Doom cheat code encryption

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'''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. Twitter user [https://twitter.com/Foone @Foone] described it in a 2019 thread.
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'''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 [https://twitter.com/Foone @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.
 
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.
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== Details ==
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As described by @Foone, who has [https://twitter.com/Foone/status/1190656026342637569 allowed these descriptions to be released as CC0] so they can be used here:
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:So Doom (1993) has a neat bit of encryption in it. It's not very strong encryption, but it's still encryption.
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: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.
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:So here's the code I'm talking about, the macro SCRAMBLE
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:It looks annoyingly complicated but it's not, really.
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:It's taking an 8-bit number and shifting around some of the bits.
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:[[File:Doom-cheat-code-encryption-01.png]]
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:If you diagram out what's happening, it makes slightly (BUT ONLY SLIGHTLY) more sense.
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: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.
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:[[File:Doom-cheat-code-encryption-02.jpg]]
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:So, how is this code used?
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: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:
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:[[File:Doom-cheat-code-encryption-03.png]]
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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 ==
 
== Links ==

Latest revision as of 00:56, 3 November 2019

File Format
Name Doom cheat code encryption
Ontology
Released 1993

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.
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.
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:
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.

[edit] Links

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