCRAMBLE

It looks annoyingly complicated but it's not, really.

It's taking an 8-bit number and shifting around some of the bits.

If you diagram out what's happening, it makes slightly (BUT ONLY SLIGHTLY) more sense.

It kinda looks like they started with a a "reverse the order of these bits" function but then switched it so the 4 and 32 don't get switched, they just go straight through.

So, how is this code used?

Well, in m_cheat.c, it's used to build a lookup table which has all the values pre-encrypted, so it can quickly look them up later. Then, when you press a key, it translates it through this table:

The thread goes on with more discussion of how these codes were used and discovered. It's worthwhile reading for people into this sort of trivia.

[edit] Links

• Twitter thread unrolled