103
Windows Defender Anti-virus Bypassed Using Direct Syscalls & XOR Encryption
(cybersecuritynews.com)
this post was submitted on 12 Apr 2025
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XOR cleartext once with a key you get ciphertext. XOR the ciphertext with the same key you get the original cleartext. At its core this is the way the old DES cipher works.
A bit of useful trivia: If you XOR any number with itself, you get all zeros. You can see this in practice when an assembly programmer XOR's a register with itself to clear it out.
That's how it was done in the old days to save a few cycles in Z80 assembly. XOR A instead of LD A, 0.
I use that daily in my accelerator work.
Once you learn the trick, you just use it naturally.
Apple had this undocumented function for screenshotting back on iOS 3.1, and kind of let you use it while waiting for better frameworks in iOS 4.0
At some point they started rejecting your app automatically if they found the symbol for that function in your app. I didn't want to leave my 3.1 users in the dust for no reason, so I did the same trick to obfuscate the symbol name before dynamically linking it in.
It worked right up until they stopped supporting iOS 3.1 completely.
Ftfy. not all CPUs have an xor register with itself instruction.
There are a lot more architectures than just x86 that are capable of XORing a register with itself (ie. ARM and RISC-V), and if you took OP to mean the accumulation register specifically, pretty much all CPUs going back as far as I can think have had that functionality.
Yes, but it's not universal that xoring a register with itself is more performant than simply loading it with 0.
I never made that claim, nor did the person you corrected.
Yes, but that's why x86 assembly programmers do it...
No argument there. It's also why it's done in ARM, 8080, SM83, z80, 6502, and basically every other assembly language. It's only not done in RISC-V because you can fold 0 into any instruction as an operand, therefore eliminating the need to clear a register before an instruction.
So why correct the person with a more narrow claim that makes it seem like xor being faster than loading zero is a rarity in CPU architectures? If I said "birds can fly", and your response was "eagles can fly. Ftfy. Not all birds can fly", it would be both true and utterly unhelpful.
Hey look, I'm good at something.