Doom cheat code encryption

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.

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.


 * [[File:Doom-cheat-code-encryption-01.png]]


 * 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.


 * [[File:Doom-cheat-code-encryption-02.jpg]]


 * 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:


 * [[File:Doom-cheat-code-encryption-03.png]]

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.

Links

 * Twitter thread unrolled